Lewisboro · ADU cost
What it costs to build an ADU in Lewisboro, NY
An 800 sq ft accessory dwelling unit in Lewisboro 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 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 Lewisboro maps 13 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 Lewisboro
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 Lewisboro 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 Residence | 2 ac | — | 35 ft | Special permit |
| One-Family Residence | 4 ac | — | 35 ft | Special permit |
| One-Family Residence | 0.5 ac | — | 35 ft | Special permit |
| One-Family Residence | 1 ac | — | 35 ft | Special permit |
| One-Family Residence | 0.25 ac | — | 35 ft | Special permit |
| Two-Family Residence | 0.2296 ac | — | 35 ft | Special permit |
| Multi-Family Residence | 0.3444 ac | — | 35 ft | Occupancy restricted |
| Two-Family Residence | 0.1722 ac | — | 35 ft | Occupancy restricted |
| Campus Commercial | 20 ac | — | 60 ft | Occupancy restricted |
| General Business | 0.5 ac | — | 30 ft | Occupancy restricted |
| General Business District | 0.5 ac | — | 30 ft | Occupancy restricted |
| Retail Business | 0.5 ac | — | 30 ft | Occupancy restricted |
| Service Utility | 10 ac | — | 30 ft | Occupancy restricted |
Size limits on record in Lewisboro
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 Lewisboro:
- One-Family ResidenceACCESSORY RESIDENCE DWELLING is SP in R-2A: a detached 600–1,500 sq ft second house by Planning Board special permit (§ 220-40.1), no occupancy conditions, but requiring twice the district's minimum lot area and subdivision-capable siting. ACCESSORY APARTMENT is marked A (permitted accessory use) under § 220-40, conditioned on owner-occupancy of one unit as a full-time legal residence and a four-person occupancy cap; 300 sq ft minimum, the lesser of 25% of the building or 800 sq ft, max two bedrooms. Mutually exclusive (§ 220-40.1G).
- One-Family ResidenceACCESSORY RESIDENCE DWELLING is SP in R-4A: a detached 600–1,500 sq ft second house by Planning Board special permit (§ 220-40.1), with no owner-occupancy or kinship condition, but requiring twice the district's minimum lot area — eight acres here — and siting that would permit future subdivision. ACCESSORY APARTMENT is marked A (permitted accessory use) under § 220-40, conditioned on owner-occupancy of one unit as a full-time legal residence and a four-person cap; 300 sq ft minimum, the lesser of 25% of the building or 800 sq ft, max two bedrooms. Mutually exclusive (§ 220-40.1G).
- One-Family ResidenceACCESSORY RESIDENCE DWELLING is SP in R-1/2A: a detached 600–1,500 sq ft second house by Planning Board special permit (§ 220-40.1), with no owner-occupancy or kinship condition, but requiring twice the district's minimum lot area and subdivision-capable siting. ACCESSORY APARTMENT is separately marked A (permitted accessory use) under § 220-40, conditioned on the owner occupying one unit as their full-time legal residence and a four-person occupancy cap; 300 sq ft minimum, the lesser of 25% of the building or 800 sq ft, max two bedrooms, principal dwelling or accessory building. Mutually exclusive (§ 220-40.1G).
- One-Family ResidenceACCESSORY RESIDENCE DWELLING is SP in R-1A: a detached 600–1,500 sq ft second house by Planning Board special permit (§ 220-40.1), no owner-occupancy or kinship condition, but twice the district's minimum lot area and siting that permits future subdivision. ACCESSORY APARTMENT is marked A (permitted accessory use) under § 220-40, with owner-occupancy of one unit as a full-time legal residence and a four-person cap; 300 sq ft minimum, the lesser of 25% of the building or 800 sq ft, max two bedrooms. Mutually exclusive (§ 220-40.1G).
- One-Family ResidenceTwo routes. ACCESSORY RESIDENCE DWELLING — marked SP in R-1/4A in the Table of Use Regulations: a detached second house of 600–1,500 sq ft by Planning Board special permit under § 220-40.1, with NO owner-occupancy and NO kinship requirement, but requiring a lot of twice the district's individual-lot minimum and siting that would permit future subdivision. This is the least-conditioned route, and it is what the recorded value reflects. ACCESSORY APARTMENT — marked A (permitted accessory use, no special permit) under § 220-40, but the owner must occupy one of the units as their legal full-time residence and occupancy is capped at four persons; 300 sq ft minimum and the lesser of 25% of the building or 800 sq ft; max two bedrooms; allowed in the principal dwelling or a permitted accessory building. The two are mutually exclusive (§ 220-40.1G).
- Two-Family ResidenceACCESSORY RESIDENCE DWELLING is SP in R-2F-10 — the only two-family district where it is available — giving a detached 600–1,500 sq ft second house by Planning Board special permit (§ 220-40.1) with no occupancy condition, subject to twice the district's minimum lot area and subdivision-capable siting. ACCESSORY APARTMENT is marked A (permitted accessory use) under § 220-40 but requires owner-occupancy of one unit as a full-time legal residence and caps occupancy at four persons. Note that a two-family dwelling is separately available here as a principal use — that is a second principal unit, not an accessory one. The two accessory routes are mutually exclusive (§ 220-40.1G).
- Multi-Family ResidenceThe Table of Use Regulations marks “Accessory residence dwelling” X in R-MF, so only the ACCESSORY APARTMENT is available — marked A, a permitted accessory use under § 220-40 with no special permit, but conditioned on the owner occupying one of the units as their legal full-time residence and a four-person occupancy cap (§ 220-40A(1), (2)). One per one-family lot and not available where more than one dwelling unit already exists (§ 220-40B(3)), which limits it in a multifamily district to lots carrying a single-family house. 300 sq ft minimum and the lesser of 25% of the building or 800 sq ft; max two bedrooms.
- Two-Family ResidenceThe Table of Use Regulations marks “Accessory residence dwelling” X — not permitted — in R-2F-7.5, so the detached-second-house route is unavailable here. What remains is the ACCESSORY APARTMENT, marked A (a permitted accessory use, no special permit) under § 220-40 — but § 220-40A(1) requires the owner to occupy one of the units as their legal full-time residence and § 220-40A(2) caps occupancy at four persons, so it is not as-of-right. May be in the principal dwelling or a permitted accessory building; 300 sq ft minimum and the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building); max two bedrooms; one per one-family lot, and not available where more than one dwelling unit already exists. A two-family dwelling is separately available as a principal use.
- Campus CommercialCC-20 is the Campus Commercial district. The Table of Use Regulations marks “Accessory apartments on single-family lots” A — a permitted accessory use — here as in every district, but “Accessory residence dwelling” X. So the accessory apartment under § 220-40 is the only route: it attaches to a single-family lot, requires the owner to occupy one of the units as their legal full-time residence, and caps occupancy at four persons (§ 220-40A(1), (2)). 300 sq ft minimum and the lesser of 25% of the building or 800 sq ft; max two bedrooms; not available where more than one dwelling unit already exists.
- General BusinessGB is the General Business district. The Table of Use Regulations marks “Accessory apartments on single-family lots” A here, and “Accessory residence dwelling” X. The accessory apartment under § 220-40 attaches only to a single-family lot and requires the owner to occupy one of the units as their legal full-time residence, with occupancy capped at four persons. 300 sq ft minimum and the lesser of 25% of the building or 800 sq ft; max two bedrooms; one per lot; not available where more than one dwelling unit already exists.
- General Business DistrictGB is the General Business district. The Table of Use Regulations marks “Accessory apartments on single-family lots” A here, and “Accessory residence dwelling” X. The accessory apartment under § 220-40 attaches only to a single-family lot and requires the owner to occupy one of the units as their legal full-time residence, with occupancy capped at four persons. 300 sq ft minimum and the lesser of 25% of the building or 800 sq ft; max two bedrooms; one per lot; not available where more than one dwelling unit already exists.
- Retail BusinessRB is the Retail Business district. The Table of Use Regulations marks “Accessory apartments on single-family lots” A here, and “Accessory residence dwelling” X. The accessory apartment under § 220-40 attaches only to a single-family lot and requires owner-occupancy of one of the units as a legal full-time residence, with occupancy capped at four persons (§ 220-40A(1), (2)). 300 sq ft minimum and the lesser of 25% of the building or 800 sq ft; max two bedrooms; one per lot.
- Service UtilitySU is the Service Utility district. The Table of Use Regulations marks “Accessory apartments on single-family lots” A here — the table applies the accessory apartment across all twelve districts — while “Accessory residence dwelling” is X. The accessory apartment under § 220-40 attaches only to a single-family lot and requires the owner to occupy one of the units as their legal full-time residence, with occupancy capped at four persons. 300 sq ft minimum and the lesser of 25% of the building or 800 sq ft; max two bedrooms.
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 rentalsShort-term-rental status not confirmed. Draft STR law (proposed Chapter 220) referred to Planning Board Feb 2026; public hearings held March 23 and April 27, 2026. Additional hearing may be required if changes made. No adoption confirmed as of Jun 2026. Separate question from whether you can build the unit.
- Building dept(914) 763-3060
Sizing up a specific parcel in Lewisboro?
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Building a full house instead?
A standard-finish house in Lewisboro runs $242–318/sq ft, about $435k–570k at 1,800 sq ft , plus roughly $585 for a conforming lot. See what it costs to build a house in Lewisboro.
ADU cost in nearby towns
All Westchester County towns: Harrison (City) · Mount Vernon (City) · New Rochelle (City) · Peekskill (City) · Rye (City) · White Plains (City) · Yonkers (City) · Bedford · Cortlandt · Eastchester · Greenburgh · Mamaroneck · Mount Pleasant · New Castle · North Castle · North Salem · Ossining · Pound Ridge · Somers · Yorktown · Ardsley (Village) · Briarcliff Manor (Village) · Bronxville (Village) · Buchanan (Village) · Croton-on-Hudson (Village) · Dobbs Ferry (Village) · Elmsford (Village) · Hastings-on-Hudson (Village) · Irvington (Village) · Larchmont (Village) · Mamaroneck (Village) · Mount Kisco (Village) · Ossining (Village) · Pelham (Village) · Pelham Manor (Village) · Pleasantville (Village) · Port Chester (Village) · Rye Brook (Village) · Scarsdale (Village) · Sleepy Hollow (Village) · Tarrytown (Village) · Tuckahoe (Village)
More on Lewisboro: Can you build an ADU here? · Zoning for new construction · Westchester County build cost
ADU cost in Lewisboro: common questions
How much does it cost to build an ADU in Lewisboro, NY?
An 800 sq ft accessory dwelling unit in Town of Lewisboro 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 Lewisboro?
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 Lewisboro?
Town of Lewisboro has 13 districts. 7 districts allow ADUs with occupancy restrictions, 6 require a special-use permit. Check the specific address with the free ADU checker or confirm with the building department at (914) 763-3060 before you spend money on design.
Can I rent an ADU in Lewisboro short term?
Short-term-rental status not confirmed for Town of Lewisboro. 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 Town of Lewisboro vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-06-15; verify with the building department at (914) 763-3060 before relying on any answer. Sources: Offsite / Aldo cost engine (costEngine.js + constants.js); zoning dataset (municipalData.js).