ADU Rules in Cohasset, MA
Yes. Cohasset allows ADUs by-right up to 900 sq ft under state law. The friction is not the zoning vote. It is coastal flood-zone construction, a local wetlands bylaw stricter than the state, grinder-pump capacity, and an older local bylaw that state law has already overridden.
Call BuildX: (781) 627-7000
Cohasset ADUs are legal under the town's adopted ADU bylaw
ADU bylaw adopted (§300-4.3B) · By-right up to 900 sq ft, plus ADU-L by special permitCohasset adopted an Accessory Dwelling Unit bylaw (§300-4.3B, amended at the December 9, 2024 Special Town Meeting, Art. 15) [Bylaw §300-4.3B]. One ADU up to 900 sq ft is allowed by-right on a single-family lot, with no special permit and no owner-occupancy requirement, consistent with state law [760 CMR 71] [MGL 40A]. The town also allows a larger ADU-L ("ADU Large") by special permit from the Zoning Board of Appeals, with no stated size cap, where the owner occupies the property and the principal dwelling is at least 1,250 sq ft and at least 10 years old [Bylaw §300-4.3B(1.1)]. Detached, attached, and interior-conversion ADUs are all allowed, subject to the building code, septic capacity, and the town's coastal flood and wetlands review. Short-term rental of an ADU is prohibited.
Can I build an ADU in Cohasset, MA?
Yes. Cohasset's adopted ADU bylaw (§300-4.3B) allows one ADU up to 900 sq ft by-right on a single-family lot, with no special permit and no owner-occupancy requirement. A larger ADU-L is allowed by special permit (no stated size cap) where the owner lives on the property and the principal dwelling is at least 1,250 sq ft and at least 10 years old. Conservation Commission filings and Floodplain Administrator certification still apply where the parcel sits in a wetland buffer or coastal flood zone, and short-term rental of an ADU is prohibited.
Show Cohasset's adopted ADU bylaw: by-right ADU, ADU-L by special permit, and what still applies
Cohasset adopted its Accessory Dwelling Unit bylaw at §300-4.3B of the Zoning Bylaw, amended 6-3-2024 ATM Art. 18 and 12-9-2024 STM Art. 15, with the definition at §300-2.1 [Bylaw §300-4.3B]. The older "Accessory Apartment" special-permit framework was replaced.
ADU, by-right (§300-4.3B(1)). One ADU is allowed by-right within the principal dwelling or an accessory building in a single-family residential district, up to 900 sq ft. No special permit and no owner-occupancy are required. At least one off-street parking space is required, except none is required within 0.5 miles of a commuter rail, subway, ferry, or bus station. The ADU must keep a residential exterior, have adequate water and sewer per the Board of Health, and may not be used as a short-term rental.
ADU-L, by special permit (§300-4.3B(1.1)). A larger "ADU Large" is allowed by special permit from the Zoning Board of Appeals, with no stated size cap. It requires owner-occupancy of the principal dwelling or the ADU-L, a principal single-family dwelling of at least 1,250 sq ft net floor area that has existed for at least 10 years, and a ZBA finding that the ADU-L will not be more detrimental to the neighborhood. Short-term rental is prohibited.
What still applies on top of the bylaw. Conservation Commission review under the local Wetlands Protection Bylaw (Chapter 260) and Floodplain Administrator certification for Zone V and Zone A parcels still run if your parcel sits within a wetland buffer or a FEMA flood zone. Sewer Commission review of added wastewater flow, including grinder-pump capacity, also still applies.
Confirm the current application process and any ADU-L thresholds with the Building & Inspections Department at (781) 383-4100 ext. 5100 [Building & Inspections] before you sign anything. For most homeowners the by-right path at or under 900 sq ft is straightforward; what drives timeline and cost is upstream of zoning: coastal flood elevation and engineer certification, the local wetlands buffer, watershed overlay review, and grinder-pump or Title 5 capacity.
Garage conversion ADU
A common variation of either the detached or attached ADU path, depending on the garage configuration. State law treats the ADU by its relationship to the principal dwelling, not as a separate garage-conversion category. Classification turns on whether the converted structure is structurally attached to the principal dwelling.
- If the garage is attached to the house: Treated as an attached ADU under state law
- If the garage is detached from the house: Treated as a detached ADU under state law
- Best for: Properties with a structurally sound existing garage; reuse of foundation, walls, and roof; faster construction than ground-up
- Watch for in Cohasset: Coastal flood-zone elevation if the garage sits in Zone V or A, foundation adequacy, separate-entrance and bathroom plumbing additions, grinder-pump or septic capacity for the added bathroom, and Conservation Commission filing if within a wetland buffer
Cohasset ADU Rules at a Glance
The six things most Cohasset homeowners ask first. Tap "View all rule details" for the complete data set.
View all Cohasset rule details
- ADU types allowed: Detached, attached, interior conversion (by-right under state law)
- Districts allowed in: Residential RA, RB, RC (any zone that allows single-family dwellings)
- Local ADU bylaw status: Adopted §300-4.3B (12-9-2024 STM Art. 15): by-right ADU plus ADU-L by special permit
- Owner-occupancy: Not required for a by-right ADU; required for an ADU-L (§300-4.3B(1.1))
- Max size: By-right 900 sq ft or 50% of principal dwelling, whichever is smaller; larger by ADU-L special permit (no stated cap)
- Parking: One space maximum under state law; none required within 0.5 mi of Cohasset Station
- Sewer status: Mixed. 60+ miles of pipeline plus grinder pumps in many areas; coastal and peripheral parcels on Title 5 septic
- Watershed overlay: Watershed Resource District (protects town water supply)
- Coastal flood zones: FEMA Zone V (high-hazard) and Zone A across much of coastal Cohasset
- Local wetlands bylaw: Chapter 260, stricter than state; 200-foot riverfront area
- Dimensional standards (RA, RB, RC): Per Chapter 300 Article 5. Confirm minimum lot, frontage, coverage, height, and setbacks with the Cohasset Planning Department at (781) 383-4100 ext. 5127
- Large House Review trigger: Planning Board review if residential gross floor area exceeds the greater of 4,000 sq ft or 10% of lot area
- Permitting platform: Cohasset Electronic Permitting (registration mandatory; paper plans still required for review)
- Dual role: Building Inspector also serves as Floodplain Administrator (§300-9.16)
- Bylaw sources: Chapter 300 (zoning), Chapter 260 (wetlands), Article 9.4 (floodplain overlay, amended March 5, 2025)
Why BuildX Knows Cohasset ADUs
Cohasset is BuildX home territory. We are your neighbors, not a regional contractor passing through. We pull three maps before we draw anything on a Cohasset lot: FEMA flood zones, the Watershed Resource District, and the Chapter 260 wetlands buffer.
BuildX in Cohasset
Cohasset is BuildX home territory. We are not a regional contractor visiting Cohasset, we are your neighbors. BuildX is currently building one ADU in Cohasset:
- Williams: 900 GFA · 1-Bed, 1-Bath · Under Construction
Real families. Real builds. Real ADUs on the South Shore. See more BuildX ADU builds →
See One in Person
Walk through a real, finished BuildX ADU before you sign anything. Private showings are available by appointment.
Day 1 Property Review
Before quoting, we check zoning, sewer or septic, FEMA flood zone, wetlands, watershed overlay, and historic district status.
What Can Complicate a Cohasset ADU?
Cohasset is a coastal town: roughly two-thirds of its area is water, and much of the land sits in a FEMA flood zone. Your parcel's overlays, not the zoning question, are usually what move project cost and timeline. The factors that come up most often:
How does Cohasset's coastal flood zone affect my ADU? High flood
Likely affects you if: your parcel is along Cohasset Cove, The Gulf, Little Harbor, Beach Island, Straits Pond, Atlantic Avenue, Border Street, or any other coastal or low-lying parcel mapped in a FEMA flood zone.
This is the headline for Cohasset. Much of coastal Cohasset sits in FEMA Zone V (high-hazard) or Zone A. ADU construction in these zones must meet specific elevation, anchoring, and flood-resistant-material standards under the Massachusetts Building Code, with engineer-stamped certification that the design will not increase 100-year flood levels. A special permit is required for any encroachment in a floodway.
Cost premium: a real number to discuss honestly is 15% to 25% above standard ADU build cost for Zone V/A construction. That covers engineer-stamped certifications, elevated foundations, flood vents in enclosures, flood-resistant materials, and breakaway walls in Zone V.
One decision-maker for two reviews. The Cohasset Building Inspector also serves as the town's Floodplain Administrator under §300-9.16 [Bylaw 9.4]. The same person reviews your building permit and your floodplain compliance, so the flood-zone math and design quality need to be right the first time.
Does Cohasset's wetlands bylaw affect my ADU? Wetlands
Likely affects you if: your lot is within 100 feet of any wetland or land subject to flooding, or within 200 feet of a riverfront area. On a coastal town with this much water, many parcels qualify.
Cohasset's local Wetlands Protection Bylaw, Chapter 260, is stricter than the state Wetlands Protection Act [Cohasset Wetlands]. The local bylaw protects wetlands within 100 feet of any land subject to flooding or inundation by groundwater, surface water, or tidal action, and the local riverfront area extends 200 feet from annual mean high water, with a 100-foot inner zone and a 100-foot outer zone.
The Conservation Commission requires a permit for almost any work near these resource areas, including ADU construction. Plan for 30 to 90 days of additional review and roughly $10,000 to $25,000 in added project cost for filings, design adjustments, and abutter notification. The Stormwater Management Bylaw (Chapter 223) can also apply.
How does the Watershed Resource District affect my ADU? Watershed
Likely affects you if: your parcel falls within the Watershed Resource District overlay shown on the Cohasset Zoning Map.
Cohasset's Watershed Resource District is a zoning overlay that preserves the ground and surface water resources providing the town's water supply [Bylaw WRD]. It carries its own use restrictions and dimensional rules, so an ADU project on a parcel inside this district faces additional review on top of the base zoning.
We pull the overlay layers on Day 1 to confirm whether your parcel sits inside the district before any design work begins.
Do I need sewer or septic review for a Cohasset ADU? Septic and grinder
Likely affects you if: your home is on a grinder pump, or your parcel is in a coastal or peripheral area on private Title 5 septic.
Cohasset has 60+ miles of sewer pipeline and 10 pumping stations, but the system is not simple. Many homes use grinder pumps that send waste uphill to gravity sewers. Adding an ADU adds wastewater flow, which requires Sewer Commission review for capacity [Sewer Dept]. If your property is on a grinder pump, you must confirm the pump can handle the additional load.
The "One Strike" policy. Cohasset adopted a "One Strike" grinder-pump policy in December 2018. Misuse, such as flushing wipes, floss, tampons, or cotton swabs, that damages a grinder pump bills the homeowner for repairs. Educate your ADU tenants up front.
Coastal and peripheral parcels are often on private Title 5 septic. Coastal lots with wetlands or ledge frequently need Maximum Feasible Compliance designs. A documented example: a Beechwood Street project placed the leach field in the front yard because the rear yard had both wetlands and ledge. A Title 5 upgrade on a constrained coastal lot can add roughly $40,000 to $80,000 to design and install.
Does Cohasset Station affect my ADU parking? Transit
Likely affects you if: your parcel is along the Route 3A corridor within half a mile of Cohasset Station.
Cohasset Station opened on the Greenbush Line on October 31, 2007 at 110 Chief Justice Cushing Highway (Route 3A), west of downtown [MBTA] [Greenbush]. Under state law (760 CMR 71.03(3)(c)), no additional parking can be required for an ADU within 0.5 miles of an MBTA transit station.
An important nuance for Cohasset: the station is a park-and-ride with few pedestrian connections to neighborhoods. The half-mile radius primarily covers the Route 3A corridor, not the historic Cohasset Common village center, which is roughly 0.7 to 1.0 miles away. Many homeowners near the village assume they qualify; map your specific parcel before you count on the exemption.
Will Large House Review apply to my ADU? Planning Board
Likely affects you if: your principal dwelling plus the ADU pushes total residential gross floor area over the threshold.
Cohasset's Article 5 Dimensional Regulations require Planning Board review when residential gross floor area exceeds the greater of 4,000 sq ft or 10% of the lot area [Bylaw Art. 5]. If your principal dwelling plus a 900 sq ft ADU crosses that line, Large House Review applies, adding roughly 30 to 60 days for Planning Board review and additional submission requirements.
Many smaller principal dwellings plus a 900 sq ft ADU stay under the threshold. Larger principal dwellings are the ones that tend to trigger it. We check this against your parcel's lot area early so it is not a surprise late in the process.
What Does a Cohasset ADU Cost?
A Cohasset ADU should not be priced from square footage alone. Two 800 sq ft ADUs can price very differently depending on coastal flood elevation, the local wetlands buffer, watershed overlay review, grinder-pump or Title 5 capacity, and site access. Cohasset's cost-of-living index runs well above the national average, which lifts labor and material costs as well.
That is the full span of our 14 floor plans. Prices include the complete building package, foundation through move-in-ready finishes. Site work is priced separately after a lot evaluation.
Cohasset-specific cost premium factors
See full ADU pricing and floor plans. For a Cohasset-specific quote, call (781) 627-7000. We will not quote a Cohasset ADU until we know your specific neighborhood, your flood-zone status, your wetlands and watershed overlay status, and your sewer, septic, or grinder-pump situation.
How the Cohasset ADU Permit Process Works
A building permit at minimum, plus the overlay reviews that Cohasset's coastal geography tends to trigger. The sequencing matters.
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1
Parcel review
1 to 2 weeks
BuildX maps your parcel against zoning, sewer or septic, grinder-pump status, FEMA flood zone, the Chapter 260 wetlands buffer, the Watershed Resource District, and historic or scenic overlay status before quoting. On a coastal town, this is the step that determines whether the rest of the timeline is straightforward or extended.
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2
Pre-file overlay reviews (where triggered)
Conservation 30 to 90 days; flood certification 2 to 4 weeks
If your parcel falls within a wetland buffer or 200-foot riverfront area, file the Conservation Commission notice under the local Wetlands Protection Bylaw (Chapter 260). If your parcel is in Zone V or A, obtain engineer-stamped flood elevation, anchoring, and flood-resistant-materials certification for the Floodplain Administrator. Coordinate Sewer Commission review of grinder-pump or Title 5 capacity in parallel.
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3
Building permit and floodplain compliance
Submitted via Cohasset Electronic Permitting
Register on the Cohasset Electronic Permitting platform (mandatory) and submit the building permit; paper plans are still required for plan review. The Building Inspector, who also serves as Floodplain Administrator under §300-9.16, reviews construction-code compliance and flood-zone compliance together. No work begins until you physically pick up the issued permit and post it on site.
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4
Large House Review (if triggered)
Add 30 to 60 days when triggered
If your principal dwelling plus the ADU exceeds the greater of 4,000 sq ft or 10% of lot area, the Planning Board conducts Large House Review under Article 5. This runs as a separate review and can extend the critical path.
Overall planning assumption: a non-overlay Cohasset parcel can move faster, but plan for a longer runway than a non-coastal town. Coastal-overlay ADUs commonly take meaningfully longer once Conservation Commission and flood-zone certification are in the path. Confirm current turnaround with the Building Department.
After permits issue, construction inspections run through the Cohasset Building & Inspections Department. Confirm current administrative posture at (781) 383-4100 ext. 5100.
What Cohasset Zoning Districts Allow ADUs?
Under state law, ADUs are by-right in any Cohasset zone that allows single-family dwellings, which means the residential RA, RB, and RC districts. Cohasset's dimensional standards (minimum lot, frontage, coverage, height, and setbacks) live in Chapter 300 Article 5. We are not publishing guessed dimensional numbers here; confirm the exact figures with the Planning Department, since the published code table is the controlling source.
RA · Residential A. Largest lots, often coastal.
- Min lot: Per Article 5. Confirm with Planning Department
- Min frontage: Per Article 5. Confirm with Planning Department
- Max coverage: Per Article 5. Confirm with Planning Department
- Max height: Per Article 5. Confirm with Planning Department
Where: Larger lots, frequently along the coast.
ADU rules: Detached, attached, and interior-conversion ADUs allowed by-right under state law up to 900 sq ft. Dimensional standards are set by Chapter 300 Article 5 [Bylaw Art. 5].
What this means for your ADU: larger coastal lots give room to site a detached ADU, but the dominant constraints on RA parcels tend to be the FEMA flood zone, the wetlands buffer, and septic capacity, not lot dimensions. Confirm your Article 5 figures with the Planning Department at (781) 383-4100 ext. 5127.
RB · Residential B. The most common residential district. Common Cohasset residential zone
- Min lot: Per Article 5. Confirm with Planning Department
- Min frontage: Per Article 5. Confirm with Planning Department
- Max coverage: Per Article 5. Confirm with Planning Department
- Max height: Per Article 5. Confirm with Planning Department
Where: the most common residential designation in town.
ADU rules: Detached, attached, and interior-conversion ADUs allowed by-right under state law up to 900 sq ft.
What this means for your ADU: RB is the most common Cohasset ADU case. Setbacks and lot coverage from Article 5, plus your sewer, septic, or grinder-pump status, drive most of the design decisions. Confirm the Article 5 dimensional figures with the Planning Department before committing to a footprint.
RC · Residential C. Denser village and downtown-adjacent areas.
- Min lot: Per Article 5. Confirm with Planning Department
- Min frontage: Per Article 5. Confirm with Planning Department
- Max coverage: Per Article 5. Confirm with Planning Department
- Max height: Per Article 5. Confirm with Planning Department
Where: denser village and downtown-adjacent areas.
ADU rules: Detached, attached, and interior-conversion ADUs allowed by-right under state law up to 900 sq ft.
What this means for your ADU: on tighter RC lots, setbacks, lot coverage, and parking layout are the constraints to solve first. An interior conversion or attached ADU is often the right answer. If your parcel is in or near the Cohasset Common Historic District, design review can also apply to work visible from a public way.
Overlay districts. Watershed, Floodplain, TOD, Historic.
Cohasset layers several overlays over the base residential zoning: the Watershed Resource District [Bylaw WRD], the Article 9.4 Floodplain & Watershed Protection Overlay (amended March 5, 2025) [Bylaw 9.4] [Planning Board], and the TOD Overlay around Cohasset Station [Bylaw TOD]. Overlays modify, but do not prevent, an ADU.
Overlay analysis happens on Day 1 with the town's mapping layers. We pull your parcel's overlays before quoting.
Source: Cohasset Zoning Bylaw Chapter 300 Article 5 (Dimensional Regulations). Confirm current figures with the Planning Department at (781) 383-4100 ext. 5127.
Sewer, Septic, or Grinder Pump? Cohasset Is Mixed.
Wastewater is one of the first things to settle on a Cohasset ADU. Cohasset has 60+ miles of sewer pipeline and 10 pumping stations, but many homes are on grinder pumps, and coastal or peripheral parcels are often on private Title 5 septic. Adding an ADU adds wastewater flow, which the Sewer Commission reviews for capacity.
Show grinder pump, Title 5, and water utility details
- Water utility: Cohasset Water Department; wastewater treatment facility operated by Woodard and Curran, Inc.
- Grinder-pump service: F.R. Mahony & Associates Inc., (508) 765-0051 (for residents on grinder pumps)
- Sewer Commission review: Required for any ADU adding wastewater flow; confirm grinder-pump capacity before design
- "One Strike" policy: Adopted December 2018; misuse that damages a grinder pump bills the homeowner for repairs
- Title 5 septic upgrade: $40,000 to $80,000 typical on constrained coastal lots
- Sewer Department: (781) 383-4100 ext. 5121 [Sewer Dept]
Much of Cohasset is sewered, but grinder pumps and coastal septic add steps that most towns do not have. We do not quote a Cohasset ADU until we know whether your parcel is on gravity sewer, a grinder pump, or Title 5 septic, and whether the system has capacity for the added flow.
Cohasset's Environmental and Historic Layers
Cohasset is a coastal town with deep open space and historic character. Three layers come up most often in ADU planning beyond the flood zone and wetlands buffer covered above.
Do historic or scenic rules apply to my Cohasset ADU?
Cohasset Common is a historic village center anchored by the First Parish Meeting House (circa 1750). Design review can apply to ADU work that is visible from a public way in the Common, and to parcels in the Historic Village Business Overlay District. Most residential ADUs outside those areas are not subject to historic design review, but scenic-overlay protection exists along parts of the coast and the historic village.
If your parcel is in or near the Cohasset Common Historic District, expect an architectural-design conversation about massing, materials, and fenestration for anything visible from the street.
How does Cohasset's open space affect my ADU?
Cohasset borders significant protected open space. Wompatuck State Park (roughly 3,500 acres in southwest Cohasset) and the Whitney & Thayer Woods Reservation sit on the inland side of town, with the 1.5-mile Whitney Spur Rail Trail connecting Cohasset Station to the park. Wheelwright Park sits near Little Harbor.
Parcels abutting protected open space or conservation land can draw additional Conservation Commission attention for work near resource areas. Inland lots near Wompatuck tend to be quieter and larger, but still warrant a buffer-zone check on Day 1.
Cohasset geography and county context
Cohasset is a Norfolk County exclave, separated from the rest of Norfolk County by Hingham and Hull, which became Plymouth County in 1803 [Wikipedia]. The town was founded in 1770 from Hingham. Roughly 68% of its 31.4 square miles is water, which is part of why coastal flood and wetlands rules sit at the center of nearly every Cohasset ADU conversation [Census].
Cohasset Is Four ADU Markets in One
Cohasset's neighborhoods have materially different ADU experiences. A coastal ADU on Atlantic Avenue is a different project from an inland ADU near Wompatuck. The four areas that come up most often:
Cohasset ADU Feasibility by Area (At a Glance)
| Cohasset Area | Common ADU Issue | BuildX Note |
|---|---|---|
| Cohasset Common / Village | Historic design review, dense lots, likely outside transit exemption | Design context drives decisions; verify parking-exemption distance to the station |
| Cohasset Harbor / Coastal | FEMA Zone V/A, rocky ledge, mix of sewer and septic | Coastal flood compliance and engineer certification dominate the budget |
| Route 3A / Station Area | Park-and-ride corridor, municipal sewer | Most likely area to qualify for the MBTA parking exemption |
| Inland / Wompatuck Border | Larger lots, septic and grinder pumps, conservation abutters | Buffer-zone check matters; quieter and larger inland lots |
Cohasset Common / Village Mixed
Historic district; First Parish Meeting House circa 1750; older homes on denser lots. Design review can apply to work visible from a public way.
Transit: roughly 0.7 to 1.0 mi from Cohasset Station, likely outside the parking-exemption radius.
Cohasset Harbor / Coastal Mixed sewer/septic Zone V/A
Cohasset Cove, The Gulf, Little Harbor, Beach Island; rocky coastline with ledges; high-end coastal homes. FEMA Zone V/A is the dominant design driver.
Flood risk: high. Engineer certification required; cost premium 15% to 25%.
Route 3A / Station Area Sewer
Park-and-ride station, commercial corridor, some residential. Within 0.5 mi of Cohasset Station for many parcels.
Transit: the area most likely to qualify for the MBTA parking exemption.
Inland / Wompatuck Border Septic and grinder
Larger lots near the 3,500-acre Wompatuck State Park and Whitney & Thayer Woods. Mix of municipal sewer, septic, and grinder pumps.
Flood risk: lower than the coast, but conservation review applies for parcels abutting open space.
Cohasset Building & Inspections Department
Cohasset Building & Inspections Department
- AddressTown Hall, 41 Highland Avenue, Cohasset, MA 02025
- Phone(781) 383-4100 ext. 5100
- Dual roleThe Building Inspector also serves as the town's Floodplain Administrator under §300-9.16 [Building & Inspections]. One decision-maker reviews both your building permit and your flood-zone compliance.
- Online permittingCohasset Electronic Permitting (registration mandatory). Paper plans are still required for plan review [Electronic Permitting].
- Permit postingNo work begins until the applicant physically picks up the issued permit and posts it on site.
- Building & Inspectionscohassetma.gov/163/Building-Inspections
- Assessor databasecohassetma.gov/216/Online-Property-Database
Related departments for Cohasset ADU projects: Sewer Department at (781) 383-4100 ext. 5121 [Sewer Dept], Planning Board at (781) 383-4100 ext. 5127 [Planning Office], and the Conservation Commission Office via the Town Hall main line [Conservation Commission]. For grinder-pump service, F.R. Mahony & Associates Inc. at (508) 765-0051.
Questions to ask when you call Cohasset
- Is my parcel in a residential district (RA, RB, RC) where single-family dwellings are allowed?
- Is there a pending Town Meeting article to adopt a Protected Use ADU Bylaw aligned with state law?
- Is my property on gravity sewer, a grinder pump, or private Title 5 septic?
- Is my parcel in FEMA Zone V or A, and what does the Floodplain Administrator require?
- Is my parcel within the Chapter 260 wetlands buffer or the 200-foot riverfront area?
- Will my principal dwelling plus the ADU trigger Large House Review?
Explore Cohasset ADU Topics
Jump to deep-dive sections on this page.
Cohasset ADU FAQs
Can I build a detached ADU on my Cohasset property?
Yes. Under Cohasset's adopted ADU bylaw (§300-4.3B), detached, attached, and interior-conversion ADUs up to 900 sq ft are allowed by-right on single-family lots, with no special permit and no owner-occupancy requirement. A larger ADU is available by special permit (ADU-L) where the owner occupies the property and the principal dwelling is at least 1,250 sq ft and at least 10 years old. Conservation Commission filings and Floodplain Administrator certification still apply where the parcel sits in a wetland buffer or coastal flood zone.
Do I need a special permit for a Cohasset ADU?
Not for a by-right ADU. Under §300-4.3B(1), an ADU up to 900 sq ft is by-right, with no special permit. A special permit from the Zoning Board of Appeals is required only for a larger ADU-L (§300-4.3B(1.1)). Either way you still need a building permit, plus Conservation Commission and Floodplain Administrator review if your parcel falls within a wetland buffer or a coastal flood zone. Confirm the current process with the Building & Inspections Department at (781) 383-4100 ext. 5100.
Do I have to live on the property?
Not for a by-right ADU. Cohasset's bylaw does not require owner-occupancy for a by-right ADU under §300-4.3B(1), so you may live in either unit and rent the other on a long-term basis. Owner-occupancy is required for a larger ADU-L (§300-4.3B(1.1)(a)). Note that short-term rental of an ADU is prohibited (§300-4.3B(1)(i)). Confirm current operating posture with the Building & Inspections Department at (781) 383-4100 ext. 5100 before signing a lease.
How does Cohasset's coastal flood zone affect my ADU?
Significantly. Much of coastal Cohasset, including Cohasset Cove, The Gulf, Little Harbor, Beach Island, Atlantic Avenue, and Border Street, sits in FEMA Zone V (high-hazard) or Zone A. ADU construction in these zones requires engineer-stamped certification that the design will not increase 100-year flood levels, plus elevated foundations, flood-resistant materials, and breakaway walls in Zone V. The real cost premium is 15% to 25% above standard ADU build cost. The Building Inspector also serves as the town's Floodplain Administrator under §300-9.16, so one decision-maker reviews both your building permit and your floodplain compliance.
Is my Cohasset property on sewer, septic, or a grinder pump?
It can be any of the three. Cohasset has 60+ miles of sewer pipeline and 10 pumping stations, but many homes use grinder pumps that send waste uphill to gravity sewers, and coastal or peripheral parcels are often on private Title 5 septic. Adding an ADU adds wastewater flow, which requires Sewer Commission review for capacity. If your home is on a grinder pump, confirm it can handle the additional load, and contact F.R. Mahony & Associates Inc. at (508) 765-0051 for grinder-pump service. Educate ADU tenants on Cohasset's "One Strike" policy from December 2018: flushing wipes, floss, tampons, or cotton swabs that damage a grinder pump bills the homeowner for repairs.
How does Cohasset's Wetlands Protection Bylaw affect my ADU?
Cohasset Code Chapter 260 is a local Wetlands Protection Bylaw stricter than the state Wetlands Protection Act. The local bylaw protects wetlands within 100 feet of any land subject to flooding or inundation by groundwater, surface water, or tidal action, and the local riverfront area extends 200 feet from annual mean high water, with a 100-foot inner zone and a 100-foot outer zone. A Conservation Commission permit is required for almost any work near these resource areas, including ADU construction. Plan for 30 to 90 days of additional review and roughly $10,000 to $25,000 in added project cost. The Stormwater Management Bylaw (Chapter 223) can also apply.
Will I qualify for the MBTA parking exemption near Cohasset Station?
Maybe. Cohasset Station opened on the Greenbush Line on October 31, 2007, at 110 Chief Justice Cushing Highway (Route 3A), west of downtown. Under state law (760 CMR 71.03(3)(c)), no additional parking can be required for an ADU within 0.5 miles of an MBTA transit station. The nuance: Cohasset Station is a park-and-ride with few pedestrian connections, so the half-mile radius primarily covers the Route 3A corridor, not the historic Cohasset Common village center, which is roughly 0.7 to 1.0 miles from the station. Map your specific parcel before assuming the exemption applies.
What is Large House Review and does it apply to my ADU?
Cohasset's Article 5 Dimensional Regulations require Planning Board review when the resulting residential gross floor area exceeds the greater of 4,000 sq ft or 10% of the lot area. If your principal dwelling plus the ADU pushes you over that threshold, Large House Review applies, adding roughly 30 to 60 days for Planning Board review and additional submission requirements. Many smaller principal dwellings plus a 900 sq ft ADU stay under the threshold; larger principal dwellings tend to trigger it.
What does a Cohasset ADU cost?
Our floor plans range from $284,301 to $525,861. That price includes the complete building package, foundation through move-in-ready finishes; site work is priced separately after a lot evaluation. In Cohasset, the site-work side is where the budget moves most: coastal flood Zone V/A construction adds 15% to 25%, a Conservation Commission filing adds roughly $10,000 to $25,000 plus 30 to 90 days, a Title 5 septic upgrade on a constrained coastal lot can add $40,000 to $80,000, and a grinder-pump upgrade can add $5,000 to $15,000. Large House Review adds 30 to 60 days when triggered. See full ADU pricing and floor plans, and for a property-specific quote, call (781) 627-7000.
Who do I call about building an ADU in Cohasset?
For official town process questions (permits, fees, electronic permitting registration, floodplain requirements, building code interpretation), call the Cohasset Building & Inspections Department at (781) 383-4100 ext. 5100.
For parcel-level feasibility, cost estimates, design options, neighborhood-specific considerations, and construction planning, call BuildX at (781) 627-7000.
The two are different conversations. The town can tell you what is administratively required. BuildX can tell you what is realistic on your specific Cohasset parcel and what it will actually cost to build.
Sources Reviewed
Last verified: September 20, 2026. Grouped by source type so you can see which claims come from town law, state law, town process, and local constraints.
View all 24 sources, grouped
- Town of Cohasset. Chapter 300, The Town of Cohasset Zoning Bylaw. Recodification April 30, 2018 ATM Art. 19; retitled May 1, 2023 ATM Art. 20.
- Town of Cohasset. Chapter 300 Use Regulations. Accessory dwelling unit regulations §300-4.3B (by-right ADU and ADU-L by special permit), amended 12-9-2024 STM Art. 15.
- Town of Cohasset. Chapter 300 Article 5, Dimensional Regulations. Large House Review threshold and setback rules.
- Town of Cohasset. Chapter 300 Article 9.4, Floodplain & Watershed Protection Overlay. Amended March 5, 2025.
- Town of Cohasset. Chapter 300 TOD Overlay District.
- Town of Cohasset. Watershed Resource District Overlay. Preserves ground and surface water for the town water supply.
- Town of Cohasset. Chapter 300 Administration & Enforcement (Article 12).
- Town of Cohasset. Chapter 260 Wetlands Protection Bylaw. Local bylaw stricter than the state Wetlands Protection Act; 200-foot riverfront area.
- Town of Cohasset. Code of Town Bylaws (table of contents). Legislation through May 12, 2025.
- Town of Cohasset. Planning Board. Article 9.4 amendments approved March 5, 2025.
- Commonwealth of Massachusetts. Accessory Dwelling Units (760 CMR 71.00). Protected-use ADU regulations under the Affordable Homes Act.
- Massachusetts General Laws. M.G.L. Chapter 40A, Section 3. As amended by Chapter 150 of the Acts of 2024 (Affordable Homes Act).
- Town of Cohasset. Building & Inspections Department. Building Inspector serves as Floodplain Administrator per §300-9.16; (781) 383-4100 ext. 5100.
- Town of Cohasset. Electronic Permitting, Building. Mandatory online registration; paper plans still required for plan review.
- Town of Cohasset. Sewer Department. (781) 383-4100 ext. 5121; grinder-pump system and "One Strike" policy.
- Town of Cohasset. Conservation Commission Office. Administers Chapter 260 Wetlands Protection Bylaw.
- Town of Cohasset. Planning and Zoning Office. Includes MBTA Communities Overlay District information; Planning Board (781) 383-4100 ext. 5127.
- MBTA. Cohasset Station (Greenbush Line). 110 Chief Justice Cushing Highway; opened October 31, 2007.
- Wikipedia. Greenbush Line. Restored as Big Dig environmental mitigation; opened 2007. General reference; verify against primary sources.
- Wikipedia. Cohasset Station. Whitney Spur Rail Trail; 3,500-acre Wompatuck State Park connection. General reference; verify against primary sources.
- Wikipedia. Cohasset, Massachusetts. Population, area, county-exclave history, founding 1770 from Hingham. General reference; verify against primary sources.
- U.S. Census Bureau / Census Reporter. Cohasset town profile. 8,381 population (2020); 31.4 sq mi (about 68% water); median household income $187,060.
- Mass by Train. Cohasset Commuter Rail Station. Whitney Spur Rail Trail, 1.5 miles to Wompatuck State Park. Travel and information reference.
- F.R. Mahony & Associates Inc. Grinder Pump Service for Cohasset residents, (508) 765-0051. Operational service reference.
Nearby South Shore ADU Rules
Cohasset borders Hingham to the west and Scituate to the south and east, with Norwell and Marshfield close by to the southwest. All four are BuildX home territory.
ADU Rules by City in the BuildX Service Area
BuildX serves 40 towns across four areas of southeastern Massachusetts. Each city has its own ADU bylaw, sewer or septic split, overlay map, and Building Department contact. Jump to the rules for your town:
South Shore (22)
South Coast (9)
Bridgewater Area (5)
Thinking about an ADU in Cohasset?
BuildX can review your Cohasset parcel for zoning, sewer or septic, FEMA flood zone, wetlands, watershed overlay, and historic status before you spend money on design or permitting. Cohasset is BuildX home territory, so we know the coastal flood rules, the Chapter 260 wetlands buffer, and the grinder-pump system that drive your project, and we know that state law has already cleared the by-right path.
See our ADU project portfolio or visit the ADU Home TourDisclaimer: The information on this page is drawn from live interviews, podcast episodes, municipal bylaws, planning board materials, town websites, and other publicly available sources, and reflects what was accurate as of the original recording or verification date. Pricing, timelines, materials, regulations, zoning rules, permit processes, fees, and overlay district boundaries can change -- sometimes without public notice -- and no two towns interpret or apply the rules the same way. This page is for informational purposes only and should not be relied upon as the basis for any purchasing, construction, or investment decision. BuildX makes no guarantee that the information is current or complete for your specific parcel. Before making any decisions, contact the town directly or call BuildX at (781) 627-7000 or schedule a Project Clarity Call to review your specific situation.