Ardsley · ADU cost
What it costs to build an ADU in Ardsley, NY
An 800 sq ft accessory dwelling unit in Ardsley runs roughly $235k–310k, or about $296–388/sq ft, and that per-square-foot figure sits deliberately above the $242–318/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 village 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 Village of Ardsley maps 10 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 Ardsley
Standard finish, slab on grade, on a flat and clear part of the lot, general-contractor margin included. These are structure costs at Westchester County pricing, before the service and septic work a second dwelling can trigger.
| Size | Type | $ / sq ft | Estimated total |
|---|---|---|---|
| 400 sq ft | Studio | $392–$514 | $155k–205k |
| 600 sq ft | One bedroom | $328–$430 | $195k–260k |
| 800 sq ft | One bedroom, larger | $296–$388 | $235k–310k |
| 1,000 sq ft | Two bedroom | $277–$362 | $275k–360k |
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 | $9–$13 | $9 | — |
| Foundation | $19–$27 | $19 | — |
| Framing | $30–$38 | $30 | — |
| Envelope - roof, siding, windows | $38–$49 | $38 | — |
| Plumbing | $33–$40 | $19 | 55% |
| Electrical | $22–$28 | $15 | 35% |
| HVAC | $27–$34 | $17 | 45% |
| Insulation | $9–$11 | $9 | — |
| Interior finishes | $38–$51 | $32 | 15% |
| Millwork & cabinetry | $22–$33 | $13 | 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 Ardsley 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 |
|---|---|---|---|---|
| One-Family Residential District | 0.92 ac | 40 / 30 / 40 ft | 30 ft | Occupancy restricted |
| Conservation or Clustered Housing District | 8 ac | 20 / 15 / 20 ft | 30 ft | No |
| Conservation or Clustered Housing District | 8 ac | 20 / 15 / 20 ft | 30 ft | No |
| One-Family Residential District | 0.23 ac | 30 / 20 / 30 ft | 30 ft | Occupancy restricted |
| One-Family Residential District | 0.17 ac | 20 / 15 / 20 ft | 30 ft | Occupancy restricted |
| Residence Office District | 0.17 ac | 20 / 15 / 20 ft | 30 ft | No |
| Residential District | — | 30 / 30 / 30 ft | 30 ft | No |
| General Business District | 0.11 ac | 10 / 0 / 0 ft | 45 ft | No |
| Special Business District | 0.11 ac | 25 / 10 / 25 ft | 45 ft | No |
| Shopping Center Business District | 4 ac | 50 / 20 / 40 ft | 25 ft | No |
One-Family Residential District - An accessory building must keep the same front, side and rear setbacks shown for the district, and may abut the main building (§ 200-12).
Conservation or Clustered Housing District, Conservation or Clustered Housing District - A development here must be 8 to 12 contiguous acres with 200 ft of frontage on a major collector street, and the 12% building coverage is measured against the whole district rather than the lot.
- A buffer at least 75 ft deep runs inward from every property line, street lines included, and no building, parking area or use may sit in it, though driveways and walkways may cross it (§ 200-36).
- Assisted, independent and special needs living units come only by special permit and site plan approval of the Board of Trustees, on a site of at least 300,000 square feet.
- That special permit site must hold at least 1,800 square feet for every living unit and have 500 ft of frontage on a state highway.
- Buildings and parking on that site must stand 75 ft back from the state highway, 30 ft from any other road frontage and 25 ft from any other lot line.
- The number of dwelling units may not exceed the number of lots the land would yield if it were subdivided to the R-3 district standard of 7,500 square feet.
- At least 30% of the land must stay open space, no more than six may be attached or clustered together, and clustered structures must stand at least 30 ft apart.
One-Family Residential District - An accessory building must be set back at least 30 ft from the front, 20 ft from the side and 20 ft from the rear, and may abut the main building (§ 200-20).
One-Family Residential District - An accessory building must be set back at least 20 ft from the front, 15 ft from the side and 15 ft from the rear, and may abut the main building (§ 200-28).
Residence Office District - On site plan approval an office or studio, or a multifamily building, takes the General Business district standards instead: a lot of at least 5,000 square feet and a front yard of 10 ft, with side and rear yards of zero ft.
- A side or rear yard that is provided must still be at least 6 ft (§ 200-70B).
- The district's own rule keeps the R-3 district standards but takes height to 40 ft and four stories and building coverage to 40% (§ 200-64.1B).
- A multifamily building may put residential use on the ground floor, which the General Business district otherwise forbids (§ 200-64.1E).
- Site plan approval from the Board of Trustees is required for multifamily and nonresidential uses (§ 200-82F).
- A principal one-family building must cover at least 800 square feet of ground on one story, or 600 square feet on two, not counting garage area (§ 200-27).
- Lot area leaves out 75% of any wetland, water body or watercourse, and on a lot created after 2003 half of any steep slope as well (§ 200-2).
Residential District - The district publishes no lot minimum of its own: density is one dwelling unit for each 4,000 square feet of lot area. The 30 ft side and rear yards shown are the ones owed next to a residence district, and drop to 20 ft where the lot line adjoins a business district.
- A one-family house instead takes the R-3 district standards: a lot of at least 7,500 square feet, 60 ft wide, with 20 ft front and rear yards, 15 ft side yards and coverage up to 28%.
- A buffer of 10 ft next to a business district or 20 ft next to a residential district is required on top of the yards, unless the Board of Trustees accepts alternate screening (§ 200-29.1B(9)).
- Buildings must stand at least 15 ft apart and no building may exceed 8,000 square feet of gross floor area (§ 200-29.1B(4) and (5)).
- Site plan approval from the Board of Trustees is required before any use or development of land in the district (§ 200-29.1C).
- The R-4A overlay covers the same land and requires that at least 20% of the units in a new multifamily development be workforce housing and another 20% affordable.
- Lot area leaves out 75% of any wetland, water body or watercourse, and on a lot created after 2003 half of any steep slope as well (§ 200-2).
General Business District - A one-family house is held to the R-3 district standards rather than the figures shown: a lot of at least 7,500 square feet with 60 ft of street frontage, 20 ft front and rear yards, 15 ft side yards, a height limit of 30 ft and two stories, and coverage of up to 28%.
- No side or rear yard is required, but one that is provided must be at least 6 ft (§ 200-70B).
- The 65% coverage counts all principal and accessory buildings (§ 200-69).
- Where the lot abuts a lot in a residential district, the yard along that line must equal the yard that residential district requires, with 10 ft of it set aside for ornamental planting (§ 200-87).
- Where one side of a street between two intersecting streets is split between residential and business zoning, the business front yard must equal the residential front yard (§ 200-88).
- A nonresidential use over 20,000 square feet of floor area needs a special permit from the Board of Trustees (§ 200-65B(12)).
- Every plan, use and condition in the district needs site plan approval of the Board of Trustees after a public hearing (§ 200-82F).
Special Business District - A lot whose side yard borders a residence district must instead meet that residence district's own minimum side yard, if it is larger than 10 ft (§ 200-77).
Shopping Center Business District - A lot must be at least 4 acres, with coverage capped at 20% and height capped at 1 story and 25 ft (§ 200-80.1E).
- A side yard bordering a residence district must instead meet that residence district's own minimum width, if it is larger than 20 ft.
Size limits on record in Ardsley
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 Ardsley:
- One-Family Residential DistrictThe unit must be 300 to 800 sq ft and no more than 30% of the main house, with at most two bedrooms (§ 200-100.23).
- One-Family Residential DistrictThe unit must be 300 to 800 sq ft and no more than 30% of the main house (§ 200-100.23).
- One-Family Residential DistrictThe unit must be 300 to 800 sq ft and no more than 30% of the main house.
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 Con Edison 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 Village of Ardsley has no local short-term rental law, and that is not the same as permission. Confirm with the building department before counting on the use. Separate question from whether you can build the unit.
- Zoning codeecode360.com/AR0005
- Building dept(914) 693-1550
Sizing up a specific parcel in Ardsley?
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Building a full house instead?
A standard-finish house in Ardsley runs $242–318/sq ft, about $435k–570k at 1,800 sq ft . See what it costs to build a house in Ardsley.
ADU cost in nearby towns
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More on Ardsley: Can you build an ADU here? · Zoning for new construction · Westchester County build cost
ADU cost in Ardsley: common questions
How much does it cost to build an ADU in Ardsley, NY?
An 800 sq ft accessory dwelling unit in Village of Ardsley runs roughly $235k–310k at a standard finish, general-contractor margin included. A 400 sq ft studio runs about $155k–205k. 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 Ardsley?
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 $296–388/sq ft against $242–318/sq ft for a full house, even though it is the same trades doing the same work.
Are ADUs allowed in Ardsley?
Village of Ardsley has 10 districts. 7 districts prohibit ADUs, 3 allow them with occupancy restrictions. Check the specific address with the free ADU checker or confirm with the building department at (914) 693-1550 before you spend money on design.
Can I rent an ADU in Ardsley short term?
No local short-term-rental law on record - which is not the same as permission for Village of Ardsley. 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 Westchester 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 Village of Ardsley vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-09-27; verify with the building department at (914) 693-1550 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.