Ossining · ADU cost
What it costs to build an ADU in Ossining, NY
An 800 sq ft accessory dwelling unit in Ossining 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 Ossining maps 17 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 Ossining
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 Ossining 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 | 0.69 ac | 35 / 18 / 36 ft | 35 ft | Occupancy restricted |
| One-Family Residence | 0.92 ac | 40 / 20 / 38 ft | 35 ft | Occupancy restricted |
| One-Family Residence | 0.23 ac | 25 / 12 / 30 ft | 35 ft | Occupancy restricted |
| One-Family Residence | 0.34 ac | 30 / 14 / 32 ft | 35 ft | Occupancy restricted |
| One-Family Residence | 0.46 ac | 30 / 16 / 34 ft | 35 ft | Occupancy restricted |
| One-Family Residence | 0.46 ac | 30 / 16 / 34 ft | 35 ft | Occupancy restricted |
| One-Family Residence | 0.11 ac | 25 / 8 / 26 ft | 35 ft | Occupancy restricted |
| One-Family Residence | 0.17 ac | 25 / 10 / 28 ft | 35 ft | Occupancy restricted |
| One-Family Residence | — | — | — | — |
| Multifamily | 0.46 ac | 25 / 50 / 40 ft | 35 ft | Occupancy restricted |
| Office-Research Manufacturing | 10 ac | 100 / 100 / 100 ft | 35 ft | — |
| General Business | 0.46 ac | 30 / 0 / 0 ft | 35 ft | No |
| General Business-1 | 0.23 ac | 30 / 0 / 0 ft | 35 ft | No |
| Neighborhood Commercial | 0.11 ac | 30 / 0 / 0 ft | 35 ft | No |
| Business Education | 40 ac | 100 / 100 / 100 ft | 35 ft | No |
| Multifamily-Inn | 0.46 ac | 50 / 50 / 40 ft | 35 ft | — |
| Office-Research Education | 10 ac | 100 / 100 / 100 ft | 35 ft | — |
One-Family Residence, One-Family Residence, One-Family Residence, One-Family Residence, One-Family Residence - The maximum floor area of a one-family residence is 5,000 sq ft plus 0.052521 of the lot area above 43,560 sq ft, § 200-21C. Impervious surface is capped separately, on a sliding scale that grows with lot area.
- Lots may also be laid out as a cluster development, which for detached houses follows the R-5 standards and needs Planning Board subdivision and site plan approval.
- On a street with a right-of-way narrower than 50 feet the front yard is measured from the street center line, with 25 feet added.
- A lot held separately in ownership when the chapter took effect may be built on for a one-family residence without meeting the lot area, lot width and side yard minimums.
- A residential density bonus of 20% on 10 acres or more, 30% above 5 acres and 40% on 5 acres or less is granted where half the bonus units are below market rate.
- An accessory building up to 15 feet high may stand in a required side or rear yard if it is set back 10 feet from every lot line and all such buildings cover no more than 30% of that required yard.
- Creating more than 100 sq ft of impervious surface needs a permit from the Building Inspector.
One-Family Residence, One-Family Residence - The maximum floor area of a one-family residence is 5,000 sq ft plus 0.052521 of the lot area above 43,560 sq ft, § 200-21C. Impervious surface is capped separately, on a sliding scale that grows with lot area.
- Converting an existing residential structure to a two-family dwelling needs a special permit from the Board of Appeals, a lot of at least 10,000 sq ft and 750 sq ft of living floor area in each unit.
- Lots may also be laid out as a cluster development, which for detached houses follows the R-5 standards and needs Planning Board subdivision and site plan approval.
- On a street with a right-of-way narrower than 50 feet the front yard is measured from the street center line, with 25 feet added.
- A lot held separately in ownership when the chapter took effect may be built on for a one-family residence without meeting the lot area, lot width and side yard minimums.
- A residential density bonus of 20% on 10 acres or more, 30% above 5 acres and 40% on 5 acres or less is granted where half the bonus units are below market rate.
- An accessory building up to 15 feet high may stand in a required side or rear yard if it is set back 10 feet from every lot line and all such buildings cover no more than 30% of that required yard.
One-Family Residence - The maximum floor area of a one-family residence is 5,000 sq ft plus 0.052521 of the lot area above 43,560 sq ft, § 200-21C. Impervious surface is capped separately, on a sliding scale that grows with lot area.
- Converting an existing residential structure to a two-family dwelling needs a special permit from the Board of Appeals, a lot of at least 10,000 sq ft and 750 sq ft of living floor area in each unit.
- On a street with a right-of-way narrower than 50 feet the front yard is measured from the street center line, with 25 feet added.
- A lot held separately in ownership when the chapter took effect may be built on for a one-family residence without meeting the lot area, lot width and side yard minimums.
- A residential density bonus of 20% on 10 acres or more, 30% above 5 acres and 40% on 5 acres or less is granted where half the bonus units are below market rate.
- An accessory building up to 15 feet high may stand in a required side or rear yard if it is set back 10 feet from every lot line and all such buildings cover no more than 30% of that required yard.
- Creating more than 100 sq ft of impervious surface needs a permit from the Building Inspector.
One-Family Residence - The code sets no lot, width, depth, yard, height or coverage standard under the R-80 name. The Building Department can say which current district a parcel here carries.
Multifamily - The figures shown are for row and attached dwellings, which need at least 4,000 sq ft of lot per unit plus 1,500 sq ft per bedroom. A multiple dwelling needs a front yard of at least 50 ft on a lot at least 100 ft wide and 150 ft deep.
- The 50 ft one-side and 100 ft combined side yards apply only between buildings and side lot lines.
- Livable floor area per unit is 450 sq ft for a studio or efficiency, 675 for one bedroom, 750 for two, 1,000 for three and 1,200 for four.
- At least 1/3 of the net site area must be permanent open space or recreation, with 1,500 sq ft of undeveloped open space per bedroom and a 400 sq ft play area for each dwelling unit.
- No building may exceed 150 feet in length, principal buildings must stand twice the height of the taller apart, and a principal and an accessory building 20 feet apart.
- A group of row dwellings may hold no more than six units.
- A residential density bonus of 20% on 10 acres or more, 30% above 5 acres and 40% on 5 acres or less is granted where half the bonus units are below market rate.
Office-Research Manufacturing - The district list in the code names no Office-Research Manufacturing district, and the figures shown are the ones § 200-24 publishes for the Office-Research Business district. Every use is subject to site plan approval.
- Side and rear yards are 200 feet on a lot along a residence district boundary, including one in another municipality.
- A parcel of 10 acres or more may be subdivided, and each principal building then needs a lot of at least two acres, with side and rear yards of 50 feet.
- Each lot in such a subdivision needs 200 feet of street frontage, which the Planning Board may reduce to 100 feet on a cul-de-sac or a curve.
- Parking and loading must stand at least 200 feet from a residence district boundary and 100 feet from any other lot line.
- In a parking lot of an acre or more at least 5% of the lot must be interior landscaping.
- Buildings must stand at least as far apart as the taller one is high and never less than 30 feet.
General Business - Multiple dwellings and row or attached dwellings are allowed on a lot of at least 20,000 sq ft under the multifamily standards of § 200-22. No side or rear yard is required here, but each is at least 30 feet on a lot along a residence district boundary.
- A multiple dwelling needs a front yard of at least 50 feet, a lot at least 100 feet wide and 150 feet deep, side yards of at least 50 feet and 100 feet combined, with coverage capped at 20%.
- A row or attached dwelling needs a front yard of at least 25 feet, side yards of at least 50 feet and 100 feet combined and a rear yard of at least 40 feet, with coverage capped at 20%.
- For those dwellings the Planning Board may modify the lot width, lot depth, yard, usable open space and building coverage requirements of § 200-22.
- Dwelling unit sizes are 450 sq ft for a studio or efficiency, 675 for one bedroom, 750 for two, 1,000 for three and 1,200 for four, and no dwelling unit may be in a basement or cellar.
- A residential density bonus of 20% on 10 acres or more, 30% above 5 acres, 40% above 2 acres and 100% below 2 acres is granted where half the bonus units are below market rate.
- No side or rear yard is required where that yard abuts an operating railroad right-of-way.
General Business-1 - The Planning Board may increase the required front, side and rear setbacks by up to a factor of two, or reduce or eliminate them, on site plan review. No side or rear yard is required, but each is at least 30 feet on a lot along a residence district boundary.
- That change rests on the Planning Board's findings about topography, slopes, grades, sight distances and highway conditions, and a finding of no adverse impact on abutting properties.
- Every use in the district is subject to site plan approval.
- There is no outdoor storage of any kind and no outdoor parking of construction equipment.
- No side or rear yard is required where that yard abuts an operating railroad right-of-way.
- Where § 200-18.1 conflicts with another section of the chapter, § 200-18.1 controls.
Neighborhood Commercial - Aggregate floor area on any lot is capped at 12,000 sq ft, and on a lot along a residence district boundary the side and rear yards must each be at least 30 ft.
- A one-family dwelling is regulated as in the R-5 district and a two-family dwelling by the R-10 figures.
- Multiple and row or attached dwellings need a lot of at least 15,000 sq ft with 5,000 sq ft of lot area per unit, and no group of row dwellings may hold more than six units.
- A mixed use needs a lot of at least 20,000 sq ft, one dwelling unit per 10,000 sq ft of lot area and no more than 4,800 sq ft of nonresidential floor area per acre.
- Artist live and work spaces are allowed at one per 5,000 sq ft of lot area, each at least 800 sq ft with no more than 30% of its floor area residential.
- A buffer at least 20 feet wide with native landscape screening is required between a nonresidential use and a residentially used or zoned lot.
- Principal buildings must stand twice the height of the taller apart, and a principal and an accessory building 20 feet apart.
Business Education - On a lot along a residence district boundary, in this town or another municipality, the side and rear yards must each be at least 150 ft.
- The subdivision option that reaches the Office-Research Business and Office-Research Education districts does not reach this one, so the 40 acre minimum is not reduced.
- The height may rise to 50 feet where a substantial part of the added height houses ventilators, water tanks, bulkheads and similar rooftop features in harmony with the building.
- Other rooftop features may rise above the limit if they cover no more than 20% of the roof and stand no more than 15 feet above it.
- Parking and loading must stand at least 100 feet from a residence district boundary and 30 feet from any other lot line.
- In a parking lot of an acre or more at least 5% of the lot must be interior landscaping.
- The amount of off-street parking and loading is set by the Planning Board on site plan approval.
Multifamily-Inn - Lot area per dwelling unit is 4,000 sq ft plus 1,500 sq ft for each bedroom, on a lot of at least 20,000 sq ft. The 50 foot single and 100 foot combined side yards apply only between buildings and side lot lines.
- A row or attached dwelling needs 20 feet of lot width, 100 feet of lot depth and a 25 foot front yard.
- Every use the R-40 district permits is also permitted here and regulated as in R-40, so a one-family house needs a 150 foot lot width, a 40 foot front yard and 20 foot side yards.
- An inn or a bed-and-breakfast establishment, with accessory shops, eating and drinking establishments, recreation facilities and spas, is a conditional use needing Planning Board approval.
- A bed-and-breakfast establishment may hold no more than eight guest bedrooms and needs a lot of at least 20,000 sq ft for two guest bedrooms, 30,000 for three and 40,000 for four.
- It must be owner occupied, must look like a one-family dwelling from outside, and no guest may stay longer than two weeks.
- At least 1/3 of the net site area must be permanent open space or recreation, with 1,500 sq ft of undeveloped open space per bedroom and a 400 sq ft play area for each dwelling unit.
Office-Research Education - One-family dwellings and every use the R-20 district permits are allowed here, and the code states no separate lot, yard or coverage standard for them. Side and rear yards are 200 feet on a lot along a residence district boundary in any municipality.
- A parcel of 10 acres or more may be subdivided, and each principal building then needs a lot of at least two acres, with side and rear yards of 50 feet.
- Each lot in such a subdivision needs 200 feet of street frontage, which the Planning Board may reduce to 100 feet on a cul-de-sac or a curve.
- Parking and loading must stand at least 100 feet from a residence district boundary and 30 feet from any other lot line, and the amount of parking is set by the Planning Board on site plan approval.
- In a parking lot of an acre or more at least 5% of the lot must be interior landscaping.
- Buildings must stand at least as far apart as the taller one is high and never less than 30 feet.
- Rooftop ventilators, water tanks, bulkheads and chimneys may rise above the height limit if they cover no more than 20% of the roof and stand no more than 15 feet above it.
Size limits on record in Ossining
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 Ossining:
- One-Family ResidenceIt must be 300 to 800 sq ft and no more than 33% of the principal building.
- One-Family ResidenceIt must be 300 to 800 sq ft and no more than 33% of the principal building.
- One-Family ResidenceIt must be 300 to 800 sq ft and no more than 33% of the principal building.
- One-Family ResidenceIt must be 300 to 800 sq ft and no more than 33% of the principal building.
- One-Family ResidenceIt must be 300 to 800 sq ft and no more than 33% of the principal building.
- One-Family ResidenceIt must be 300 to 800 sq ft and no more than 33% of the principal building.
- One-Family ResidenceIt must be 300 to 800 sq ft and no more than 33% of the principal building.
- One-Family ResidenceIt must be 300 to 800 sq ft and no more than 33% of the principal building.
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 Town of Ossining 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/OS0797
- Building dept(914) 762-8419
Sizing up a specific parcel in Ossining?
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Building a full house instead?
A standard-finish house in Ossining runs $242–318/sq ft, about $435k–570k at 1,800 sq ft . See what it costs to build a house in Ossining.
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 · Lewisboro · Mamaroneck · Mount Pleasant · New Castle · North Castle · North Salem · 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 Ossining: Can you build an ADU here? · Zoning for new construction · Westchester County build cost
ADU cost in Ossining: common questions
How much does it cost to build an ADU in Ossining, NY?
An 800 sq ft accessory dwelling unit in Town of Ossining 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 Ossining?
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 Ossining?
Town of Ossining has 17 districts. 9 districts allow ADUs with occupancy restrictions, 4 do not. 4 districts have incomplete data - verify with the building department at (914) 762-8419. Check the specific address with the free ADU checker before you spend money on design.
Can I rent an ADU in Ossining short term?
No local short-term-rental law on record - which is not the same as permission for Town of Ossining. 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 Ossining vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-09-26; verify with the building department at (914) 762-8419 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.