The State's Accessory Dwelling Unit Law Stops at the Boston Line

Can you put a small unit in your yard now that the state legalized them? I get asked that a lot, and the answer turns on which side of the Boston line the yard sits on. Here’s what the statute says, and why the city is carved out of it.

What the state law says

The state Zoning Act, M.G.L. c. 40A, defines an accessory dwelling unit as a self contained housing unit with its own sleeping, cooking and sanitary facilities on the same lot as a principal dwelling. It has to maintain a separate entrance, either from outside or through a shared hall, sufficient to meet the state building code for safe egress. It can’t be larger in gross floor area than half the gross floor area of the principal dwelling or 900 square feet, whichever is smaller.

Section 3 of the same chapter is where the change bites. No zoning ordinance or by-law may prohibit, unreasonably restrict, or require a special permit or other discretionary approval for a single accessory dwelling unit, or the rental of one, in a single family residential zoning district. The unit may still be subject to reasonable regulation, including site plan review, setbacks, bulk and height rules, septic rules where those apply, and restrictions on short term rental. What a town may not do is require that you occupy either the main house or the unit, and it may not require more than one additional parking space. Within half a mile of a commuter rail station, subway station, ferry terminal or bus station it may not require any additional parking at all. For more than one accessory unit, a special permit is still required.

Why Boston is different

Boston’s ADU page answers this directly. Every municipality in Massachusetts except Boston derives its power to enact and enforce zoning through c. 40A. Boston’s zoning power comes from a separate act passed in 1956. Because the state’s accessory dwelling unit provision works by amending c. 40A, the change doesn’t apply to Boston. The city notes it wasn’t specifically excluded, but that Boston has to be specifically included for a statewide zoning change to reach it.

What Boston allows instead

Internal accessory dwelling units may be built on all owner occupied lots that contain one, two or three family homes. Detached units, and units built as external additions to the existing home, are only allowed without special zoning approval in parts of Mattapan, and only on parcels inside both the Greater Mattapan Neighborhood Zoning District and the PLAN: Mattapan study area. Anywhere else in the city, those go through the Zoning Board of Appeal on a variance. That’s the position on the city’s ADU page. The Planning Department is separately drafting a Neighborhood Housing rezoning, beginning in Hyde Park, Roslindale and West Roxbury, that would make a detached unit easier. As of early September that draft is still out for public comment, with meetings scheduled for the middle of the month, and the Zoning Commission has not adopted it.

What it costs and how long it takes

The city publishes a rough benchmark: generally between $75,000 and $100,000 to build an internal unit, and between $250,000 and $350,000 for a detached one. Its ADU Technical Assistance Grant reimburses design and permitting costs up to a combined $7,500, paid after you have an approved building permit and have enrolled in the Boston Home Center’s ADU Loan Program. The grant and the loan are two separate things, and you need to be in the loan program to collect the reimbursement. A licensed architect has to stamp the plans. Once a complete permit package is in and the plans examiner flags nothing, review runs about 5 weeks. The city also says yes, an accessory unit will raise your property tax once a new assessment happens, and that you must register the unit even if nobody is paying rent.

Wondering whether your lot could take one? Send me the address and I’ll pull the zoning before you pay anyone for drawings. Diana Kim, REALTOR®, eXp Realty.