How Much Does an ADU Cost?
A straight answer, the arithmetic behind it, and an honest account of what moves the number on your lot.
Our starting ballpark
That is where we start the conversation for a new ADU on the North Shore of Massachusetts. Most builders will not put a number on a page at all, and we would rather you have one than have to guess. But be clear about what it is: a planning figure that tells you whether an ADU is worth exploring on your property. It is not a quote, and it cannot see your lot.
The real number comes from a site visit and a set of drawings. That visit is free, it carries no obligation, and it is the only honest way to price a building.
What That Works Out To
Straight arithmetic at $350 per square foot, at the three sizes homeowners ask about most.
Studio
400 sq ft
~$140,000
Compact one-room layout with a full kitchen and bath. The smallest footprint most homeowners consider.
One-bedroom
600 sq ft
~$210,000
A separate bedroom plus living space. The most common ADU size we get asked about.
Maximum by-right
900 sq ft
~$315,000
The largest ADU Massachusetts protects by right, subject to the half-the-main-house rule.
Ballpark figures for planning only. Not a quote, not an offer, and not adjusted for your site.
What a Construction Ballpark Does Not Cover
A per-square-foot figure covers building the unit. Several real costs on an ADU project are charged by other parties and vary by town and by lot, so we identify them for your specific property rather than folding a guess into a headline number:
- Design and engineering fees, billed by the architect or engineer who draws the plans.
- Town permit fees and any inspection fees, set by your municipality.
- Utility connection or service upgrade fees charged by the utility.
- Septic design, upgrade, or replacement, where a site needs it.
- Site work beyond the building itself — driveways, extensive grading, landscaping, fencing.
What Actually Moves the Number
Two identical sets of drawings can price differently on two lots a mile apart. These are the conditions we look at on a site visit, roughly in the order they tend to matter.
Site access
A backyard an excavator can reach from the street is a different job than one where every yard of concrete and every stick of lumber gets moved by hand or over a neighbor's easement. Tight access adds labor to nearly every phase.
Septic vs. sewer
On sewer, tying in is usually routine. On septic, MassDEP's Title 5 guidance is what governs — a system serving both the main house and an ADU generally needs a multi-compartment tank or two tanks in series, and your town may look at capacity. An upgrade or a replacement system is a real line item that a per-square-foot number cannot see.
Foundation and what is under the yard
Ledge, high water table, or poor soil changes the excavation and the foundation. We build ICF foundations, which cost more up front than a poured wall and pay it back in insulation and durability. A slab, a crawlspace, and a full basement are three different budgets.
Energy code tier
Massachusetts towns sit on one of three tiers — base code, the Stretch Energy Code, or the Specialized Opt-in Code. The stricter tiers mean more insulation, tighter air sealing, better mechanicals, and third-party verification by a rater. Same drawings, different cost, purely because of where the lot is.
Utility runs
Distance matters. Water, sewer or septic, electrical service, and gas all have to reach a detached ADU from wherever they are now. Thirty feet across an open lawn and a hundred and fifty feet around a mature tree line are not the same trench.
Which type of ADU you build
A detached cottage carries its own foundation, roof, and full envelope. An attached addition shares a wall and often shares mechanical runs. An internal conversion reuses a structure that already exists. The three sit at genuinely different price points.
Finish level
Cabinetry, counters, flooring, tile, fixtures, and trim have an enormous spread between builder-grade and custom. Two identical 600 sq ft shells can finish tens of thousands of dollars apart, and this is the driver you have the most direct control over.
Size efficiency
A 400 sq ft studio still needs a complete kitchen and a complete bathroom — the two most expensive rooms in any dwelling — spread over far fewer square feet. Small units tend to carry a higher cost per square foot than large ones. It is one of the reasons a per-square-foot figure is a starting point rather than a formula.
Your Town Changes the Math
Of the 25 North Shore municipalities in our lookup tool, 22 have adopted the Stretch Energy Code or the Specialized Opt-in Code, which set a higher performance bar than base code and require verification by a third-party rater. That is real money on a new build, and it is decided entirely by which side of a town line your lot sits on. Local septic rules, historic district review, and parking requirements vary the same way.
ADU Cost Questions
How much does an ADU cost in Massachusetts?
Don Perry Construction's starting ballpark for a new ADU on the North Shore is about $350 per square foot. At that rate a 400 sq ft studio lands near $140,000, a 600 sq ft one-bedroom near $210,000, and a 900 sq ft unit near $315,000. This is a starting point for planning, not a quote — site conditions such as access, septic, foundation, utility runs, and your town's energy code tier move the number in both directions. A site visit is what turns the ballpark into a real number.
Is the $350 per square foot figure a quote?
No. It is a ballpark that helps you decide whether an ADU is worth exploring on your property. Every ADU is priced from the actual lot and the actual drawings. We visit the site, look at access, utilities, septic or sewer, soil, and your town's requirements, and then give you a real number in writing. The site visit is free and there is no obligation.
What is not covered by a per-square-foot construction ballpark?
A construction ballpark covers building the unit. It does not cover fees charged by other parties: design and engineering, town permit and inspection fees, utility connection or service upgrade charges, septic design or replacement where a site needs it, and site work beyond the building itself such as driveways, grading, and landscaping. Those vary by town and by lot, so we identify them for your specific property rather than guessing at them.
Why does the same ADU cost more in one town than the next?
The largest town-to-town swing is the energy code. Massachusetts municipalities sit on one of three tiers — base code, the Stretch Energy Code, or the Specialized Opt-in Code — and the stricter tiers require more insulation, tighter air sealing, higher-performing mechanicals, and third-party verification. Local septic rules, historic district review, and parking requirements also differ. Our free town lookup shows what your municipality requires.
How big an ADU can I build in Massachusetts?
Under the Affordable Homes Act and its implementing regulations at 760 CMR 71, effective February 2, 2025, one Protected Use ADU is allowed by right on a single-family lot. It may be no larger in gross floor area than half the gross floor area of the principal dwelling, or 900 square feet, whichever is smaller. So if your house is 1,400 sq ft, your by-right ADU tops out at 700 sq ft rather than 900.
Does a smaller ADU always cost less?
Less in total, yes — but usually more per square foot. A studio needs a full kitchen and a full bathroom just like a larger unit does, and those are the two most expensive rooms in a dwelling. Shrinking the footprint does not shrink them. That is why we price from drawings and a site visit rather than from square footage alone.
Sources
Regulatory statements on this page come from the following official sources. Bylaws and codes change — verify against the source before relying on any of it for a permit.
Get a Real Number for Your Lot
We'll walk your property, check access, utilities, and your town's requirements, and put a written estimate in your hands. Free, no obligation.