Cortlandt · ADU cost
What it costs to build an ADU in Cortlandt, NY
An 800 sq ft accessory dwelling unit in Cortlandt 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 Cortlandt maps 19 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 Cortlandt
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 Cortlandt 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 | 3.6731 ac | — | 35 ft | Occupancy restricted |
| Single-Family Residential | — | — | — | Occupancy restricted |
| Single-Family Residential | 0.9183 ac | — | 35 ft | Occupancy restricted |
| Single-Family and Two-Family Residential | 0.9183 ac | — | 35 ft | Occupancy restricted |
| Single-Family Residential | 0.2296 ac | — | 35 ft | Occupancy restricted |
| Single-Family Residential | 0.3444 ac | — | 35 ft | Occupancy restricted |
| Single-Family Residential | 0.4591 ac | — | 35 ft | Occupancy restricted |
| General Residential | 0.1722 ac | — | 40 ft | Occupancy restricted |
| Planned Village Development | — | — | — | — |
| Designed Industrial | — | — | 40 ft | No |
| Community Commercial | 0.2755 ac | — | 35 ft | Occupancy restricted |
| Designed Commercial | 1.8365 ac | — | 35 ft | No |
| Designed Commercial | 1.8365 ac | — | 35 ft | — |
| Highway Commercial | 0.4591 ac | — | 35 ft | Occupancy restricted |
| Highway Commercial/Multi-Family | — | — | — | Occupancy restricted |
| Light Industrial | 0.4591 ac | — | 40 ft | No |
| Camp Smith Reuse B | 0.1148 ac | — | — | — |
| Conservation, Recreation and Open Space | — | — | — | Occupancy restricted |
| Parks, Recreation, and Open Space | — | — | — | No |
Size limits on record in Cortlandt
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 Cortlandt:
- One-Family ResidentialTable of Permitted Uses (Ch. 307 Attachment 2): ADU attached = P, ADU detached = SP in R-160. Owner must occupy one of the units as their principal residence and file a declaration of covenants terminating the permit on death, transfer of title, or the owner ceasing to occupy (§ 307-45C(2), C(3)) — which is why this is special-provisions rather than as-of-right. Detached units need a Planning Board special permit (§ 307-45B(3)). One ADU per lot, max two units on the lot; 800 sq ft or 40% of the house, whichever is less; max two bedrooms. No short-term rental and no renting of individual bedrooms. ADUs certified after 8-1-2025 need an operating permit renewed every three years.
- Single-Family ResidentialTable of Permitted Uses: ADU attached = P, ADU detached = SP in R-80. Owner-occupancy of one unit plus a recorded covenant terminating on death, transfer or cessation of occupancy (§ 307-45C(2), C(3)). Detached units require a Planning Board special permit. 800 sq ft or 40% of the house, whichever is less; max two bedrooms; one ADU per lot. No short-term rental; no renting of individual bedrooms. Operating permit renewable every three years for units certified after 8-1-2025.
- Single-Family ResidentialTable of Permitted Uses: ADU attached = P, ADU detached = SP in R-40. Owner-occupancy of one unit plus recorded covenant (§ 307-45C(2), C(3)). Detached units need a Planning Board special permit. 800 sq ft or 40% of the house, whichever is less; max two bedrooms; one ADU per lot. No short-term rental. Operating permit renewable every three years for units certified after 8-1-2025.
- Single-Family ResidentialTable of Permitted Uses: ADU attached = P, ADU detached = SP in R-10. Owner-occupancy of one unit plus recorded covenant (§ 307-45C(2), C(3)). Detached units need a Planning Board special permit. 800 sq ft or 40% of the house, whichever is less; max two bedrooms; one ADU per lot. No short-term rental.
- Single-Family ResidentialTable of Permitted Uses: ADU attached = P, ADU detached = SP in R-15. Owner-occupancy of one unit plus recorded covenant (§ 307-45C(2), C(3)). Detached units need a Planning Board special permit. 800 sq ft or 40% of the house, whichever is less; max two bedrooms; one ADU per lot. No short-term rental.
- Single-Family ResidentialTable of Permitted Uses: ADU attached = P, ADU detached = SP in R-20. Owner-occupancy of one unit plus recorded covenant terminating on death, transfer or cessation of occupancy (§ 307-45C(2), C(3)). Detached units need a Planning Board special permit. 800 sq ft or 40% of the house, whichever is less; max two bedrooms; one ADU per lot. No short-term rental.
- Community CommercialCC is the Community Commercial district. Table of Permitted Uses: ADU attached = P, ADU detached = SP in CC, and single-family dwellings are permitted subject to note (1), so an ADU can arise here as an accessory to a house. Owner-occupancy of one unit plus recorded covenant (§ 307-45C(2), C(3)); detached units need a Planning Board special permit; 800 sq ft or 40% of the house, whichever is less.
- Highway CommercialHC is the Highway Commercial district. Table of Permitted Uses: ADU attached = P, ADU detached = SP in HC, with single-family dwellings permitted subject to note (2). Owner-occupancy of one unit plus recorded covenant (§ 307-45C(2), C(3)); detached units need a Planning Board special permit; 800 sq ft or 40% of the house, whichever is less.
- Highway Commercial/Multi-FamilyThis GIS label is the HC-9A column of the Table of Permitted Uses (the Route 9A portion of the Highway Commercial district). ADU attached = P, ADU detached = SP. Owner-occupancy of one unit plus recorded covenant (§ 307-45C(2), C(3)); detached units need a Planning Board special permit; 800 sq ft or 40% of the house, whichever is less; max two bedrooms.
- Conservation, Recreation and Open SpaceCROS is the Conservation, Recreation and Open Space district (§ 307-5F). The Table of Permitted Uses marks ADU attached = P and ADU detached = SP in CROS, with single-family dwellings permitted by right — this district protects sensitive land but is not use-barren. Owner-occupancy of one unit plus a recorded covenant terminating on death, transfer or cessation of occupancy (§ 307-45C(2), C(3)); detached units need a Planning Board special permit; 800 sq ft or 40% of the house, whichever is less.
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 rentals allowed with local rules. Transient lodging requires a Town operating permit under Chapter 281, Transient and Nontransient Lodging, adopted 8-13-2019 by L.L. No. 3-2019. § 281-2 makes an operating permit a condition of continued use, with application, inspection and fee requirements at §§ 281-3 through 281-5. Westchester County's 3% hotel occupancy tax also applies to short stays. New York State's 2025 short-term rental registry (Ch. 99 of the Laws of 2025) separately requires every host statewide to register the unit regardless of local law. Confirm with Town of Cortlandt at (914) 734-1010 before listing. Separate question from whether you can build the unit.
- Building dept(914) 734-1010
Sizing up a specific parcel in Cortlandt?
Aldo pulls the exact zoning district, setbacks, and build feasibility for any parcel — before you buy. Enter your email to open Aldo and run your free report.
Building a full house instead?
A standard-finish house in Cortlandt runs $242–318/sq ft, about $435k–570k at 1,800 sq ft , plus roughly $7,657 for a conforming lot. See what it costs to build a house in Cortlandt.
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 · Eastchester · Greenburgh · Lewisboro · 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 Cortlandt: Can you build an ADU here? · Zoning for new construction · Westchester County build cost
ADU cost in Cortlandt: common questions
How much does it cost to build an ADU in Cortlandt, NY?
An 800 sq ft accessory dwelling unit in Town of Cortlandt 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 Cortlandt?
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 Cortlandt?
Town of Cortlandt has 19 districts. 12 districts allow ADUs with occupancy restrictions, 4 do not. 3 districts have incomplete data - verify with the building department at (914) 734-1010. Check the specific address with the free ADU checker before you spend money on design.
Can I rent an ADU in Cortlandt short term?
Short-term rentals allowed with local rules for Town of Cortlandt. 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 Cortlandt vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-06-08; verify with the building department at (914) 734-1010 before relying on any answer. Sources: Offsite / Aldo cost engine (costEngine.js + constants.js); zoning dataset (municipalData.js).