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 12 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 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 District | 2 ac | 50 / 40 / 50 ft | 35 ft | Special permit |
| One-Family Residence District | 4 ac | 50 / 50 / 50 ft | 35 ft | Special permit |
| One-Family Residence District | 0.5 ac | 30 / 15 / 30 ft | 35 ft | Special permit |
| One-Family Residence District | 1 ac | 40 / 30 / 40 ft | 35 ft | Special permit |
| One-Family Residence District | 0.25 ac | 25 / 12 / 25 ft | 35 ft | Special permit |
| Two-Family Residence District | 0.23 ac | 30 / 10 / 20 ft | 35 ft | Special permit |
| Multifamily Residence District | 15 ac | 50 / 40 / 50 ft | 35 ft | Occupancy restricted |
| Two-Family Residence District | 0.17 ac | 25 / 8 / 20 ft | 35 ft | Occupancy restricted |
| Campus Commercial District | 20 ac | 150 / 100 / 100 ft | 60 ft | Occupancy restricted |
| General Business District | 0.5 ac | 20 / 15 / 15 ft | 30 ft | Occupancy restricted |
| Retail Business District | 0.5 ac | 20 / 15 / 15 ft | 30 ft | Occupancy restricted |
| Service Utility District | 10 ac | 150 / 100 / 100 ft | 30 ft | Occupancy restricted |
One-Family Residence District - Each new lot needs 40,000 sq ft of contiguous land likely to be buildable, at least 50 ft wide and clear of wetlands, watercourses, floodplain and slopes of 15% or more, and the house and any septic system must sit on it.
- The front yard is also at least 75 ft from the street center line.
- Each new lot must be able to contain a circle 200 ft in diameter (§ 220-25A).
- Where a new lot is deeper than 450 ft, its required lot area rises by the ratio of its actual depth to 450 ft (§ 220-25B).
- In an approved cluster development, lots may be reduced to the R-1A standards (further only with Town Board authorization), with no more lots than a conventional subdivision would yield.
One-Family Residence District - Each new lot needs 50,000 sq ft of contiguous land likely to be buildable, at least 50 ft wide and clear of wetlands, watercourses, floodplain and slopes of 15% or more, and the house and any septic system must sit on it.
- The front yard is also at least 75 ft from the street center line.
- Each new lot must be able to contain a circle 250 ft in diameter (§ 220-25A).
- Where a new lot is deeper than 700 ft, its required lot area rises by the ratio of its actual depth to 700 ft (§ 220-25B).
- In an approved cluster development, lots may be reduced to the R-1A standards (further only with Town Board authorization), with no more lots than a conventional subdivision would yield.
One-Family Residence District - Each new lot needs 20,000 sq ft of contiguous land likely to be buildable, at least 50 ft wide and clear of wetlands, watercourses, floodplain and slopes of 15% or more, and the house and any septic system must sit on it.
- The front yard is also at least 55 ft from the street center line.
- An accessory building no more than 12 ft high needs only 5 ft from any lot line in the rear yard.
- Each new lot must be able to contain a circle 100 ft in diameter (§ 220-25A).
- Where a new lot is deeper than 225 ft, its required lot area rises by the ratio of its actual depth to 225 ft (§ 220-25B).
- A lot in a cluster development must still meet at least the R-1A standards (§ 220-88B).
One-Family Residence District - Each new lot needs 35,000 sq ft of contiguous land likely to be buildable, at least 50 ft wide and clear of wetlands, watercourses, floodplain and slopes of 15% or more, and the house and any septic system must sit on it.
- The front yard is also at least 65 ft from the street center line.
- Each new lot must be able to contain a circle 150 ft in diameter (§ 220-25A).
- Where a new lot is deeper than 300 ft, its required lot area rises by the ratio of its actual depth to 300 ft (§ 220-25B).
- The R-1A standards are also the smallest a lot in a cluster development in any district may have, unless the Town Board authorizes a further reduction (§ 220-88B).
One-Family Residence District - Each new lot needs 10,000 sq ft of contiguous land likely to be buildable, at least 50 ft wide and clear of wetlands, watercourses, floodplain and slopes of 15% or more, and the house and any septic system must sit on it.
- The front yard is also at least 50 ft from the street center line.
- An accessory building no more than 12 ft high needs only 5 ft from any lot line in the rear yard.
- Each new lot must be able to contain a circle 75 ft in diameter (§ 220-25A).
- Where a new lot is deeper than 150 ft, its required lot area rises by the ratio of its actual depth to 150 ft (§ 220-25B).
- A lot in a cluster development must still meet at least the R-1A standards (§ 220-88B).
Two-Family Residence District - The figures shown apply to one-family and two-family dwellings alike. Each new lot needs 9,300 sq ft of contiguous land likely to be buildable, at least 50 ft wide and clear of wetlands, watercourses, floodplain and slopes of 15% or more.
- The house and any septic system must sit on that buildable land (§ 220-10E(2)).
- By special permit, an existing building on a lot of at least 0.60 acre may be converted to a multifamily dwelling of no more than four units, with ACARC review (§ 220-43.8).
- That conversion allows at most one density unit per 0.15 acre of net lot area, and needs two parking spaces per unit plus one for each unit with two or more bedrooms.
- The front yard is also at least 55 ft from the street center line.
- An accessory building no more than 12 ft high needs only 5 ft from any lot line in the rear yard.
- Each new lot must be able to contain a circle 75 ft in diameter (§ 220-25A).
Multifamily Residence District - On a lot within and served by a Town of Lewisboro public water and sewer district, the minimum lot area is 15,000 sq ft. Multifamily dwellings need site plan approval and may not exceed four density units per acre of net lot area.
- The Planning Board may raise that density by up to 50% where at least one third of the added units are affordable units (§ 220-26B(2)).
- Within a density transition area along a single-family district boundary, as deep as that district's minimum lot width, density may not exceed one density unit per acre.
- At least 30% of the gross lot area stays permanent open space, free of buildings and parking, plus at least 300 sq ft of common recreation area per density unit (§ 220-26D).
- Along a line shared with an adjoining single-family residence district, each yard must be at least twice the setback that district requires from that line, and never less than the figures shown.
- A multifamily or nonresidential property keeps a natural or planted buffer 30 ft wide along lines that abut or face across a street land in a residence district (§ 220-15C).
- The front yard is also at least 75 ft from the street center line.
Two-Family Residence District - The figures shown apply to one-family and two-family dwellings alike. Each new lot needs 7,125 sq ft of contiguous land likely to be buildable, at least 50 ft wide and clear of wetlands, watercourses, floodplain and slopes of 15% or more.
- The house and any septic system must sit on that buildable land (§ 220-10E(2)).
- By special permit, an existing building on a lot of at least 0.60 acre may be converted to a multifamily dwelling of no more than four units, with ACARC review (§ 220-43.8).
- That conversion allows at most one density unit per 0.15 acre of net lot area, and needs two parking spaces per unit plus one for each unit with two or more bedrooms.
- The front yard is also at least 50 ft from the street center line.
- An accessory building no more than 12 ft high needs only 5 ft from any lot line in the rear yard.
- Each new lot must be able to contain a circle 60 ft in diameter (§ 220-25A).
Campus Commercial District - Each lot needs frontage on a state highway, with no access drive to local streets. No site improvement may be closer than 200 ft to a residence district boundary, and a 200 ft buffer inside the lot along every line abutting one stays natural.
- The 60 ft height applies to a principal building the Planning Board finds reduces site and visual impacts, on a site of at least twice the minimum lot area (§ 220-24B(3)(d)).
- That building also needs at least 90% of required parking inside it below adjacent grade, and a roof no higher than elevation 370 on the USGS datum.
- Site coverage counts all buildings, structures, parking and loading areas, drives, patios, terraces, walks and other improved surfaces.
- The floor area ratio is figured on net lot area, deducting 0.75 of the area of wetlands, watercourses and steep slopes.
- The front yard is also at least 175 ft from the street center line.
- Multifamily dwellings need site plan approval and may not exceed four density units per acre of net lot area (§ 220-26B(1)).
General Business District, Retail Business District - No single structure may cover more than 10,000 sq ft. Where a lot line abuts land in a residence district, that residence district's setbacks govern.
- Site coverage counts all buildings, structures, parking and loading areas, drives, patios, terraces, walks and other improved surfaces.
- A multifamily or nonresidential property keeps a natural or planted buffer 30 ft wide along lines that abut or face across a street land in a residence district (§ 220-15C).
- That buffer is 15 ft wide along lines abutting a nonresidence district and 20 ft wide across a street from one.
- Multifamily dwellings need site plan approval and may not exceed four density units per acre of net lot area (§ 220-26B(1)).
- A multifamily site needs at least two acres of gross lot area. Within a Town public water and sewer district that drops to 15,000 sq ft, but the stricter 1/2 acre district minimum governs.
- The front yard is also at least 45 ft from the street center line.
Service Utility District - No site improvement may be closer than 100 ft to a residence district, and a perimeter buffer 100 ft wide, kept in its natural state, also applies (§ 220-24C(4)).
- The front yard is also at least 175 ft from the street center line.
- Site coverage counts all buildings, structures, parking and loading areas, drives, patios, terraces, walks and other improved surfaces.
- A multifamily or nonresidential property keeps a natural or planted buffer 30 ft wide along lines that abut or face across a street land in a residence district (§ 220-15C).
- Multifamily dwellings need site plan approval and may not exceed four density units per acre of net lot area (§ 220-26B(1)).
- The Planning Board may raise that density by up to 50% where at least one third of the added units are affordable units (§ 220-26B(2)).
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 Residence DistrictThe apartment must be at least 300 sq ft and at most the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building), with at most two bedrooms.
- One-Family Residence DistrictThe apartment must be at least 300 sq ft and at most the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building), with at most two bedrooms.
- One-Family Residence DistrictThe apartment must be at least 300 sq ft and at most the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building), with at most two bedrooms.
- One-Family Residence DistrictThe apartment must be at least 300 sq ft and at most the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building), with at most two bedrooms.
- One-Family Residence DistrictThe apartment must be at least 300 sq ft and at most the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building), with at most two bedrooms.
- Two-Family Residence DistrictA detached second house of 600 to 1,500 sq ft, an accessory residence dwelling, needs a Planning Board special permit (§ 220-40.1) and a lot of at least twice the district's minimum lot area, with no occupancy condition.
- Multifamily Residence DistrictThe apartment must be at least 300 sq ft and at most the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building), with at most two bedrooms.
- Two-Family Residence DistrictThe apartment must be at least 300 sq ft and at most the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building), with at most two bedrooms.
- Campus Commercial DistrictThe apartment must be at least 300 sq ft and at most the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building), with at most two bedrooms.
- General Business DistrictThe apartment must be at least 300 sq ft and at most the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building), with at most two bedrooms.
- Retail Business DistrictThe apartment must be at least 300 sq ft and at most the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building), with at most two bedrooms.
- Service Utility DistrictThe apartment must be at least 300 sq ft and at most the lesser of 25% of the building or 800 sq ft (800 sq ft in an accessory building), with at most 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. The short-term rental rule in Lewisboro is not confirmed. Confirm with the building department before counting on the use. Separate question from whether you can build the unit.
- Zoning codeecode360.com/LE1300
- 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: 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 · Harrison (Village) · 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 12 districts. 6 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 2026-09-21; verify with the building department at (914) 763-3060 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.