Amherst · ADU cost
What it costs to build an ADU in Amherst, NY
An 800 sq ft accessory dwelling unit in Amherst 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 Amherst maps 25 districts that treat accessory units differently from one another, so read the table below as a starting point rather than a verdict.
ADU cost by size in Amherst
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 Amherst 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 |
|---|---|---|---|---|
| Rural Residential District | 3 ac | 50 / - / - ft | 35 ft | Occupancy restricted |
| Suburban Agricultural District | 1 ac | 50 / - / - ft | 35 ft | Occupancy restricted |
| Residential District One | 0.5 ac | 30 / 8 / 35 ft | 35 ft | Occupancy restricted |
| Residential District Two | 0.264 ac | 30 / 8 / 35 ft | 35 ft | Occupancy restricted |
| Residential District Three | 0.194 ac | 30 / 5 / 30 ft | 35 ft | Occupancy restricted |
| Cluster Residential District Three-A | 0.115 ac | 25 / - / 30 ft | 35 ft | Occupancy restricted |
| Traditional Single-Family Residential District | 0.103 ac | 25 / 5 / 25 ft | 35 ft | No |
| Residential District Four | 0.134 ac | 30 / 8 / 30 ft | 35 ft | Occupancy restricted |
| Multifamily Residential District Four-A | 0.138 ac | 25 / 5 / 20 ft | 35 ft | Occupancy restricted |
| Multifamily Residential District Five | 0.083 ac | — | 35 ft | Occupancy restricted |
| Multifamily Residential District Six | 0.138 ac | 25 / - / - ft | 35 ft | Occupancy restricted |
| Multifamily Residential District Seven | 0.138 ac | — | — | No |
| Manufactured Home Residential District Eight | 0.092 ac | 30 / 15 / 15 ft | — | No |
| Office Building District | — | 20 / - / - ft | — | No |
| Neighborhood Business District | 1 ac | - / 10 / 15 ft | 30 ft | No |
| General Business District | — | 20 / - / - ft | 65 ft | No |
| Commercial Service District | 1 ac | 20 / 10 / 10 ft | 40 ft | No |
| Motor Service District | — | 20 / 10 / 25 ft | 65 ft | No |
| Shopping Center District | 10 ac | 25 / 50 / 50 ft | 65 ft | No |
| Research and Development District | 1 ac | 20 / - / - ft | 65 ft | No |
| Science Technology District | 2 ac | 20 / - / - ft | 45 ft | No |
| General Industrial District | 1 ac | 20 / - / - ft | 65 ft | No |
| AG Agricultural District | 5 ac | 50 / 30 / 30 ft | 35 ft | No |
| TND Traditional Neighborhood Development District | — | - / 15 / 15 ft | 50 ft | No |
| LW-1 Live-Work District 1 | — | 25 / 5 / 25 ft | 35 ft | No |
Size limits on record in Amherst
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 Amherst:
- Rural Residential DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Suburban Agricultural DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Residential District OneMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Residential District TwoMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Residential District ThreeMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Cluster Residential District Three-AMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Traditional Single-Family Residential DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Residential District FourMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Multifamily Residential District Four-AMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Multifamily Residential District FiveMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Multifamily Residential District SixMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Multifamily Residential District SevenMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Manufactured Home Residential District EightMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Office Building DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Neighborhood Business DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- General Business DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Commercial Service DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Motor Service DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Shopping Center DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Research and Development DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- Science Technology DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- General Industrial DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- AG Agricultural DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- TND Traditional Neighborhood Development DistrictMinimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
- LW-1 Live-Work District 1Minimum 500 sq ft; the unit may not exceed 49% of the principal dwelling's gross floor area up to 900 sq ft, whichever is less restrictive
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 rentalsShort-term rentals allowed with registration. A short-term rental requires Town registration. § 44-3 of Chapter 44, Rental Unit Registration, makes it unlawful to let a dwelling unit unless the owner has first obtained EITHER a short-term rental dwelling unit registration certificate OR an ordinary rental dwelling unit registration — so the short-term case has its own certificate. § 44-2 defines SHORT TERM RENTAL, and § 44-7 sets the registration term. The requirement is outside the zoning chapter. New York State's 2025 short-term rental registry (Ch. 99 of the Laws of 2025) separately requires every host statewide to register the unit regardless of local law. Confirm with Town of Amherst at (716) 631-7080 before listing. Separate question from whether you can build the unit.
- Building dept(716) 631-7080
Sizing up a specific parcel in Amherst?
Aldo pulls the exact zoning district, setbacks, and build feasibility for any parcel — before you buy. Enter your email to open Aldo and run your free report.
Building a full house instead?
A standard-finish house in Amherst runs $204–268/sq ft, about $370k–480k at 1,800 sq ft , plus roughly $3,901 for a conforming lot. See what it costs to build a house in Amherst.
ADU cost in nearby towns
All Erie County towns: Buffalo (City) · Lackawanna (City) · Tonawanda (City) · Alden · Aurora · Boston · Brant · Cheektowaga · Clarence · Colden · Collins · Concord · Eden · Elma · Evans · Grand Island · Hamburg · Holland · Lancaster · Marilla · Newstead · North Collins · Orchard Park · Sardinia · Tonawanda · Wales · West Seneca · Akron (Village) · Alden (Village) · Angola (Village) · Blasdell (Village) · Depew (Village) · East Aurora (Village) · Farnham (Village) · Gowanda (Village) · Hamburg (Village) · Kenmore (Village) · Lancaster (Village) · North Collins (Village) · Orchard Park (Village) · Sloan (Village) · Springville (Village) · Williamsville (Village)
More on Amherst: Can you build an ADU here? · Zoning for new construction · Erie County build cost
ADU cost in Amherst: common questions
How much does it cost to build an ADU in Amherst, NY?
An 800 sq ft accessory dwelling unit in Town of Amherst 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 Amherst?
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 Amherst?
Town of Amherst has 25 districts. 15 districts prohibit ADUs, 10 allow them with occupancy restrictions. Check the specific address with the free ADU checker or confirm with the building department at (716) 631-7080 before you spend money on design.
Can I rent an ADU in Amherst short term?
Short-term rentals allowed with registration for Town of Amherst. 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 Amherst vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-08-06; verify with the building department at (716) 631-7080 before relying on any answer. Sources: Offsite / Aldo cost engine (costEngine.js + constants.js); zoning dataset (municipalData.js).