A PRACTICAL GUIDE FOR MASSACHUSETTS HOMEOWNERS
What they cost, what permits you need, what the 2025 ADU law changed, and what actually decides whether your property can support the project. Written by Andrew Mercurio, a fourth-generation builder working in Worcester County.
Most homeowners start this process with the same three questions and get vague answers to all of them. What will it cost. How long will it take. Am I even allowed to build it.
This guide answers those directly, using Massachusetts rules and Central Massachusetts numbers rather than national averages that do not reflect what anything costs here. It also covers the constraint that decides more projects in this part of the state than zoning ever does, which is what your septic system is rated for.
If you want to skip to a specific service, we have detailed pages on home additions, ADUs and in-law suites, sunrooms and four season rooms, and garage and basement conversions.
This is the biggest change to Massachusetts residential building rules in a generation, and most homeowners still have not caught up with it.
Governor Healey signed the Affordable Homes Act in August 2024, and its accessory dwelling unit provisions took effect on February 2, 2025. The law requires every city and town in the Commonwealth to allow at least one accessory dwelling unit by right on lots in single-family zoning districts. It applies to 350 of the 351 municipalities in Massachusetts. Boston runs its own separate program.
By right means the use is permitted without discretionary approval. No special permit. No zoning board of appeals hearing. No vote a neighbor can derail. You still need a building permit and the associated trade permits, and your town may require site plan review, but the town processes your application the way it would process one for a single-family home rather than treating it as a question it gets to answer.
What the law allows:
An ADU can be up to 900 square feet or half the gross floor area of the principal dwelling, whichever is smaller. That second clause catches people. On a 2,400 square foot house you get the full 900. On a 1,600 square foot house your cap is 800. On a 1,200 square foot ranch it is 600.
The unit must be self-contained, with its own entrance, kitchen, and bathroom. Attached additions, detached cottages, garage conversions, basement conversions, and attic conversions all qualify.
What your town cannot do:
It cannot require that you or a family member occupy either unit. It cannot require a family relationship between the people living in the ADU. It cannot apply design standards it would not apply to a single-family home in the same district. It cannot require more than one additional parking space, and it cannot require any if your property sits within a half mile of a commuter rail station, subway station, ferry terminal, or bus station. It cannot ban modular or prefabricated units that meet the state definition.
What the law does not allow:
A protected-use ADU cannot be used as a short-term rental of under 31 days. Long-term rental is fully protected. If the plan was an Airbnb, this is not the route to it.
One clarification worth knowing: the state has confirmed the by-right allowance applies to lots in single-family zoning districts, so even if you own a multifamily property, if it sits in a single-family zone the law still applies. Check your town's zoning map rather than assuming from what is currently built there.
Bump-outs. Extending a single room by a few feet. Often the most cost-effective fix for a cramped kitchen or a bathroom you cannot turn around in. Sometimes cantilevered off the existing structure without a new foundation, which is why the cost is so much lower than other options.
Single-story additions. A family room, a primary suite, a home office, an expanded kitchen. New foundation, new roof, tied into the existing house at one wall.
Two-story additions. Substantial square footage on both levels at once, usually the best value per square foot on a larger project because the foundation and roof are shared across two floors.
Second-story additions. Building up rather than out when the lot will not allow expansion at grade. The most disruptive and the most expensive per square foot, because the roof comes off and the existing structure below has to be reinforced to carry the new load.
ADUs and in-law suites. Covered in detail on our ADU page. The distinction from a regular addition is that an ADU is a separate dwelling with its own kitchen and entrance, which is what brings it under the state law and its protections.
Sunrooms and four season rooms. Including conversions of screened porches past their useful life. The critical decision is three season versus four season, because in Massachusetts a three season room is unusable from November through March.
Garage, basement, and attic conversions. Adding living space without changing the footprint. Usually the least expensive route to square footage, though rarely as cheap as homeowners expect once code requirements for foundations, egress, and ceiling height are accounted for.
Septic capacity, and this is the big one. Massachusetts sizes septic systems by bedroom count under Title 5. Any project that adds a bedroom, which includes most additions and effectively every ADU, has to be measured against your system's rated capacity. If the system will not carry the additional load, the project becomes a septic upgrade plus an addition, which is a materially different budget.
Whether this affects you depends entirely on your town, and the variation across Worcester County is dramatic:
Sterling has no municipal sewer anywhere in town. Every property is on private septic, and the Board of Health reviews wastewater capacity before a building permit is issued.
Princeton is almost entirely septic and private wells.
Boylston is mixed, with only a portion of the town sewered.
Holden is partly sewered, with the trunk lines owned and operated by the state Department of Conservation and Recreation.
West Boylston is roughly 85 percent sewered with mandatory connection inside the sewered district.
Shrewsbury is approximately 90 percent sewered, which makes it one of the easier places in the county to build an ADU.
Setbacks, lot coverage, and height. Still apply, and on a tight lot a detached ADU may simply not fit where you pictured it. Your town applies the same dimensional rules it would apply to a single-family home.
The existing foundation. On an attached addition, what you are tying into determines a great deal. Older homes across this county sit on fieldstone and rubble foundations laid by hand, and the connection between old and new is usually the hardest part of the job.
Watershed regulations. If you are in Boylston, Clinton, Holden, Leominster, Paxton, Princeton, Sterling, West Boylston, or Worcester, your property sits in the Wachusett Reservoir watershed, and the Massachusetts Watershed Protection Act regulates land use within critical areas near tributaries, wetlands, and surface water. On most lots it never comes up. Near a stream it can affect where you build and how much impervious surface you add.
Wetlands. Conservation Commission review applies near wetlands in every town, and on the larger wooded lots common in this area it comes up more often than people expect.
These are Central Massachusetts numbers. Greater Boston generally runs 20 to 30 percent higher for identical scope.
Additions, by type:
A bump-out of 100 to 200 square feet typically runs $25,000 to $60,000. A single-room addition of 250 to 400 square feet runs $60,000 to $140,000. A primary suite addition runs $90,000 to $180,000. A kitchen extension runs $75,000 to $180,000. A two-story addition of 500 to 800 square feet runs $150,000 to $320,000. A full second-story addition runs $200,000 to $500,000 or more.
Per square foot, most additions land between $150 and $350 fully finished. Second-story additions run $250 to $400 because of the structural reinforcement and roof removal involved.
ADUs, by type:
A basement or interior conversion generally runs $95,000 to $180,000. A garage conversion runs $95,000 to $175,000. An attached ADU built as an addition runs $150,000 to $280,000. A detached ADU, which is effectively a small house, runs $200,000 to $380,000, and detached new construction in Massachusetts generally prices between $275 and $450 per square foot.
The line items that surprise people:
Utility connections to a detached unit run $10,000 to $40,000, and trenching water and sewer roughly sixty feet out to a detached garage runs $10,000 to $25,000 of that.
Foundation work on a garage conversion runs $15,000 to $40,000. Most garage slabs in this area were poured four inches thick with no frost footings and no insulation, because a slab holding a car does not need any of that. Habitable space in Massachusetts does.
Septic upgrades, where Title 5 capacity requires them, vary widely by system and site.
Permits you will need. Any project that changes the footprint or the structure of your home requires a building permit under the Massachusetts State Building Code. Most towns also require zoning review for setbacks, lot coverage, and height. Depending on scope you will also need electrical, plumbing, and mechanical permits, each with its own inspections. Massachusetts requires that licensed electricians and licensed plumbers perform their respective work. Work near wetlands requires Conservation Commission review.
Who should pull the permit. Your contractor. Under Massachusetts law, a homeowner who pulls their own building permit gives up access to the state Guaranty Fund, which exists to protect consumers against contractor default. Saving a small fee is not worth surrendering that protection.
Timeline. Design and permitting typically run one to three months depending on the town and the complexity of the project. Construction on most additions runs three to six months. A bump-out on an existing foundation moves faster. A two-story addition requiring a new foundation, structural modification to the existing house, and utility rerouting takes longer.
Massachusetts weather is a real scheduling factor. Foundation work is difficult to schedule between December and March, and at elevation, in towns like Princeton sitting on and around Mount Wachusett, the window is shorter still. A contractor who promises a January foundation pour in the hills is either working with heated enclosures and charging for them, or is going to be late.
Most of the housing stock in Worcester County predates 1960, and a major project is when everything a house has been quietly living with surfaces at once. Knob and tube wiring. An electrical service sized for a 1955 household. Cast iron waste lines corroded from the inside. Galvanized supply pipes narrowed by corrosion. Plaster over lath. Framing on irregular spacing that does not match modern dimensions. Sills with rot behind the siding.
And lead, which has legal consequences most homeowners do not know about.
Massachusetts law requires that a home built before 1978 occupied by a child under six be deleaded or brought under interim control. It applies to owner-occupied homes, not just rentals. Critically, a new owner of such a home where a child under six will live has 90 days from taking title to comply. If you are buying a fixer-upper with young children, that clock starts at closing.
Separately, the federal Renovation, Repair and Painting rule governs the work itself. It applies to renovation disturbing more than six square feet of painted surface indoors or twenty square feet outdoors in pre-1978 housing, and requires certified renovators and certified firms using lead-safe work practices. Massachusetts is not an authorized RRP state, so the EPA administers that program directly here.
There is also a Massachusetts tax credit for deleading, up to $1,500 per unit for full compliance and up to $500 per unit for interim control, with unused credit carrying forward up to seven years. Most homeowners never hear about it.
Verify two credentials, and know that they are different.
Home Improvement Contractor registration is required for most residential work in Massachusetts and gives you access to the state Guaranty Fund if a registered contractor defaults.
A Construction Supervisor License is what legally qualifies someone for structural work. Additions, ADUs, and any project removing a load-bearing wall fall under it. This is the credential that separates a general contractor from a remodeling company, and it is why many companies that will happily quote your kitchen will not touch an addition.
Both are verifiable through the Commonwealth's public license lookup. Ask for the numbers and check them. Any contractor who hesitates is telling you something.
Know the deposit rule. Under Massachusetts home improvement contractor law, a deposit cannot exceed one third of the total contract price, or the cost of special order materials, whichever is greater. A contractor asking for half up front is outside the rules.
Get the contract right. Massachusetts requires a written contract for residential work over $1,000, with the registration number, a payment schedule tied to stages of work, and start and completion dates. Any change to the contract has to be in writing and signed by both parties before the work proceeds. That last requirement protects you specifically.
Ask who actually does the work. On an addition, the foundation determines whether everything above it holds. If your contractor subcontracts the foundation, the masonry, the framing, and the finish work to four different companies, the schedule depends on four companies' availability rather than one.
ARM Construction & Management Inc is a general contracting company based in Princeton, Massachusetts, serving homeowners throughout Worcester County.
Andrew Mercurio is a fourth-generation builder. He started in masonry, the way his family did, and grew ARM Construction into a full general contracting company that takes projects from the first site walk through design, permitting, construction, and final inspection. That background is why we self-perform the foundation and structural work most remodeling companies subcontract out.
Andrew holds Massachusetts Construction Supervisor License #CS-093044, issued in 2011. ARM Construction & Management Inc holds Home Improvement Contractor Registration #153199. Both are active and verifiable through the Commonwealth.
We work in Worcester, Holden, Princeton, Sterling, West Boylston, Boylston, Shrewsbury, Leominster, Westborough, Northborough, Southborough, and Grafton.
This guide is general information about Massachusetts law and construction practice, not legal advice. Requirements vary by municipality and change over time. Confirm current requirements with your local building department. Pricing reviewed August 2026.
What Our Customers Say
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Most additions in Central Massachusetts run $150 to $350 per square foot fully finished, including design, permitting, and standard finishes. Second-story additions run $250 to $400 because the roof has to come off and the existing structure has to be reinforced. By project type, a bump-out typically lands between $25,000 and $60,000, a single-room addition between $60,000 and $140,000, a primary suite addition between $90,000 and $180,000, and a two-story addition between $150,000 and $320,000. Greater Boston generally runs 20 to 30 percent higher than Central Massachusetts for identical scope.
Yes. Since February 2, 2025, one accessory dwelling unit is allowed by right on any lot in a single-family zoning district, with no special permit and no public hearing. The unit can be up to 900 square feet or half the gross floor area of the main house, whichever is smaller. The law applies to 350 of the 351 cities and towns in Massachusetts, with Boston operating its own program. Towns cannot require owner occupancy, cannot require a family relationship between occupants, and cannot require more than one additional parking space, or any at all within a half mile of a transit station.
In the towns north and west of Worcester, it is septic capacity. Massachusetts sizes septic systems by bedroom count under Title 5, so any project adding a bedroom has to be measured against the existing system's rated capacity. Exceeding it turns the project into a septic upgrade plus an addition. Sterling and Princeton have no municipal sewer at all, while Shrewsbury is roughly 90 percent sewered and West Boylston around 85 percent. Setbacks, lot coverage, and watershed regulations are the other common constraints.
Two credentials matter and they are different. Home Improvement Contractor registration is required for most residential work and gives you access to the state Guaranty Fund. A Construction Supervisor License is what legally qualifies someone for structural work, which includes additions, ADUs, and any project that removes a load-bearing wall. Both are verifiable through the Commonwealth's public license lookup. Also know that a deposit cannot exceed one third of the contract price, or the cost of special order materials, whichever is greater.
Almost certainly, and it is worth planning for. Massachusetts assesses property at full and fair cash value, and a permitted addition or ADU adds finished square footage that gets picked up at the next assessment. Your building permit is what puts the assessor on notice. The increase applies to the new value rather than triggering a reassessment of your entire house, and the exact impact depends on your town's rate and how the assessor values the addition. Your local assessor's office will discuss it with you before you commit, and that call is worth making. A three-season room, which is not counted as living space, is treated differently from a four-season room, which is.
An addition adds space to your existing home. An ADU is a separate dwelling with its own entrance, kitchen, and bathroom, whether it is attached to the house or standing on its own. The distinction matters because ADUs fall under the state law that took effect in February 2025 and get its protections, including by-right approval and the ban on owner-occupancy requirements. An attached ADU and a single-story addition can look nearly identical from the street. What separates them is the kitchen and the separate entrance.
Massachusetts allows one ADU by right on most single-family lots. Here is what the law permits. ...more
Additions & ADUs
August 05, 2026•7 min read
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131 Houghton Road
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