Massachusetts changed its ADU rules on February 2, 2025. One accessory dwelling unit is now a protected use on any single-family lot, with no special permit and no variance required. The size cap is 900 square feet or half the gross floor area of your house, whichever is smaller. Your town also can’t require you to live on the property.
So the zoning argument is settled. The money question is the one nobody answers, and two things changed there this year that most homeowners have not heard about. Fannie Mae began allowing ADU rent toward qualifying income in March, and MassHousing announced a construction loan built for these projects in January. Below is the rule set, and then the financing.
| Question | What the state protects | What your town can still control |
|---|---|---|
| How many | One ADU on a single-family lot, by right | A second unit is a local question |
| Size | Up to 900 sq ft, or half the main home’s gross floor area, whichever is smaller | Additional size limits, as long as the protected ADU is still allowed |
| Form | Interior conversion, attached addition, garage or basement or attic conversion, or a detached cottage | Setbacks, height, and other dimensional rules that shape the design |
| Who lives there | No owner-occupancy requirement in either unit | Use rules that stop short of owner-occupancy |
| Parking | One extra space at most, and none within a half mile of a transit station | How the required parking is laid out on the site |
| Permitting | No special permit or variance required for a compliant ADU | Site plan review, and limits on short-term rental use |
What the Law Changed
Before February 2025, an ADU in most Massachusetts towns meant a hearing, a special permit, and a neighbor with an opinion. The Affordable Homes Act made one compliant ADU a protected use on single-family lots statewide. A town can no longer prohibit them outright, and can’t make you win a variance to build one.
The size formula catches people out, so run it before you fall in love with a floor plan. Your cap is 900 square feet or half your house, whichever number is smaller. A 1,400 square foot ranch supports a 700 square foot ADU. Only a house of 1,800 square feet or more reaches the full 900.
Form is wide open. An apartment carved out of the second floor counts. So does an attached addition, a garage conversion, an attic buildout, or a cottage in the back yard. A finished basement with its own door counts too. The unit needs its own entrance, kitchen, bathroom and sleeping area to count as an ADU rather than a bedroom with a hot plate.
Two rules matter more than most homeowners realize. Your town can’t require the owner to live in either unit, so building one doesn’t chain you to the address for twenty years. On parking, a town may require at most one additional space. Within a half mile of a transit station it may require none, and state guidance counts commuter rail stations and ferry terminals as transit stations.
Where Your Town Can Still Say No
By right sets the permit path, and design still has to clear the rules a town kept. Site plan review survived. So did setbacks, height limits, and dimensional rules generally, along with the building and health code requirements that apply to any new living space. A town may also add its own size restrictions and may restrict or prohibit renting the ADU short term.
Reading your town’s bylaw online can mislead you here. Municipalities were never required to amend their bylaws to match the new law. A page on a town website may still describe an owner-occupancy requirement or a special permit process that stopped being enforceable on February 2, 2025. The text is stale; the state law governs.
Before you pay anyone for drawings, call the building or planning department and ask two questions. Which of your rules apply to a by-right ADU on my lot, and which dimensional rule sends the most ADU plans back for redesign? Ask for the answer by email. Twenty minutes of that saves a five-figure redraw.
What a Build Costs
By right on paper is still six figures in practice, and this is where projects stall. Massachusetts ADU construction commonly runs $150 to $400 per square foot in 2026 contractor and industry cost guides, with Greater Boston closer to the $250 to $400 end. Site conditions move those numbers hard, so treat the ranges as planning figures rather than quotes.
By project type, those same 2026 guides put the ranges at:
- Basement or garage conversion: roughly $100,000 to $250,000
- Attached unit: roughly $150,000 to $325,000
- Detached backyard cottage: roughly $200,000 to $360,000
- Architectural design: another $3,000 to $8,000 on top
Turn whichever range fits your plan into a monthly payment before you go further. Our mortgage calculator will do that in a minute. The number it returns is the one your household lives with, whatever the unit rents for.
The Money Built for ADU Projects
MassHousing opened applications for a dedicated ADU construction loan on March 17, 2026, and the design of it tells you a lot about what the state expects. The program finances construction costs up to $250,000 for a detached ADU and up to $150,000 for an attached one. The structure is a fixed rate second mortgage amortizing over twenty years, matched with additional funding at zero percent interest and deferred repayment. That pairing lowers the blended cost and lets a homeowner reach a larger total.
Eligibility runs up to 135 percent of area median income, according to the state’s launch announcement. Published examples are $205,335 in Eastern Massachusetts, $165,345 in Worcester County and $129,870 in Hampden County. The loans come through MassHousing’s statewide network of partner lending institutions, and because the money is construction-to-permanent, the program expects you to have designs and permits in hand. GoldCoast partners with MassHousing programs and can walk you through what the current guidelines require.
Notice what that $250,000 and $150,000 split does to your plan. If your financing lands nearer the attached limit, a detached cottage may never pencil out, and you would rather learn that before an architect draws it. Most homeowners run this backwards, paying for design and permitting first and discovering the borrowing limit last. Price the money, then size the design to it.
MassHousing’s program doesn’t fit every project. Your household may earn more than its income limit. You may want a longer fixed-rate term than MassHousing’s 20 years, or the build may need more than $250,000. For those cases, GoldCoast can arrange a fixed-rate second mortgage from $250,000 up to $500,000, with qualifying debt-to-income ratios up to 50 percent. Your full file decides if you qualify, so have that conversation before the architect starts drawing.
How Lenders Count ADU Rent Now
Rent from an ADU used to be a story you told yourself rather than income a lender would recognize. That changed on March 21, 2026. Fannie Mae began allowing ADU rental income toward qualifying income on a one-unit principal residence, for purchases and limited cash-out refinances. Rent may come from only one ADU even if the property has two, and the amount used can’t exceed 30 percent of your total qualifying income. The appraisal package needs the standard 1004 plus a Single-Family Comparable Rent Schedule, Form 1007. The appraiser has to state that the market rent estimate covers an ADU on a one-unit principal residence.
FHA got there first, under a 2023 policy that still governs. Where an existing ADU has no rental history, FHA can count 75 percent of the appraiser’s fair market rent or the rent in a signed lease, whichever is lower. The same 30 percent cap applies. Building an ADU through the Standard 203(k) rehabilitation program is treated more cautiously, with only 50 percent of the projected rent counted.
Ask any loan officer two direct questions before you budget around rental income. Will this file include a 1007 for the ADU, and after the 30 percent cap, how much of that rent counts toward what I qualify for? The gap between the rent a tenant would pay and the rent an underwriter will credit is where plans fall apart. You want that number early. Whether you go through a bank or a broker who can shop the file across many lenders changes which programs are even on the table.
Buying a House With an ADU Already Built
These properties are no longer unusual. Massachusetts municipalities approved 854 ADUs in the first six months of 2026. That brings the total to 2,084 since the start of 2025, by the state’s own count. Shop the North Shore or the Merrimack Valley this year and you’ll meet listings with an in-law apartment, a converted garage, or a permitted cottage out back.
An ADU that already exists puts you in a different position than someone planning a build. There’s no construction risk, no permitting timeline, and the rent can help you qualify inside the caps above. The catch is documentation. The appraiser has to support a market rent for the unit, and the loan program has to recognize the income.
If you have been telling yourself that the rental unit will cover the payment, get the file priced before you write an offer. A house with an ADU can be more reachable than its sticker price suggests, and it can also be a stretch dressed up as a bargain. The difference is a number, and you can have it in a day.
FAQ: Massachusetts ADU Rules and Financing
How big can an ADU be in Massachusetts?
Up to 900 square feet, or half the gross floor area of your main house, whichever is smaller. A 1,400 square foot house supports a 700 square foot protected ADU. Towns may add size rules, so confirm locally before you design.
Does by right mean my town has to approve my plan?
No. A compliant ADU on a single-family lot is a protected use, so the town can’t ban it or require a special permit for it. Your plan still has to satisfy site plan review, setbacks, height and building code. Confirm the local process with the building or planning department.
Can a Massachusetts town make me live in the house to have an ADU?
No. Owner-occupancy requirements for either unit stopped being enforceable on February 2, 2025. Some town bylaws still carry that language because amending them was optional, but the state law controls.
How much parking does an ADU require?
At most one additional off-street space. Within a half mile of a transit station, which state guidance defines to include commuter rail stations and ferry terminals, none at all.
Can ADU rent help me qualify for a mortgage?
Sometimes, within limits, and only when the file documents the rent the way the program requires. Fannie Mae has allowed it on a one-unit principal residence since March 21, 2026, capped at 30 percent of qualifying income and supported by Form 1007. FHA generally counts 75 percent of an existing unit’s appraised or leased rent, and 50 percent for an ADU built through Standard 203(k).
Where We Come In
GoldCoast Mortgage Service is an independent mortgage broker in Beverly, licensed by the Massachusetts Division of Banks, MB 1841, NMLS #1841. We’re licensed in Massachusetts, New Hampshire and Maine. John Donlon, who owns the company, has spent more than 25 years financing homes across the North Shore and the Merrimack Valley. A bank offers you the loans that bank sells. A broker prices your file across many wholesale lenders and brings back what they say.
An ADU project touches almost every part of a mortgage file at once. What you can borrow, what the appraiser will support, which program counts the rent, and what the payment does to your monthly picture. We’ll pre-qualify you at no cost and lay out the routes that fit your property, including the state programs a national call center will never mention. You can see the towns we cover on our service areas page. If the answer is that the project should wait a year, we’ll say that too.