Boston · ADU cost
What it costs to build an ADU in Boston, NY
An 800 sq ft accessory dwelling unit in Boston runs roughly $200k–260k, or about $249–327/sq ft, and that per-square-foot figure sits deliberately above the $204–268/sq ft a full house costs here. An ADU still needs one whole kitchen, one whole bath, its own service panel and its own heating plant no matter how small you draw it, and those costs do not shrink with the floor plan, so they land on half the square footage and push the rate up. Whether the town will let you build one is a separate question from what it costs, and it gets answered parcel by parcel rather than townwide, since the Town of Boston maps 11 districts that treat accessory units differently from one another, so read the table below as a starting point rather than a verdict.
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ADU cost by size in Boston
Standard finish, slab on grade, on a flat and clear part of the lot, general-contractor margin included. These are structure costs at Erie County pricing, before the service and septic work a second dwelling can trigger.
| Size | Type | $ / sq ft | Estimated total |
|---|---|---|---|
| 400 sq ft | Studio | $331–$433 | $130k–175k |
| 600 sq ft | One bedroom | $276–$362 | $165k–215k |
| 800 sq ft | One bedroom, larger | $249–$327 | $200k–260k |
| 1,000 sq ft | Two bedroom | $233–$305 | $235k–305k |
400 sq ft — One room plus a bath. Garage conversions and small backyard cottages land here.
600 sq ft — The most common detached build - a real bedroom, full kitchen, one bath.
800 sq ft — Room for a proper living area. Still a single bath and one kitchen.
1,000 sq ft — About as large as most codes allow an accessory unit to go.
Why the rate is higher than a house
The systems below are priced for an 800 sq ft unit. The middle column is what each system would cost if it simply shrank with the floor plan; the left column is what it actually costs once you account for the equipment a dwelling needs one of regardless of size. The gap between the two columns is the whole reason small builds price the way they do.
| System | $ / sq ft in an ADU | If it scaled with floor area | Fixed per unit |
|---|---|---|---|
| Site work | $7–$11 | $7 | — |
| Foundation | $16–$23 | $16 | — |
| Framing | $25–$32 | $25 | — |
| Envelope - roof, siding, windows | $32–$41 | $32 | — |
| Plumbing | $28–$34 | $16 | 55% |
| Electrical | $18–$24 | $13 | 35% |
| HVAC | $23–$28 | $15 | 45% |
| Insulation | $7–$9 | $7 | — |
| Interior finishes | $32–$43 | $27 | 15% |
| Millwork & cabinetry | $18–$28 | $11 | 55% |
The fixed-per-unit share is a modeling assumption, not a figure from the code: it is our estimate of how much of each system's budget buys equipment a dwelling needs exactly one of - a kitchen rough-in, a bath group, a service panel, a heating plant - and therefore does not shrink when the unit does. Site work, foundation, framing, envelope and insulation are treated as scaling straight with floor area, which understates the premium slightly, because a small building carries more envelope per square foot of floor than a large one.
What the Boston code says about accessory units
Whether you can build one, and how large, is decided by the district your parcel sits in rather than by the town as a whole. The table shows how each mapped district treats ADUs alongside the lot and setback standards a second unit still has to fit inside. District boundaries and conditions vary, so treat it as a starting point and confirm the district that applies to your parcel.
| District | Min lot | Setbacks F / S / R | Max height | ADU |
|---|---|---|---|---|
| R-1 Single-Family Residential District | 0.46 ac | 50 / 10 / 30 ft | 35 ft | No |
| R-2 Single-Family Residential District | 4 ac | 50 / 10 / 30 ft | 35 ft | Occupancy restricted |
| R-3 Two-or-More Family Residential District | 0.34 ac | 50 / 10 / 30 ft | 35 ft | No |
| R-A Residential-Agricultural District | 3 ac | 50 / 10 / 30 ft | 35 ft | No |
| General Residence-Restricted Business District | 0.34 ac | 50 / 10 / 30 ft | 35 ft | Special permit |
| Local Retail Business District | 0.34 ac | 50 / 0 / 30 ft | 35 ft | No |
| General Commercial District | 0.34 ac | 50 / 0 / 30 ft | 35 ft | No |
| Industrial Park Research and Development District | 1 ac | 50 / 15 / 25 ft | 30 ft | No |
| General Industrial District | 1 ac | 50 / 15 / 25 ft | 35 ft | No |
| Mobile Home Court District | — | — | — | No |
| Planned Unit Development District | 100 ac | — | — | — |
General Residence-Restricted Business District - Lot width is additive and sewer moves it: on public sanitary sewer, 75 ft for the first dwelling unit or other principal building and a further 25 ft for each unit beyond, capped at 200 ft. Without public sewer, add 25 ft to whatever that comes to.
- The 10 ft side yard is for dwellings only: any other principal building takes 30 ft or the height of the building if greater, and 15 ft or half the height beside a lot outside a residential district
- A dwelling for three or more families must stand fifty feet from any property line
- The rear yard is 30 ft or the height of the principal building, whichever is greater, so the figure shown is a floor
- Two principal buildings on one lot must stand 30 ft apart
- The width at the front lot line must equal the width at the building line, so a flag lot is not available here
Local Retail Business District - No side yard is required for a building that is not a dwelling, but one actually provided must be at least five feet, 20 ft or the building height against a residential district boundary, and 25 ft where used for vehicular access. A dwelling takes ten feet.
- Height splits by use: 30 ft for any dwelling and 35 ft for anything else
- The rear yard is 30 ft or the height of the principal building, whichever is greater
- A dwelling for three or more families needs 20,000 sq ft per structure plus a further 3,000 sq ft for each dwelling unit, and must stand at least 30 ft from every property line
- Lot width is additive: 75 ft for the first dwelling unit or other principal building, a further 25 ft for each unit beyond, capped at 200 ft, and 25 ft more without public sanitary sewer
- The width at the front lot line must equal the width at the building line, so a flag lot is not available here
- A 1997 amendment marks the one- and two-family figures here as for historical purposes only, so confirm with the town before relying on them
General Commercial District - No side yard is required for a building that is not a dwelling, but one actually provided must be at least ten feet, 20 ft or the building height against a residential district boundary, 12 ft where used for vehicular access and 25 ft where used for both.
- A dwelling takes ten feet on the side
- The rear yard is 30 ft or the height of the principal building, whichever is greater
- A dwelling for three or more families needs 20,000 sq ft per structure plus a further 3,000 sq ft for each dwelling unit, and must stand at least 30 ft from every property line
- Lot width is additive: 75 ft for the first dwelling unit or other principal building, a further 25 ft for each unit beyond, capped at 200 ft, and 25 ft more without public sanitary sewer
- A 1997 amendment marks the one- and two-family figures here as for historical purposes only, so confirm with the town before relying on them
Industrial Park Research and Development District - The 30 ft height is the by-right ceiling, and the section says so in terms: above it a special permit from the Town Board is needed, with no upper figure named for that case.
- The 50 ft front yard rises to sixty where the lot lies opposite a residential district or where the front yard holds accessory parking
- The 15 ft side yard rises to fifty, or the height of the principal building if greater, against a residential district boundary, and the rear yard follows the side-yard figure there
- Retail, restaurant and commercial recreation uses are accessory only, filling no more than 15% of the principal building's gross floor area and planned as an integral part of it
General Industrial District - The 35 ft height is the by-right ceiling, and the section says so in terms: above it a special permit from the Town Board is needed, with no upper figure named for that case.
- The 50 ft front yard rises to sixty where the lot lies opposite a residential district or where the front yard holds accessory parking
- The 15 ft side yard rises to seventy-five, or the height of the principal building if greater, against a residential district boundary, and the rear yard follows the side-yard figure there
Mobile Home Court District - Lot size, height and yards inside a licensed mobile home court are set by the Town's Mobile Home Court Law in Ch. 70 of the Town Code rather than by the zoning chapter.
Planned Unit Development District - Density is four dwelling units per gross acre, and gross acres count developable land only: wetlands and streams come out first, so a wet parcel yields far fewer units than its deed acreage suggests.
- At least 30% of the site must be kept as open space
- The 100 acre threshold may be relaxed by the Planning Board where the applicant shows the objectives are met and the land is contiguous to an existing planned unit development
- Yards, height and coverage are settled in the approved plan rather than published in advance
Size limits on record in Boston
Where the code states a size cap, it decides which row of the cost table above actually applies to you. These are the caps recorded for Boston:
- R-1 Single-Family Residential DistrictAn accessory apartment of 640 to 800 sq ft, made inside or as an addition to a single-family house, needs a special use permit in R-2 and R-C. The owner must live in one of the two units when applying. The other districts do not allow one.
- R-2 Single-Family Residential DistrictAn accessory apartment of 640 to 800 sq ft, made inside or as an addition to a single-family house, needs a special use permit in R-2 and R-C. The owner must live in one of the two units when applying. The other districts do not allow one.
- R-3 Two-or-More Family Residential DistrictAn accessory apartment of 640 to 800 sq ft, made inside or as an addition to a single-family house, needs a special use permit in R-2 and R-C. The owner must live in one of the two units when applying. The other districts do not allow one.
- R-A Residential-Agricultural DistrictAn accessory apartment of 640 to 800 sq ft, made inside or as an addition to a single-family house, needs a special use permit in R-2 and R-C. The owner must live in one of the two units when applying. The other districts do not allow one.
- General Residence-Restricted Business DistrictAn accessory apartment of 640 to 800 sq ft, made inside or as an addition to a single-family house, needs a special use permit in R-2 and R-C. The owner must live in one of the two units when applying. The other districts do not allow one.
- Local Retail Business DistrictAn accessory apartment of 640 to 800 sq ft, made inside or as an addition to a single-family house, needs a special use permit in R-2 and R-C. The owner must live in one of the two units when applying. The other districts do not allow one.
- General Commercial DistrictAn accessory apartment of 640 to 800 sq ft, made inside or as an addition to a single-family house, needs a special use permit in R-2 and R-C. The owner must live in one of the two units when applying. The other districts do not allow one.
- Industrial Park Research and Development DistrictAn accessory apartment of 640 to 800 sq ft, made inside or as an addition to a single-family house, needs a special use permit in R-2 and R-C. The owner must live in one of the two units when applying. The other districts do not allow one.
- General Industrial DistrictAn accessory apartment of 640 to 800 sq ft, made inside or as an addition to a single-family house, needs a special use permit in R-2 and R-C. The owner must live in one of the two units when applying. The other districts do not allow one.
- Mobile Home Court DistrictAn accessory apartment of 640 to 800 sq ft, made inside or as an addition to a single-family house, needs a special use permit in R-2 and R-C. The owner must live in one of the two units when applying. The other districts do not allow one.
Costs an ADU adds that a house does not
- Septic and water A second dwelling adds bedrooms to the design flow, which on an unsewered parcel often means expanding or replacing the existing system rather than tapping it. This is the single most common reason an ADU budget moves after design, and it is site-specific enough that no model should quote it - get it evaluated early.
- Utility service National Grid territory. A detached unit needs power run to it, and whether it goes on its own meter or subfeeds the main house changes both the cost and the permit path.
- Access and parking Many codes require an added parking space for the accessory unit, and a detached unit usually needs a driveway extension or a path, both of which count against a lot-coverage or impervious-surface cap.
- Short-term rentalsNo local short-term-rental law on record - which is not the same as permission. The town has no local short-term rental law. That is not the same as permission, so confirm with the town before counting on renting a home short term. Separate question from whether you can build the unit.
- Zoning codeecode360.com/9002443
- Building dept(716) 941-6113
Sizing up a specific parcel in Boston?
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Building a full house instead?
A standard-finish house in Boston runs $204–268/sq ft, about $370k–480k at 1,800 sq ft , plus roughly $3,440 for a conforming lot. See what it costs to build a house in Boston.
ADU cost in nearby towns
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More on Boston: Can you build an ADU here? · Zoning for new construction · Erie County build cost
ADU cost in Boston: common questions
How much does it cost to build an ADU in Boston, NY?
An 800 sq ft accessory dwelling unit in Town of Boston runs roughly $200k–260k at a standard finish, general-contractor margin included. A 400 sq ft studio runs about $130k–175k. Those are structure costs on an easy site and do not include the septic, water or electrical service work a second unit often triggers.
Why does an ADU cost more per square foot than a house in Boston?
Because a lot of a dwelling's cost is per dwelling, not per square foot. One kitchen, one bath group, one service panel and one heating plant cost about the same in an 800 sq ft cottage as in a 1,800 sq ft house, so on the smaller unit they get spread over less than half the floor area. That is why an ADU here prices at $249–327/sq ft against $204–268/sq ft for a full house, even though it is the same trades doing the same work.
Are ADUs allowed in Boston?
Town of Boston has 11 districts. 8 districts prohibit ADUs, 1 allows them with occupancy restrictions, 1 requires a special-use permit. 1 district has incomplete data - verify with the building department at (716) 941-6113. Check the specific address with the free ADU checker before you spend money on design.
Can I rent an ADU in Boston short term?
No local short-term-rental law on record - which is not the same as permission for Town of Boston. ADU rules and short-term-rental rules are set separately, so a unit you are allowed to build is not automatically one you are allowed to rent nightly. Confirm both before you underwrite the build on short-term-rental income.
ADU costs are modeled estimates in current 2026 dollars at the Erie County regional index, general-contractor margin included, derived from Offsite's cost engine with a stated adjustment for the equipment a dwelling needs one of regardless of size. They are planning numbers for decision-support only - not a bid, quote, or guarantee, and not legal, architectural or engineering advice. They cover the structure and exclude septic expansion, utility service runs, site work beyond a flat and clear pad, and permit fees. ADU eligibility, size, owner-occupancy and permit conditions in the Town of Boston vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-09-10; verify with the building department at (716) 941-6113 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.