Straight answers about building an accessory dwelling unit in Massachusetts — what the law allows, what it costs, how long it takes, and what your property needs to support one.
Massachusetts changed its ADU law in 2024, and the new rules took effect on February 2, 2025. Most homeowners have more options than they think.
In almost all of Massachusetts, yes. The 2024 Affordable Homes Act requires every city and town to allow at least one ADU by right on lots zoned for single-family use. Your town cannot ban them outright or force you through a special permit hearing for a qualifying unit.
Boston operates under its own ordinance and works differently. Local rules on setbacks, height and design still apply everywhere.
Under the state's by-right rules, up to 900 square feet or half of your home's floor area, whichever is smaller. So if your house is 1,500 square feet, your by-right ADU can be up to 750.
Some towns allow larger units than the state minimum requires. Worth checking before you assume 900 is your ceiling.
Not for a qualifying by-right ADU. That was the whole point of the law change — no discretionary approval, no public hearing, no neighbors voting on your project.
You still need a building permit and the usual inspections. That is a process, not a hearing, and we handle it.
Generally yes, and for many homeowners rental income is the whole reason to build. An ADU is a complete, independent dwelling with its own kitchen, bath and entrance.
Short-term rentals are a separate question — many communities restrict or register them, so check locally if that is your plan.
State rules limit what towns can require for a protected-use ADU, and owner-occupancy requirements are among the things the law restricts. Boston's separate ordinance does require it in some cases.
This is one to confirm with your building department for your specific address before you plan around it.
Almost certainly. You are adding finished living space, and your assessment will reflect that. How much depends on your town's rate and how the assessor values the addition.
Your local assessor's office can give you a realistic estimate before you commit. For most homeowners renting the unit, the income comfortably exceeds the tax increase.
General guidance, not legal advice. Local zoning still governs setbacks, height, parking and design review, and towns interpret the state rules differently. Confirm the specifics for your address with your building department — or ask us and we will find out.
What can actually be built, and how long it takes.
Four common approaches: a detached unit standing on its own in the yard, an attached unit connected to your house, a garage conversion, and a basement conversion.
Detached gives the most privacy and the most rental appeal. Conversions cost less because the structure already exists.
Often, but it depends on what is there. For a garage: ceiling height, the condition of the slab and framing, and whether utilities can reach it. For a basement: ceiling height, a legal egress window or door, moisture, and headroom under ducts and beams.
A walkthrough answers this quickly. We will tell you if the answer is no.
Modular is built in a factory and craned onto your foundation, which shortens the schedule and keeps materials out of the weather — but it needs road access and room for a crane. Stick-built is framed on site and handles tight or sloped lots that a truck could never reach.
We walk through both on the modular vs. stick-built page, step by step.
Plan on the better part of a year from first conversation to move-in. Design and permitting typically run a few months, and construction a few months more, with modular usually shorter than stick-built because factory work and site work overlap.
Permitting timelines vary the most between towns. We give you a realistic schedule for your specific project once we have seen the property.
The question everyone has, answered as directly as we can without seeing your lot.
Detached 900 square foot units start at $300,000. Conversions of existing space — basements and garages — typically run less, since the structure is already there.
That is a starting point, not a quote. Site conditions move the number more than finishes do.
Site work is the big one: how far utilities have to run, whether the ground needs grading, and whether equipment can reach the spot. Septic is the other, if your property is not on sewer.
After that, size, foundation type, construction method, and the finishes you choose. A simple rectangle with durable finishes costs meaningfully less than a complex roofline with high-end materials.
Most homeowners use home equity — a HELOC, a home equity loan, or a cash-out refinance. Renovation loans that lend against the value of the finished property rather than the current one are also common, since an ADU adds value.
Massachusetts has also offered ADU-specific loan programs for income-qualified homeowners. Worth asking your lender what is currently available.
Generally yes, and a rented ADU adds value twice — once as square footage, once as income a future buyer inherits.
How much depends on your market and whether the unit is rentable. No one can promise you a return, and we will not pretend otherwise.
The site conditions that usually decide what is possible.
If you are on town sewer, this is usually straightforward. If you are on septic, adding a dwelling unit means the system has to handle the additional bedrooms, and Title 5 governs that.
Sometimes the existing system is adequate. Sometimes it needs expanding or replacing, which can be a significant cost. This is worth finding out early — it is the single most common surprise.
Not necessarily. Many ADUs run off the existing house services, though the electrical panel often needs upgrading to carry the extra load.
Separate meters make sense if you are renting and want the tenant paying their own utilities. It costs more up front and it is a decision worth making deliberately.
Less than most people assume. A 900 square foot unit is roughly the footprint of a two-car garage, and setback requirements usually determine placement more than total lot size does.
Access matters as much as space — whether equipment, and possibly a crane, can reach the build location.
Depends on the town. Some require an additional space for the ADU, some waive it, and some waive it near public transit.
It rarely stops a project, but it can affect where the unit goes on your lot.
Who you are dealing with and what we take off your plate.
We do. We pull the permits and deal with the building department, and we coordinate the architects and engineers for drawings and engineering.
You get one point of contact rather than managing three separate professionals yourself.
No. Start with a conversation about the property. If the project needs architectural drawings or structural engineering, we bring in the right people and manage that work.
Hiring an architect before you know what your lot supports often means paying for drawings twice.
Yes. Massachusetts Unrestricted Construction Supervisor License CS-094213 and Home Improvement Contractor registration HIC 151287, fully insured, with more than 23 years of construction experience behind the work.
Both numbers are verifiable through the state. We would encourage you to check them — for us and for anyone else you consider.
Massachusetts and Rhode Island.
If you are not certain whether your town is in range, ask. We will give you a straight answer rather than string you along.
A conversation. Tell us where the property is, what you want the space for, and roughly what you have in mind for a budget.
From there we look at the property — access, grade, utilities, septic and setbacks usually answer the big questions quickly.
Then we tell you. Some lots genuinely will not support one, and some budgets do not match what the site requires.
You are better off hearing that in week one than after you have spent money finding out.
If your question is about your specific property, a short conversation beats any FAQ. No charge and no obligation.
(508) 257-1308