How Much Does a BuildX ADU Cost in 2026?
Want to see what that investment will get you? Take our virtual ADU Home Tour: buildx.com/adu-home-tour. When a homeowner asks us about ADU costs, the truth is, it depends on a lot of things. We are not trying to dodge the question, but the cost of two ADUs on neighboring lots can have dramatically different pricing. There are a lot of factors that are not obvious from the street.
At BuildX, we build ADUs all over southeastern Massachusetts. We publish our starting prices for all of our floor plans because we believe that transparency is important. But always keep in mind that "starting price" means just that. The final cost depends on all kinds of property conditions, septic requirements, and finish selections.
Quick Answer: BuildX ADUs range from $284,301 for a 610 sq ft one-bedroom to $526,000 for premium designs with garages. Floor plan prices range from $284,301 to $525,861. Final cost depends on your septic system, site conditions, and finish selections.
In This Article
IMPORTANT: Does Your Property Qualify For An ADU?
Now, before we start getting into costs, the first question is whether an ADU will even work on your lot. We can do a free site check where we analyze HD satellite imagery of your property to evaluate setbacks, septic locations, access, and buildable area. This way you have a good picture of the project before you invest in design or engineering. Call us at 781-627-7000 to schedule your free site check.
The 7 Biggest ADU Cost Drivers
Our starting prices are based on very standard site conditions. These are the factors that determine whether your ADU lands at the base price or if it requires additional investment:
1. Septic System Requirements
For properties without city sewer, septic is usually the biggest cost driver. If your system is functioning and meets current regulations, that is the absolute best scenario because you can expand it or connect the ADU to it. But if your system is non-functional or non-conforming, that is the worst scenario and you basically have to start over.
It will help to know that the state considers any septic system designed before 1995 to be potentially non-conforming. Even if the system works fine, you may still need to add upgrades to accommodate an ADU. It's safe to budget between $15,000 and $50,000+ for septic work, depending on the soil conditions and the status and age of the current system.
2. Site Work and Access
It's important to understand that what is underground matters just as much as what you plan to build above it. Site conditions are one of the main things that can swing costs dramatically. Factors that determine costs include: solid rock that requires blasting or hammering, high water tables that need new drainage systems, steep slopes that need grading or retaining walls, and having limited access for equipment. We have seen site work add $10,000 on easy lots and $60,000 or more on much more challenging ones. The free site check we mentioned using the HD satellite imagery helps us identify potential issues before you decide to give the go ahead on the project.
3. Foundation Type
The foundation you choose affects the overall cost and it also determines what is possible on your site. Proper foundation prep is critically important. The walls need to be ground, holes patched, and everything scraped. If you skip this step, you could end up with major leaks and mold issues years later.
| Foundation | Cost Range | Best For |
|---|---|---|
| Slab on Grade | $15K to $25K | Level sites, simple access, budget-conscious |
| Crawl Space | $20K to $35K | Sloped sites, easier utility access |
| Full Basement | $40K to $70K | Maximum storage, future flexibility |
4. Floor Plan and Design
The specific design and how complex it is will dramatically affect the cost as well. Every custom detail adds time and money. A simple rectangle with standard ceiling heights will cost less than a design with vaulted ceilings, multiple rooflines, and custom millwork.
This is why we developed a list of standard floor plans of proven designs that ensure quality without inflating costs with custom designs.
5. Interior Finishes and Trim
Countertops, cabinets, flooring, and fixtures can vary significantly. The difference between entry-level granite and premium quartz with a waterfall edge can be $5,000 to $15,000 just for the kitchen.
6. Windows and Lead Times
Windows play a large part in your overall cost and design. Custom sizes, special shapes, and upgraded brands can add quite a bit to the pricing. More importantly, window lead times can push your project timeline out by as much as 8 to 16 weeks if not ordered early.
7. Financing Strategy
Last but not least, the method you use to pay for your ADU affects the total project cost. If you have a low-interest-rate mortgage already, the last resort would be to do a cash-out refi. Ideally you want to tap into equity through a HELOC if at all possible.
A renovation HELOC is an innovative option that uses the after-repair market value of the home. The draw period is 10 years, and even after the work is done, you can keep the home equity line, pay it down, and continue using it.
What Are The Actual Prices For BuildX ADU Floor Plans?
In Massachusetts, ADU costs vary widely based on size, site conditions, and level of finish. Most homeowners see total project costs fall within a broad market range before narrowing to specific builders.
Here is a breakdown of our current floor plans with the starting prices. Remember all prices are estimates that can change based on property evaluation. As we mentioned before, your specific property conditions will affect the final cost.
Compact ADUs (551-667 SF)
These are great for aging parents, rental income, or home offices with convenient sleeping quarters.
| Floor Plan | Size | Bed/Bath | Footprint | Starting At |
|---|---|---|---|---|
| X-2 The Hideaway | 610 SF | 1 bed / 1 bath | 20' x 30' | $284,301 |
| X-10 The Nook | 667 SF | 1 bed / 1 bath | 24' x 30' | $314,602 |
Mid-Size ADUs (725-775 SF)
A more comfortable size for living in full time, with extra room for guests or an office.
| Floor Plan | Size | Bed/Bath | Footprint | Starting At |
|---|---|---|---|---|
| X-14 The Woodland | 725 SF | 1 bed / 1.5 bath | 26' x 36' | $332,500 |
| X-6 The Nest | 900 SF | 1 bed / 1 bath | 28' x 28' | $488,600 |
| X-11 The Meadow | 775 SF | 1 bed / 1 bath | 26' x 32' | $345,505 |
Full-Size ADUs (890-900 SF)
These are the largest size allowed under Massachusetts law. Our designs really maximize living space for comfortable living or long-term rentals.
| Floor Plan | Size | Bed/Bath | Footprint | Starting At |
|---|---|---|---|---|
| X-5 The Glen | 890 SF | 2 bed / 1 bath | 42' x 24' | $358,700 |
| X-3 The Haven | 900 SF | 2 bed / 1 bath | 28' x 36' | $356,478 |
| X-8 The Garden House | 900 SF | 2 bed / 1.5 bath | 18' x 27' (2-story) | $346,502 |
| X-1 The Cottage | 900 SF | 2 bed / 1.5 bath | 28' x 36' | $359,624 |
| X-9 The Breeze | 900 SF | 2 bed / 2 bath | 30' x 36' | $372,065 |
What If I Need More Storage Space?
Here are a few options for those who want additional storage, workshop space, or covered parking.
| Floor Plan | Living SF | Features | Footprint | Starting At |
|---|---|---|---|---|
| X-7 The Gables | 900 SF | 2-car garage | 26' x 38' | $384,600 |
| X-15 The Retreat | 900 SF | 1-car + attic + basement | 31' x 48' | $405,062 |
| X-12 The Homestead | 900 SF | 2-car + sun room | 50' x 42' | $525,861 |
Remember these are estimates. After evaluating your property's specific conditions, which we explained in the cost drivers section above, we can provide accurate pricing.
Walk through completed BuildX ADUs on our virtual home tour. See the finishes, layouts, and craftsmanship before you commit.
â–¶ buildx.com/adu-home-tour
What Is Included in BuildX Starting Prices?
Our starting prices include a complete, turnkey ADU that's move-in ready. Here's a list of what is included in our pricing structure.
- All architectural and engineering plans
- Foundation
- Complete framing, roofing, siding, windows, and doors
- Full HVAC system (mini-split or ducted)
- Complete electrical with panel and fixtures
- Plumbing including water heater
- Kitchen with cabinets, countertops, and sink
- Bathroom(s) with fixtures and tile
- Interior trim, doors, and paint
- Basic landscaping (grading and seeding)
What May Add to Your Final Cost?
These items are priced separately based on your property's conditions:
- Septic work: Upgrades, expansions, or new system installation
- Utility connections: Running new water, sewer, gas, or electric from the street
- Site work beyond standard: Ledge removal, retaining walls, extensive grading
- Permit fees: Vary by town ($1,500 to $5,000 typical)
- Appliances: Refrigerator, range, washer/dryer
- Finish upgrades: Premium countertops, custom cabinets, hardwood floors
How to Get Started
- Take the virtual ADU Home Tour. See completed BuildX projects so you can really get a feel for our quality and craftsmanship. buildx.com/adu-home-tour
- Request a free site check. We look at HD satellite imagery of your property to evaluate setbacks, access, and buildable area for free.
- Get a preliminary estimate. Based on your site check, we will determine which floor plans will work and provide detailed pricing for your project.
- Schedule a consultation. Meet with our team to discuss your goals, timeline, and financing options.
Tour Completed ADUs: buildx.com/adu-home-tour
Free Site Check: buildx.com/lets-talk-about-your-project
Call Us: (781) 627-7000
Disclaimer: All prices shown are starting estimates as of January 2026 and are subject to change based on property evaluation, market conditions, and specific project requirements. Final pricing is provided after site assessment. This information is for educational purposes and does not constitute a binding quote.
