Should I Convert My Garage Into an ADU or Build a New Detached Unit?

By Buz Artiano, BuildX | June 2026

Watch: Lt. Governor Kim Driscoll | How ADUs Are Solving Massachusetts' Housing Crisis One Family at a Time

If you own a two-car garage that barely fits one car and a whole bunch of storage boxes, you've probably thought about making it a living space for Mom and Dad, or for a rental. You're not the only one thinking that. Garage conversions are one of the first things people think about when they learn about the new ADU law in Massachusetts. The logic makes sense to turn the garage into usable space. The walls and foundation are already done, so it feels like it would save you a lot of time and money.

My name is Buz Artiano, and I started BuildX. Since then we've built dozens of ADUs all over Massachusetts, but most of our builds have been around the South Shore and Plymouth County areas. We handle all the design, permitting, and construction ourselves, and we've dealt with the state's ADU regulations, septic constraints, and zoning processes on every one of those projects. Our main focus is on detached new-construction ADUs. We don't do garage conversions at all, and that's important, because we are going to give you an honest comparison, not just try and steer you toward the service we sell.

In this article we will explain what the real tradeoffs are between converting an existing garage and building a new ADU. We'll talk about cost, timeline, permitting, and livability so you can decide what makes sense for your property, your family, and your budget.

Which Option Is Right for My Property?

The honest answer is that neither option is always the correct answer. A garage conversion makes sense when you have a structurally sound garage with a high enough ceiling, a foundation that meets code, and a lot where adding another structure would be impossible due to setbacks. A detached ADU makes sense when you want an independent living space with modern energy and accessibility standards and when your property has room to put a separate structure, and when you're building for someone to live there long term, not as a quick short term thing.

The decision starts with three questions. First, what condition is your garage actually in? A garage with a slab-on-grade foundation, minimal insulation, and 7-foot ceilings, is going to need more work than people realize. The second question is does your lot have the septic or sewer capacity to support a second home regardless of which type of ADU you are planning to build. The sewer and septic question is what decides what you can build. Third, who is going to live in this space, and for how long? A unit for an aging parent who needs accessible design and long-term comfort would be designed very differently from a rental unit.

What Does a Garage Conversion Actually Involve?

The phrase "garage conversion" sounds simple, but the amount of work that has to be done depends on what your garage looks like right now. Garages are designed for vehicle storage, not for living in. Converting a garage to a code-compliant living space means addressing foundation issues, insulation, HVAC, electrical service, plumbing, ceiling height, windows, and fire separation. If the garage shares a wall with the main house, the fire separation requirements can make things a lot more complicated.

Ceiling height is one of the first deal-breakers for most of these conversion projects. Massachusetts code requires a minimum ceiling height of 7 feet for any living space. A lot of garages don't meet that requirement once you account for the finished floor, insulation above, and any mechanical routing. If your garage ceiling doesn't clear that 7 feet after all those layers are figured in, the conversion could mean that you'd have to lower the floor, raise the roof, or both. Doing either of those things changes a simple conversion into a full blown structural renovation.

Massachusetts Lieutenant Governor Kim Driscoll has acknowledged that garage conversions and attic conversions are great ways to create more ADUs in the state. She had mentioned that people who are converting garages or using existing infrastructure could get to a finished project much sooner, but it all depends on how much work has to be done. Every garage is different and the timeline and budget is based on what is needed for each one.

What Does a Detached New-Construction ADU Involve?

A detached new-construction ADU is built from the ground up on your property as a separate structure with its own foundation, utility connections, and entrance. Doing it this way gives you total control over layout, accessibility features, and energy systems without being subjected to the existing structure's limitations.

At BuildX, we guarantee a 100-day build timeline from the day we start digging. The longest project we have completed to date took 115 days, and that was on a property that had some very difficult to manage conditions. We can build you a fully finished, move-in-ready ADU in approximately four months. Lieutenant Governor Driscoll called that timeline "terrific" and she also said that she wishes all construction projects could go that fast.

It's pretty simple. New construction costs more upfront than a conversion, and it requires a longer pre-construction phase that includes design, permitting, and site prep. But what you get is a custom built living space. Every wall, window, and system is designed just for you. Otherwise you end up with a retrofitted space that was originally designed to park cars.

How Do the Costs Compare Between a Garage Conversion and a Detached ADU?

Cost is the thing that gets people thinking about doing a garage conversion in the first place. They just figure that if the structure already exists, the conversion should cost less, and a lot of times it does. But once you factor in the structural, mechanical, and other work that has to be done to bring the space up to code, you're not saving as much money as you thought.

A basic garage conversion in Massachusetts, assuming the foundation is good, the ceiling height clears code, and no major structural changes are needed, can range from $80,000 to $150,000 depending on finishes, mechanical systems, and plumbing. But if the garage has to have a new foundation, a raised roof, or major structural reinforcement, you could be looking at $175,000 and up. At that point, a lot of people start considering just building new if they're gonna have to spend that much money anyway.

A completed detached new-construction ADU in Massachusetts, built to modern code with full HVAC, plumbing, electrical, and finishes, will usually cost you $250,000 to $375,000 depending on size, site conditions, and septic requirements. Yes that is more than the $175,000, but it includes a structure that is designed from the ground up as a living space, an insulated garage will never give you that.

But it's more than the money. A garage conversion means you don't have a garage anymore. If you use that space to park cars, have a workshop, or use it for storage, you lose that function permanently. A detached ADU doesn't sacrifice any space, it only adds square footage and value to your property.

How Do the Timelines Compare?

If your garage is in good structural condition and meets those minimum requirements we talked about (like ceiling height), the construction phase of a conversion is usually going to be shorter than new construction. Depending on how big of a project it is, a simple garage conversion can be completed in 60 to 90 days from start of construction.

A detached new-construction ADU takes longer because you have to do all the excavation. At BuildX, our construction phase runs 100 to 115 days from when we first start digging to completion. Once you add the time for the pre-construction phase, which is the design, engineering, permitting, and site eval, it takes longer for new construction than it does for a garage conversion.

There are exceptions to that though. A garage conversion that uncovers structural problems, too shallow a foundation, or code-compliance issues mid-project can add weeks or months while waiting for the design to be re-done. With new construction, there are fewer surprises because you're starting from scratch and not risking finding issues with the structure you're building on to.

What Structural Surprises Should I Expect with a Garage Conversion?

We specialize in new construction, not conversions, but we've looked at enough garages on client properties to have figured out where nasty surprises often come from. The foundation is the first thing to check. A lot of garages were built on shallow frost-wall footings or floating slabs that are plenty for vehicle weight but they don't meet the code's foundation depth requirements for a living space. If the foundation does not meet code, the fix can add $30,000 to $50,000 to the project.

Insulation is the second issue. A garage built without insulation in the walls, floor, or ceiling needs to be completely insulated and sealed before it can be a living space. In Massachusetts, that means meeting the state's energy code requirements, which for new construction and substantial renovations include air sealing, continuous insulation, and blower-door testing. Retrofitting all that into an existing garage is more work than building it into a new wall from the start.

The third issue is mechanical routing. Most garages don't have plumbing, HVAC ductwork or piping, and only a couple electrical outlets or lights. Running those systems through an existing structure, especially one with a low ceiling, means we have to get creative with the routing, and that can compromise the finished ceiling height or wall layout. In a new-construction ADU, every mechanical run is planned before we start building.

Is a Garage Conversion Ever the Better Choice?

Yes. We build detached new-construction ADUs, so you might expect us to steer you toward that option. But the truth is, a garage conversion is the right call for some people.

If your garage has a full-depth foundation, a high enough ceiling, and you're on city sewer rather than septic, doing a conversion can save you real money and time. If your lot is too small or too constrained by setbacks to let you build another structure, a conversion could be your only option. If your main priority is getting it built fast because a family member needs housing in the next three to four months, then a garage conversion can move much faster as long as it's planned out correctly.

We encourage families to get an assessment of the structure and foundation of the garage before deciding which type of ADU to go with. The assessment will tell you whether the foundation, framing, and roof can support a conversion, and that's what you need in order to compare prices.

Does My Septic System Affect Which Option I Choose?

Whether you convert a garage or build a detached unit, adding additional living space to your property increases the flow to the septic system. If you are on town sewer, it's not as much of a concern. But if you're on septic, and most properties on the South Shore and Plymouth County are, the septic system's capacity is the single biggest thing that will tell you what you can and can't build.

At BuildX, we check into the septic capacity before we quote any project, because the septic system is the biggest factor in determining whether an ADU makes financial sense on a given property. When people don't do a septic evaluation they risk committing to a project and finding out right in the middle of the build that the septic system cannot handle the additional flow. At that point, you have to upgrade your system which can add $40,000 to $60,000 to the total cost.

The septic question applies whether you're doing new construction or a conversion. A garage conversion doesn't avoid the septic evaluation just because the structure already exists. The additional bathrooms create additional flow regardless of whether the walls are new or not. Massachusetts requires compliance with Title V septic regulations for any change in use that increases wastewater flow. You can review the state's ADU requirements, including how they interact with septic regulations, at mass.gov.

Are the Permitting Requirements Different for Each Option?

Under the Massachusetts Affordable Homes Act, both garage conversions and detached new-construction ADUs are both buildable by right under Massachusetts' new law as long as you stay within the size regulations. That means you can pull a building permit without a special permit or variance from your local zoning board, as long as the project stays within the parameters set by state law.

Lieutenant Governor Driscoll was clear about the intent of the new law. That applies to both conversions and new construction.

The permitting differences are simple and practical. A new-construction project requires site plan review, foundation engineering, and potentially a septic design. A conversion requires a structural assessment and may require engineering plans to demonstrate that the existing structure meets current code. Both types of ADU's require building permits, inspections, and compliance with the state's energy code.

How Do You Decide Which Path Fits Your Property?

The right choice between a garage conversion and a detached new-construction ADU comes down to the condition of your garage and how structurally sound it is. It also depends on your lot's capacity, and what you plan on using the ADU for. If your garage is sound, your lot is constrained, and you need to move fast, a conversion could be your best bet. If you want a purpose-built living space, your property has room, and you are building for the next 20 years, new construction delivers a better outcome.

Start with a structural assessment of your garage and a septic evaluation of your property. Those two data points will tell you more about which path is realistic than any cost estimate or timeline projection.