Under the Massachusetts ADU law, every city and town has been required since February 2, 2025 to allow at least one accessory dwelling unit by right on any lot zoned for single-family housing. The rule comes from the Affordable Homes Act (Chapter 150 of the Acts of 2024), signed by Governor Healey on August 6, 2024.1Mass.gov. Accessory Dwelling Units “By right” is the key phrase. If your plans meet the dimensional and building code requirements, the local zoning board cannot deny the project through a special permit process or because neighbors object.
That is the headline. The rest of the picture is less tidy, because zoning is only one of the layers an ADU has to pass through.
What Qualifies as an ADU
An ADU under the statute is a self-contained housing unit with its own sleeping, cooking, and bathroom facilities, located on the same lot as a primary home. It can be a detached backyard cottage, an addition to the main house, or a conversion of an existing garage or basement.1Mass.gov. Accessory Dwelling Units
The size cap is firm. An ADU cannot exceed half the gross floor area of the primary dwelling or 900 square feet, whichever is smaller.2General Court of Massachusetts. Massachusetts General Laws Chapter 40A Section 1A It must have a separate entrance, either directly from the outside or through a shared hallway that meets building code egress standards.1Mass.gov. Accessory Dwelling Units
What Your City or Town Cannot Require
Municipalities keep real tools: site plan review, dimensional setbacks, and height and bulk restrictions. What they cannot do is use those tools to make building an ADU effectively impossible.1Mass.gov. Accessory Dwelling Units Several specific restrictions are now off the table.
No special permit or other discretionary zoning approval can be required to build or rent the unit. No municipality can require that the owner live in either the ADU or the primary dwelling, which means both units can be rented. And parking requirements are capped. If the ADU is within half a mile of a commuter rail station, subway station, ferry terminal, or bus station, no parking can be required at all. Outside that radius, the maximum is one space.1Mass.gov. Accessory Dwelling Units
Short-Term Rentals Are a Separate Question
If your plan is to list the unit on Airbnb or a similar platform, read your local rules before you break ground. The statute explicitly allows cities and towns to restrict or ban use of an ADU as a short-term rental, defined as a stay of 31 consecutive calendar days or fewer.3Mass.gov. Accessory Dwelling Unit (ADU) – FAQs Long-term rental is protected. Short-term is not.2General Court of Massachusetts. Massachusetts General Laws Chapter 40A Section 1A
Site Plan Review Has a Clock
Municipalities can subject ADUs to site plan review, but the process cannot stall indefinitely. For by-right projects that do not require a special permit, local planning boards are expected to act within 60 days of receiving a complete site plan submission. If the board takes no action within that window, the project can proceed.
Septic Systems and Title 5
If your home is on a septic system rather than municipal sewer, this is where many ADU projects run into their biggest expense. When an ADU increases the total bedroom count on your property, your existing septic system has to meet current new-construction standards for the combined bedroom count of both the primary dwelling and the ADU.4Mass.gov. Guidance on Title 5 (310 CMR 15.000) Compliance for Accessory Dwelling Units A full septic upgrade can run tens of thousands of dollars on top of the ADU itself.
Even if the bedroom count does not change, say you reconfigure the main house to eliminate a bedroom, a Title 5 inspection is still required to confirm the system is not in failure, and a multi-compartment tank or dual-tank setup becomes mandatory. Properties in nitrogen-sensitive areas or water supply protection zones face tighter limits still. Title 5 caps residential nitrogen output at 440 gallons per day per acre in those areas, which can effectively limit how many bedrooms a lot can support.
Wetlands and Conservation
The state ADU law does not override the Wetlands Protection Act. If any part of your proposed ADU sits within 100 feet of a wetland, you will need to file with your local Conservation Commission before construction can begin. Properties near streams, ponds, or flood zones face similar review. The zoning may be clear, but the environmental overlay can add months and force design changes.
Building Code and Fire Safety
ADUs must comply with the state building code, which includes fire separation requirements. Exterior walls of a detached ADU facing the lot line or an existing structure generally need a one-hour fire-resistance rating, and the percentage of window openings in those walls depends on the distance from the lot line. For attached ADUs built behind the main house without direct street access, the fire code requires an interior throughway connection to satisfy emergency access. These rules are not negotiable locally. They come from the state building code and the International Building Code as adopted in Massachusetts.
What It Costs, and What It Does to Your Tax Bill
Building an ADU increases your property’s assessed value. Local assessors treat the added living space as additional taxable value, so your property tax bill goes up. The exact figure depends on your community’s tax rate and how much value the ADU adds, but plan for a meaningfully higher annual tax obligation.
Construction costs in Massachusetts are steep. In the greater Boston area, a detached one-bedroom ADU runs roughly $250,000 to $350,000 all-in, including site work, utilities, permits, and finishes. Larger two-bedroom units near the 900-square-foot cap can approach $400,000. Converting an existing basement or garage is less expensive, generally in the $150,000 to $250,000 range, though the final number depends heavily on what the existing structure needs. Figures vary outside the Boston metro area, but Massachusetts labor and material costs make cheap ADU construction rare anywhere in the state.
Renting the Unit Out
Because the owner-occupancy requirement is gone, you can rent either the ADU, the primary dwelling, or both. A few practical rules shape how that works.
Occupancy Limits
The state Sanitary Code sets minimum square footage per occupant. Every dwelling unit must provide at least 150 square feet of habitable floor space for the first occupant and 100 square feet for each additional occupant.5Mass.gov. 105 CMR 410.000 – Minimum Standards of Fitness for Human Habitation A 600-square-foot ADU, for example, could house up to about five people under the math, though bedroom count and practical livability usually limit occupancy well below that ceiling.
Electric Metering
If you want the tenant to pay for electricity, you will need a separate electric meter installed, maintained, and read by the utility company. Massachusetts generally prohibits electric submetering, where a landlord uses one meter and bills tenants for a share. Without a separate meter and a written rental agreement specifying tenant payment, the owner is responsible for the unit’s electricity under the state Sanitary Code.6Mass.gov. Electric Submetering
The zoning door is open in a way it has never been in Massachusetts. The septic, wetlands, building code, and cost realities are the parts of the project that will determine whether an ADU on your lot is actually feasible, and in what form.