How Much Does It Cost to Build an ADU in Boston?
Real 2026 numbers from a Greater Boston design-build firm and how to find out what your lot actually allows.
Short answer: the cost to build an ADU in Boston typically ranges from about $100,000 - $250,000 for a basement or garage conversion, $150,000 - $325,000 for an attached unit, and $200,000 - $360,000 for a detached backyard cottage, plus roughly $3,000 - $8,000 for architectural design. Where a project lands depends on the ADU type, the size, and your lot's site and utility conditions.
Key takeaways: Boston ADU costs (2026)
- Basement or garage conversion: about $100,000 - $250,000
- Attached unit: about $150,000 - $325,000
- Detached cottage: about $200,000 - $360,000
- Design $3,000 - $8,000 · survey/plot plan $3,000 - $5,000 · civil engineering (if your town requires it) $2,000 - $4,000
- ADUs under 900 sq ft are by-right in most Massachusetts towns; Boston proper follows its own city ordinance
- Typical project timeline: 5 - 8 months from design through construction
ADU costs in Greater Boston at a glance
| ADU type | Typical all-in cost |
|---|---|
| Basement or garage conversion | ~$100,000 - $250,000 |
| Attached unit | ~$150,000 - $325,000 |
| Detached cottage | ~$200,000 - $360,000 |
| Architectural design | $3,000 - $8,000 |
| Boston building permits | ~$10 - $15 per $1,000 of project cost |
| Survey / plot plan | $3,000 - $5,000 |
| Civil engineering (if your town requires it) | $2,000 - $4,000 |
Horizon ADU designs and builds stick-built, on-site ADUs exclusively, across six Greater Boston counties. Unrestricted construction supervisor's license, home improvement contractor license, fully insured.
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1. What's Included in ADU Construction Costs
An ADU budget is really five budgets:
- Planning and design: Plans, survey, and feasibility work: design and architecture $3,000 - $8,000, survey $3,000 - $5,000, civil engineering $2,000 - $4,000 where the town requires it.
- Zoning and permits: Boston building permit fees run roughly $10 - $15 per $1,000 of total project cost; other towns set their own fees.
- Site work and utilities: Excavation, foundation, and independent water, sewer, and electrical connections. The most lot-dependent category.
- Construction shell: Framing, roofing, siding, and insulation: the weathertight shell.
- Interior finishes. Kitchen, bathroom, flooring, and fixtures: where homeowners have the most control.
ADUs typically cost more per square foot than a simple addition, because they do more.
2. ADU Cost by Type: Conversion, Attached, and Detached
Match the build route to your property:
- Basement or garage conversion ($100,000 - $250,000): Often the most affordable route, because the structure already exists; ceiling height, egress, and structural condition drive where you land. See our garage conversion and basement ADU
- Attached unit ($150,000 - $325,000): Attached ADUs share a wall and often some utility runs with the main home, which brings costs down and fits tighter lots.
- Detached cottage ($200,000 - $360,000): A detached ADU is a standalone home with its own foundation, roof, and utilities. It costs the most and delivers the most privacy and independence.
The cheapest ADU is usually the one your property has already half-built.
3. ADU Cost per Square Foot in Boston and Massachusetts: Why Smaller Isn't Cheaper
An ADU's most expensive components, the kitchen, bathroom, and utility connections, cost about the same at 400 square feet as at 900, so the per-square-foot figure falls as the unit grows. In our cost guide for 900 sq ft units, the by-right maximum for most Massachusetts homeowners, a detached stick-built unit runs roughly $300 - $400 per square foot, often below the rate of a smaller unit. Our Boston ADU floor plans show layouts from 400 sq ft studios to 850 sq ft two-bedrooms.
4. Hidden Costs That Move the Number
Most ADU budgets go wrong on the site, not the building. The items we flag earliest in feasibility reviews:
- Title V septic upgrades: Often the single largest surprise: adding a bedroom on a septic property can trigger system upgrades.
- Utility connections: Water, sewer, and electrical runs, including trenching, vary by town and distance from existing service.
- Site access: Tight side yards, mature trees, and slopes change how equipment reaches the build site.
- Grading and stormwater: Several towns require stormwater management once you add impervious area.
Our guide to hidden ADU costs in Massachusetts walks through the full list. The difference between the bottom and top of every range is mostly the lot, not the building.
Find your lot's surprises early: run your address through the ADU Planner.
5. Why ADU Costs Run High in Boston
- New England conditions. Foundations below the frost line, snow-load framing, and entrances that work in winter, all required by code.
- The Massachusetts State Building Code (780 CMR) applies to every ADU, along with the state's ADU regulations (760 CMR 71.00) and tightening energy codes.
- Town-by-town variation. Since February 2025, ADUs under 900 sq ft (or 50% of the main home, whichever is smaller) are by-right in single-family districts statewide under the Affordable Homes Act, but setbacks, height, parking, and design review still vary. Our Massachusetts ADU regulations hub covers each town.
- Boston itself is the exception. The city is excluded from the statewide by-right law and follows its own ADU ordinance, a distinction most cost guides miss. Start with our Boston ADU rules guide if you're in the city.
How the rules split, at a glance:
| Most Massachusetts cities & towns (statewide law) | City of Boston | |
|---|---|---|
| ADU under 900 sq ft (or 50% of the main home) | Buildable by-right, no special permit or variance | Excluded from the statewide law; the city's own ordinance applies |
| Owner-occupancy requirement | Prohibited statewide | Set by the city ordinance (see our Boston ADU rules guide) |
| Parking | At most one added space; none within ½ mile of transit | Set by the city ordinance |
6. How to Pay for It: ADU Financing in Boston
The main options are home equity (HELOC or cash-out refinance), construction loans, and the MassHousing ADU program: up to $250,000 for a detached ADU or $150,000 for an attached unit as a second mortgage for eligible homeowners, per the state's program announcement. Compare routes on our ADU financing in Boston page, our 2026 financing guide, and our MassHousing program breakdown.
7. Is the Cost Worth It?
An honest answer: it depends on what you're building it for, and anyone who promises a specific return isn't being straight with you. Rental income, family housing, and flexibility can add meaningful long-term value, but the return depends on your lot, the unit, and the market. We go deeper in Is an ADU a Good Investment in Boston?
8. How to Get a Real Number for Your Property
Every figure on this page is a range. Here's how to pin down real numbers:
- Feasibility first. Our $200 feasibility report, ordered through the ADU Planner, confirms what your lot and your town's zoning allow before design dollars are spent.
- Design to the lot. Plans drawn for your setbacks, slope, and access, not a factory spec.
- Price the real scope. A build price that reflects your project, not a regional average.
An honest range is where planning starts. A feasibility review is where your real number starts.
Start with your address: order the $200 feasibility report through the ADU Planner.
Prefer to talk it through first? Book a free consultation call, no obligation, and we'll tell you honestly if an ADU isn't the right fit.
Frequently Asked Questions
How much does it cost to build an ADU in Boston?
Typically $100,000 - $250,000 for a basement or garage conversion, $150,000 - $325,000 for an attached unit, and $200,000 - $360,000 for a detached cottage, plus $3,000 - $8,000 for design.
What's the cheapest type of ADU to build in Massachusetts?
Generally a basement or garage conversion, at roughly $100,000 - $250,000, if the existing space has adequate ceiling height, egress, and structural condition.
How long does it take to build an ADU in Boston?
Typically 5 - 8 months: 1 - 2 months for design and survey, 1 - 2 months for permitting, and 3 - 4 months for construction. Our ADU timeline guide breaks down each stage.
Do I need a special permit to build an ADU in Massachusetts?
No, in most Massachusetts single-family districts, ADUs under 900 sq ft (or 50% of the main home, whichever is smaller) are buildable by-right under the Affordable Homes Act. Boston proper is the exception: the city follows its own ordinance. See our regulations hub.
Can I finance an ADU in Massachusetts?
Yes. Homeowners typically use home equity, a construction loan, or the MassHousing ADU program (up to $250,000 for a detached unit, for eligible homeowners). Our ADU financing page compares the options.
How do I find out what an ADU would cost on my property?
Start with a feasibility review. Our $200 feasibility report, ordered through the ADU Planner, confirms what your lot and zoning allow and flags the site conditions that determine your real cost.
Final Thoughts
The honest version of "how much does an ADU cost in Boston" is a set of ranges; the dishonest version is a single confident number quoted before anyone has looked at your property. The best projects begin with clear thinking about the lot, not with a price pulled from an average.
About Horizon ADU
Horizon ADU is a Greater Boston design-and-build company specializing exclusively in stick-built, on-site ADUs, detached backyard cottages and attached units. We do not build modular or factory-built ADUs. We help homeowners navigate zoning, access, and design to create ADUs that are both beautiful and buildable.
Phone: 774-222-0127 · Address: 77 King St, Boston, MA 02122