Grand Island · ADU cost
What it costs to build an ADU in Grand Island, NY
An 800 sq ft accessory dwelling unit in Grand Island 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 Grand Island maps 19 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 Grand Island
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 Grand Island 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 |
|---|---|---|---|---|
| Low-Density Single-Family Residential District | 3 ac | 50 / 15 / 50 ft | 35 ft | No |
| Single-Family Residential District | 0.69 ac | 50 / 12 / 50 ft | 35 ft | No |
| Single-Family Residential District | 0.46 ac | 50 / 12 / 50 ft | 35 ft | No |
| Medium-Density Single-Family Residential District | 0.29 ac | 35 / 10 / 40 ft | 35 ft | No |
| High-Density Single-Family Residential District | 0.22 ac | 30 / 8 / 40 ft | 30 ft | No |
| Attached/Detached Single-Family Residential District | 5 ac | 30 / 8 / 30 ft | 35 ft | No |
| Attached/Detached Waterfront Single-Family Residential District | 5 ac | 30 / 8 / 20 ft | 35 ft | No |
| Multiple-Family Residential District | 3 ac | 40 / 30 / 30 ft | 35 ft | No |
| General Business District | 0.46 ac | 65 / 25 / 30 ft | 40 ft | No |
| Waterfront Business District | 1 ac | 65 / 20 / 20 ft | 40 ft | No |
| Hamlet Business District | 0.46 ac | 65 / 25 / 30 ft | 40 ft | No |
| Commercial Recreational Facilities District | 10 ac | 50 / 50 / 50 ft | 35 ft | No |
| Light Industrial and Research District | 1.5 ac | 65 / 25 / 30 ft | 45 ft | No |
| Industrial District | 1.5 ac | 75 / 25 / 30 ft | 45 ft | No |
| Mining/Excavation Overlay District | 5 ac | 100 / 100 / 100 ft | — | — |
| North Business District | — | 15 / - / - ft | 40 ft | No |
| Central Business District | — | 15 / - / - ft | 40 ft | No |
| South Business District | — | 15 / - / - ft | 40 ft | No |
| Open Space District | — | — | — | No |
Low-Density Single-Family Residential District - A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater: 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road.
- On East River Road that setback is 90 ft, and on the other named roads 80 ft (§ 407-18A)
- The three acres cannot be taken in any shape: a lot must also be 140 ft wide at the street and 250 ft deep
- On a shoreline lot the setback from the top of the river bank is the average of the two adjoining structures, or a match to the single one that is built, and never less than the district rear yard
Single-Family Residential District - A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater: 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road.
- On East River Road that setback is 90 ft, and on the other named roads 80 ft (§ 407-18A)
- The size figure here is the buildable area each dwelling unit takes, 30,000 sq ft, rather than a lot area, and for a single house the two come to the same thing
- Shape is controlled separately: at least 120 ft of frontage and 250 ft of depth
- On a shoreline lot the setback from the top of the river bank is the average of the two adjoining structures, or a match to the single one that is built, and never less than the district rear yard
Single-Family Residential District - A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater: 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road.
- On East River Road that setback is 90 ft, and on the other named roads 80 ft (§ 407-18A)
- The size figure here is the buildable area each dwelling unit takes, 20,000 sq ft, rather than a lot area, and for a single house the two come to the same thing
- Shape is controlled separately: at least 100 ft of frontage and 200 ft of depth
- On a shoreline lot the setback from the top of the river bank is the average of the two adjoining structures, or a match to the single one that is built, and never less than the district rear yard
Medium-Density Single-Family Residential District - A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater: 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road.
- On East River Road that setback is 90 ft, and on the other named roads 80 ft (§ 407-18A)
- The size figure here is the buildable area each dwelling unit takes, 12,800 sq ft, rather than a lot area, and for a single house the two come to the same thing
- Shape is controlled separately: at least 80 ft of frontage and 160 ft of depth
- On a shoreline lot the setback from the top of the river bank is the average of the two adjoining structures, or a match to the single one that is built, and never less than the district rear yard
High-Density Single-Family Residential District - A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater: 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road.
- On East River Road that setback is 90 ft, and on the other named roads 80 ft (§ 407-18A)
- The size figure here is the buildable area each dwelling unit takes, 9,450 sq ft, rather than a lot area, and for a single house the two come to the same thing
- Shape is controlled separately: at least 70 ft of frontage and 135 ft of depth
- On a shoreline lot the setback from the top of the river bank is the average of the two adjoining structures, or a match to the single one that is built, and never less than the district rear yard
Attached/Detached Single-Family Residential District - The five acres is the minimum size of the development, not of a lot inside it. Yield comes from the 8,400 sq ft of buildable area each dwelling unit consumes.
- A development reached by private roads is priced lower, at 6,600 sq ft per unit on 60 ft of frontage rather than 8,400 on 70 ft
- No more than four dwelling units may sit in one contiguous structure, no structure may run more than 160 ft along its facade, and structures must stand 60 ft apart
- A common parking area must be 12 ft from any lot line, and any part of the development facing a residentially zoned lot must stand 30 ft back
- Minimum dwelling size runs 650 sq ft for one bedroom, 860 for two and 1,000 for three, with a further 120 sq ft for every bedroom beyond that
- A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater
- That road setback is 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road, 90 ft on East River Road and 80 ft on the rest
Attached/Detached Waterfront Single-Family Residential District - The five acres is the minimum size of the development, not of a lot inside it. Yield comes from the 8,400 sq ft of buildable area each dwelling unit consumes.
- The 20 ft rear yard is the yard that faces the water on a river lot
- A development reached by private roads is priced at 6,600 sq ft per unit on 60 ft of frontage instead
- No more than four dwelling units may sit in one contiguous structure, no structure may run more than 160 ft along its facade, and structures must stand 60 ft apart
- Minimum dwelling size runs 650 sq ft for one bedroom, 860 for two and 1,000 for three, with a further 120 sq ft for every bedroom beyond that
- On a shoreline lot the setback from the top of the river bank is the average of the two adjoining structures, or a match to the single one that is built, and never less than the district rear yard
- A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater
Multiple-Family Residential District - The three acres is the minimum size of the development, not of a lot inside it. Yield comes from the 3,500 sq ft of buildable area each dwelling unit consumes.
- The parcel must also be at least 200 ft wide and 200 ft deep
- No building may run more than 160 ft along its facade, main buildings must stand 60 ft apart and other buildings 20 ft
- Common parking must be 25 ft back at the front and 10 ft at the rear, excluding enclosed garages and carports
- Minimum dwelling size runs 650 sq ft for one bedroom, 860 for two and 1,000 for three, with a further 120 sq ft for every bedroom beyond that
- A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater
- That road setback is 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road, 90 ft on East River Road and 80 ft on the rest
General Business District, Hamlet Business District - A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater: 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road.
- On East River Road that setback is 90 ft, and on the other named roads 80 ft (§ 407-18A)
- The 20,000 sq ft figure is the land a dwelling consumes here, not a floor on the size of a commercial lot
- A lot must be 100 ft of frontage, 100 ft wide and 200 ft deep
- Anything within 50 ft of a residentially zoned lot must stand back that far
Waterfront Business District - A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater: 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road.
- On East River Road that setback is 90 ft, and on the other named roads 80 ft (§ 407-18A)
- The 20 ft yards also apply against a residentially zoned lot
- A lot must be 100 ft of frontage, 100 ft wide and 200 ft deep
- On a shoreline lot the setback from the top of the river bank is the average of the two adjoining structures, or a match to the single one that is built, and never less than the district rear yard
Commercial Recreational Facilities District - Frontage and width are different figures here: 100 ft of frontage on the street but 200 ft of width, so a lot may neck down where it meets the road.
- Every yard is 50 ft, including the setback from a residentially zoned lot
- The 35 ft height cap applies to buildings; an amusement park ride or attraction may reach 100 ft
- A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater
- That road setback is 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road, 90 ft on East River Road and 80 ft on the rest
Light Industrial and Research District, Industrial District - A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater: 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road.
- On East River Road that setback is 90 ft, and on the other named roads 80 ft (§ 407-18A)
- A lot must be 150 ft of frontage, 150 ft wide and 300 ft deep
- Anything within 50 ft of a residentially zoned lot must stand back that far
Mining/Excavation Overlay District - This is an overlay, applied on top of whatever district the land already carries, so its 100 ft yards sit alongside the underlying district's standards rather than replacing them. Those yards are measured to the top of the excavated area, not to a building.
- Fifty feet of frontage is the only shape requirement
- A setback measured from the centre of the right-of-way governs on eighteen named roads and overrides the district front yard wherever it is greater
- That road setback is 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road, 90 ft on East River Road and 80 ft on the rest
North Business District, South Business District - Residential density is the standard that binds here, not lot size: attached houses, duplexes and multifamily developments are permitted by right at eight units per acre or fewer, and a special permit lifts attached and multifamily to between nine and twelve.
- Lot area, width, depth, side and rear yards and coverage are settled by the design and performance standards at § 407-13 and at site plan review
- The Town Board may approve a plan with setbacks smaller than the town's named-road setback would otherwise require (§ 407-18B)
Central Business District - This district sets a minimum residential density as well as a maximum: attached houses and multifamily developments are permitted by right at no fewer than nine and no more than sixteen units per acre, so a thin scheme is as non-conforming as an overbuilt one.
- Above sixteen units per acre a special permit is required, and no upper figure is named
- Lot area, width, depth, side and rear yards and coverage are settled by the design and performance standards at § 407-13 and at site plan review
- The Town Board may approve a plan with setbacks smaller than the town's named-road setback would otherwise require (§ 407-18B)
Open Space District - Setbacks here are determined at site plan review. A setback measured from the centre of the right-of-way still governs on eighteen named roads: 115 ft on Grand Island Boulevard, 100 ft on Alt Boulevard, Baseline, Whitehaven and most of Stony Point Road.
- On East River Road that setback is 90 ft, and on the other named roads 80 ft (§ 407-18A)
Size limits on record in Grand Island
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 Grand Island:
- Low-Density Single-Family Residential DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Single-Family Residential DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Single-Family Residential DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Medium-Density Single-Family Residential DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- High-Density Single-Family Residential DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Attached/Detached Single-Family Residential DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Attached/Detached Waterfront Single-Family Residential DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Multiple-Family Residential DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- General Business DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Waterfront Business DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Hamlet Business DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Commercial Recreational Facilities DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Light Industrial and Research DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Industrial DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- North Business DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Central Business DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- South Business DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
- Open Space DistrictAccessory building floor area is capped by district, from 1,700 sq ft in R-1A to 800 sq ft in R-1E
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 prohibited. Short-term rentals are not permitted in any of the residential districts. The one exception is a bed-and-breakfast specifically authorized elsewhere in the chapter, which is regulated separately. Separate question from whether you can build the unit.
- Zoning codeecode360.com/11851931
- Building dept(716) 773-9600
Sizing up a specific parcel in Grand Island?
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Building a full house instead?
A standard-finish house in Grand Island runs $204–268/sq ft, about $370k–480k at 1,800 sq ft , plus roughly $1,360 for a conforming lot. See what it costs to build a house in Grand Island.
ADU cost in nearby towns
All Erie County towns: Buffalo (City) · Lackawanna (City) · Tonawanda (City) · Alden · Amherst · Aurora · Boston · Brant · Cheektowaga · Clarence · Colden · Collins · Concord · Eden · Elma · Evans · 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 Grand Island: Can you build an ADU here? · Zoning for new construction · Erie County build cost
ADU cost in Grand Island: common questions
How much does it cost to build an ADU in Grand Island, NY?
An 800 sq ft accessory dwelling unit in Town of Grand Island 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 Grand Island?
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 Grand Island?
Town of Grand Island has 19 districts. 18 of 19 districts prohibit ADUs. 1 district has incomplete data - verify with the building department at (716) 773-9600. Check the specific address with the free ADU checker before you spend money on design.
Can I rent an ADU in Grand Island short term?
Short-term rentals prohibited for Town of Grand Island. 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 Grand Island 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) 773-9600 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.