What an ADU Costs to Build on the North Shore
If you have asked a few contractors what an accessory dwelling unit costs, you have probably gotten three different answers, and none of them explained why. Part of the confusion is real: an ADU is not a single product with one price. A basement conversion, a garage conversion, an attached addition, and a detached new-build unit are four different construction projects that happen to share a name. Part of the confusion is that the law changed in 2025, and a lot of what homeowners on the North Shore think they know about ADUs is based on rules that no longer apply.
This page walks through what changed, what size unit you are legally allowed to build, and what real North Shore and Greater Boston cost data says about each type of ADU project, so you can tell the difference between a serious number and a guess.
What Changed on February 2, 2025
Sections 7 and 8 of Chapter 150 of the Acts of 2024 amended Massachusetts General Laws Chapter 40A, sections 1A and 3, and took effect on February 2, 2025. The implementing regulations sit at 760 CMR 71.00. Together, they made one accessory dwelling unit a by-right use on single-family lots across the state, which means:
- A town cannot require a special permit for a single ADU on a single-family lot.
A town cannot impose a minimum lot size specific to ADUs. - A town cannot cap the number of ADUs allowed community-wide.
- Owner occupancy of the main house or the ADU may not be required as a condition of approval.
- Detached ADUs are protected under the same by-right framework as attached ones.
- Many homeowners still assume a detached unit needs a special permit. It does not.
What the law does not remove is local dimensional and procedural review. Setbacks, height limits, lot coverage, stormwater, and site plan review still vary by town, and every North Shore, Cape Ann, and Andover-area community applies its own zoning bylaw on top of the state floor. Treat “by right” as a statement about whether you need a special permit, not a statement that there is nothing left to review.
How the Size Cap Actually Works
The size limit is the detail most marketing copy gets wrong, so it is worth stating precisely: an ADU may not exceed one half of the principal dwelling’s gross floor area, or 900 square feet, whichever is smaller.
That “whichever is smaller” matters. A 2,600-square-foot house gets a cap of 900 square feet, because half of 2,600 is 1,300, which is larger than 900, so the smaller number governs. A 1,400-square-foot house gets a cap of 700 square feet, because half of 1,400 is smaller than 900. The larger your existing house, the more likely 900 square feet is your real ceiling. The smaller your house, the more likely the half-of-GFA rule caps you below 900. Run your own house’s square footage through that formula before you settle on a floor plan, because it changes the whole cost conversation below.
What an ADU Actually Costs, by Type
These are planning ranges drawn from current Massachusetts and Greater Boston construction cost data, not a quote. Every one of these numbers moves based on your lot, your existing utilities, and your finish level, which is what the next section covers.
Basement or garage conversion: Generally the least expensive path because the structural shell already exists. Statewide guides put a basement or garage ADU conversion in roughly the $100,000 to $250,000 range, with the final number driven by egress work, ceiling height, insulation, and whether new plumbing has to be run to a space that never had it.
Attached addition: Sharing walls, roof structure, and often utilities with the main house typically saves 20 to 30 percent against a fully detached unit of the same size, because you are not building a complete standalone structure with its own foundation and roof from scratch.
Detached new construction is the most common request and the most expensive per square foot to start, but the per-square-foot number falls as the unit gets larger, because the kitchen, bathroom and utility connections cost roughly the same whether the unit is 400 square feet or 900. Massachusetts detached ADU construction commonly runs $250 to $450 per square foot in current cost guides, with Greater Boston and the North Shore trending toward the upper half of that range. At the 900-square-foot by-right maximum, that puts a detached stick-built unit in the rough neighborhood of $270,000 to $400,000 for construction alone, before site work, permitting and design fees.
What Moves the Number
- Site work and access. If the ADU location is far from the street, on a slope, or requires significant grading or tree removal to get equipment in, add $20,000 to $80,000 or more before the structure itself is priced.
- Utility connections. Most ADUs tie into the main house’s existing water, sewer and electrical service. Where that connection is long, undersized, or the electrical panel needs upgrading to carry a second unit, budget $10,000 to $30,000 for utility work alone.
- Septic capacity. On unsewered lots, which describes a meaningful share of Boxford, Topsfield, Essex, parts of Newbury and Manchester, an ADU adds bedroom count and occupancy to a Title 5 system that was sized for the main house only. If the existing septic system does not have capacity for the added flow, an upgrade or a new leach field can be the single largest line item on the whole project, and it has to be evaluated before a floor plan is finalized, not after.
- Finish level and design complexity. A straightforward one-bedroom layout with mid-range finishes costs far less than a unit with custom millwork, vaulted ceilings or a layout that fights the shape of the lot. This is where an ADU stops being a commodity box and starts being a small, well-designed house, and where the cost range genuinely widens in both directions.
Paying for It
As of early 2026, MassHousing offers a dedicated ADU construction loan through its network of partner lenders, financing up to $250,000 for a detached ADU and up to $150,000 for an attached one, structured as construction-to-permanent financing. The program is built around applicants who already hold design plans and permits, not around homeowners still deciding whether to build. That sequencing is worth sitting with: the state’s own financing program assumes you arrive with a completed design and an issued permit in hand, which is exactly the point at which a design-build process, rather than a contractor’s estimate, actually earns its keep. Loan terms, income limits, and program availability change, so confirm current details directly with MassHousing or a participating lender before budgeting around a specific figure.
What a Design-Build Proposal Includes That a Contractor's Bid Does Not
A contractor’s bid prices construction against drawings that already exist. It does not tell you whether the drawings are buildable at the budget you have in mind, whether the septic system can carry the added bedroom, or whether the roofline will read as an addition instead of an afterthought next to the main house. Those are design decisions, and on a design-build project they get worked out before a number is finalized, not discovered mid-framing.
A complete design-build proposal for an ADU should account for the site evaluation, including septic and utility capacity, the design and permitting process through the relevant town boards, the construction budget itself, and the finish and millwork decisions that determine what the finished unit actually feels like to live in. Any Season Design Build runs all of that under one team, with cabinetry and built-ins designed and produced in-house rather than ordered from a catalogue and adjusted on site to fit. On a small unit where every square foot of storage matters, that in-house control over millwork is often the difference between a floor plan that works on paper and one that works in daily use.
Jason DeVincenzo has spent more than twenty years building on the North Shore, and the firm carries $2,000,000 in liability insurance, workers’ compensation, and builder’s risk coverage where required, backed by a three-year warranty and licensed sub-tradespeople on every trade.
Frequently Asked Questions
Do I need a special permit to build an ADU on the North Shore?
Can I build a detached ADU, or only an addition to my existing house?
Do I have to live in the main house or the ADU myself?
How big can my ADU be?
What does an ADU cost on the North Shore?
Is septic capacity a real concern for an ADU?
Does my town have its own ADU rules on top of the state law?
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