Massachusetts Approved 2,084 In-Law Apartments. Why Is Boston Missing?

Think of a family that owns a two-family house in Dorchester and wants to turn an unused part of the property into a small apartment. It could give an aging parent somewhere nearby to live, or create an extra rental home. A family in Somerville might be considering the same idea. The floor plan could look similar, but the zoning path is not.

That difference is the story behind one of Massachusetts' biggest new housing numbers. From January 2025 through June 2026, cities and towns reported 2,084 approved accessory dwelling units, or ADUs. Somerville accounted for 50, and Revere 46. But Boston does not appear in those statewide rankings, because its zoning operates under a different legal system. Massachusetts approval report (opens in a new tab)

An ADU can be a legal basement apartment, an addition or a detached backyard cottage. The most important distinction for a renter is simple: an approval is permission to build, not proof that an apartment is finished or available to rent.

The number that surprised us most

The state recorded 854 ADU approvals in just the first six months of 2026. Because it reported 2,084 approvals across all of 2025 and the first half of 2026, the implied 2025 count is 1,230. That means six months of 2026 already produced nearly 70% of the full prior year's reported approvals. Reporting periods and survey responses can change, so this is a comparison of reported totals, not a guaranteed future pace.

The activity is not limited to rural lots with lots of land. Lowell had 53 approvals, Somerville 50 and Revere 46 in the state's January 2025-June 2026 survey. Somerville sits just three approvals behind Lowell, despite being one of the most built-up communities in Massachusetts. The state says 70% of its municipalities had approved at least one ADU. State municipality list (opens in a new tab)

But what nobody should claim from these numbers is that 2,084 new rentals are on the market. The state also received reports of 2,824 ADU applications, but approvals and applications are not necessarily a set of resolved cases, because some applications can remain pending. Its public summary does not provide a matching count of completed, legally occupied homes. State ADU data (opens in a new tab)

Why does a Boston homeowner face different rules?

The 2024 Affordable Homes Act made one qualifying ADU a protected by-right use on lots in single-family residential zoning districts. That usually removes the need for a discretionary special permit solely to add the unit. But Massachusetts has 351 cities and towns, and this law applies to 350 of them. Boston is the exception. The reason is legal: Boston is not governed by the Chapter 40A zoning statute that the new protection amended. It uses its own zoning code. Official Massachusetts FAQ (opens in a new tab)

Under the statewide protected-use rule, the ADU generally cannot exceed 900 square feet or half the main home's gross floor area, whichever is smaller. Municipalities cannot require the owner to live in either unit just because it is a protected ADU. Safety rules, permits, site constraints and allowed local requirements still apply, however. Actual ADU law (opens in a new tab)

In Boston, the city says owners of one-, two- or three-family homes may be able to add an ADU when they live on the same parcel. Depending on the home and neighborhood, an internal conversion may be simpler than a detached cottage, and some proposals need zoning relief. Certain Mattapan districts have more permissive rules for attached or detached units. It is not safe to transfer Somerville zoning advice to a Dorchester or Mattapan address. Boston ADU guidance (opens in a new tab) | Check a Boston property's options (opens in a new tab)

Boston is not doing nothing

Being excluded from the state dataset does not mean Boston approved zero ADUs. Boston's Planning Department reported 34 ADUs permitted during all of 2024, compared with 51 permitted by September 2025. That is 17 more permits in nine months than the prior full-year count, although the reporting periods are different. The Boston figures cannot be added to the state's 2,084 or compared as if they covered identical dates. Boston Planning update (opens in a new tab)

For a Boston renter, that is a more useful interpretation than a political scorecard: some homeowners are navigating the city's system and gaining permits, even while the state's published league table excludes Boston altogether. How many finished units resulted, how much they cost and how many are offered for rent remain separate questions.

The next hurdle might be a six-figure construction bill

A garage is not automatically a legal apartment. It may need new plumbing, a kitchen, fire separation, windows or exits, utility service, structural work and inspections. State housing officials say some inspectors have seen ADU projects cost as much as $500,000 because of site and building-code requirements. That is an example of high-cost projects, not the typical price of a backyard apartment. State barriers assessment (opens in a new tab)

MassHousing offers qualifying owners second-mortgage loans up to $250,000 for detached ADUs or $150,000 for attached ADUs, not cash grants. The program currently requires an owner-occupied single-family home, income eligibility and completed plans and permits before applying. Someone with a two-family Boston property should not assume that this particular financing program covers them just because the city may permit an ADU. MassHousing requirements (opens in a new tab)

Massachusetts announced another $5 million in ADU assistance on October 15, including a pilot intended to support 20 to 30 income-restricted ADUs, not 20 to 30 already delivered homes. State officials also said more than 350 low-cost feasibility studies had started or finished within eight weeks of a separate assistance program's launch. That is evidence of owners trying to work out whether the project is practical before hiring builders. State October announcement (opens in a new tab)

For renters, the missing data is the most important data

A new independent apartment near family or an Orange Line stop could be useful even if it never appears in a big apartment complex. But owners might use ADUs for relatives, or charge market rent. The law does not automatically make an ADU income-restricted, nor does it guarantee an apartment will be listed publicly.

What we would really like to see next is the number of ADUs that pass final inspection, are legally occupied, and eventually become rentals. That progression tells a more useful housing story than permit counts by themselves. It could also reveal whether these units tend to help aging residents stay nearby, adult children find independent space or strangers find a new lease.

A local decision worth watching next week

Somerville's Land Use Committee and Planning Board have scheduled a November 5 hearing on zoning amendments that include backyard-cottage provisions. A public hearing is an opportunity for comment, not a newly adopted rule. Official Somerville meeting notice (opens in a new tab)

What should you do with this information?

• If you own a home: Start with the specific city or town zoning office. Ask whether your lot and building qualify, which permits are needed, and whether the cost of an internal unit could be less than a detached cottage. Get a feasibility assessment before paying for full plans.
• If you want to rent an ADU: Check that it has the required legal occupancy and safe independent living facilities. An approved proposal is not a finished rental.
• If you care about Boston's housing supply: Judge progress by legally completed homes and access to them, not only applications or approved designs.

Massachusetts has made it easier for many owners to ask, "Could we put another home here?" The less comfortable question is whether that permission can become a reasonably priced, livable apartment. In Greater Boston, the gap between those two questions is exactly where the next housing story will be.

Have you looked into adding or renting an accessory apartment in Greater Boston? Share a question or documented project lead at hello@bostondwelling.com.