Larchmont · ADU cost
What it costs to build an ADU in Larchmont, NY
An 800 sq ft accessory dwelling unit in Larchmont 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 Larchmont maps 11 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 Larchmont
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 Larchmont 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 | 0.69 ac | 30 / 12 / 50 ft | 30 ft | No |
| One-Family Residence District | 0.46 ac | 30 / 12 / 40 ft | 30 ft | No |
| One-Family Residence District | 0.34 ac | 30 / 10 / 30 ft | 30 ft | No |
| One-Family Residence District | 0.29 ac | 30 / 10 / 30 ft | 30 ft | No |
| One-Family Residence District | 0.23 ac | 30 / 10 / 25 ft | 30 ft | No |
| One-Family Residence District | 0.17 ac | 30 / 10 / 25 ft | 30 ft | No |
| One-Family Residence District | 0.11 ac | 30 / 6 / 25 ft | 30 ft | No |
| Multifamily Residence District | 0.034 ac | 20 / 15 / 40 ft | 30 ft | No |
| Retail Business Commercial District | 0.023 ac | 10 / - / 15 ft | 25 ft | No |
| Retail Center Commercial District | 0.023 ac | - / - / 15 ft | 25 ft | No |
| Waterfront Coastal Zone District | 1.15 ac | 30 / 12 / 50 ft | 30 ft | No |
One-Family Residence District, One-Family Residence District - The 30 ft front yard is a maximum as well as a minimum: for a one-family dwelling the front and side-front yard may be no deeper than the average of the nearest four occupied lots in the same block on the same side of the street, where that average is less (§ 381-33A).
- A side-front yard of 30 ft is also required.
- A second lot coverage limit keyed to the lot's actual area applies on top of the building coverage shown, from 55% at 3,000 sq ft down to 35% at 40,001 sq ft and above (§ 381-38A).
- A one-family residence may not exceed 30 ft at any point around its perimeter, measured from the lower of existing or proposed grade (§ 381-34A).
- A sky-plane applies as well: at the minimum required side and rear setback line the height may not exceed 20 ft, rising 2 ft for each extra 1 ft of setback (§ 381-34C).
- An accessory building must be at least 5 ft from the side and rear lot lines, may cover 5% of the lot, and may not stand in the front, side or side-front yard (§ 381-32).
- The 1,500 sq ft minimum is first-story floor area for a house of 2 to 2 1/2 stories; at one story the first story must be 2,100 sq ft.
One-Family Residence District, One-Family Residence District - The 30 ft front yard is a maximum as well as a minimum: for a one-family dwelling the front and side-front yard may be no deeper than the average of the nearest four occupied lots in the same block on the same side of the street, where that average is less (§ 381-33A).
- A side-front yard of 30 ft is also required.
- A second lot coverage limit keyed to the lot's actual area applies on top of the building coverage shown, from 55% at 3,000 sq ft down to 35% at 40,001 sq ft and above (§ 381-38A).
- A one-family residence may not exceed 30 ft at any point around its perimeter, measured from the lower of existing or proposed grade (§ 381-34A).
- A sky-plane applies as well: at the minimum required side and rear setback line the height may not exceed 20 ft, rising 2 ft for each extra 1 ft of setback (§ 381-34C).
- An accessory building must be at least 5 ft from the side and rear lot lines, may cover 5% of the lot, and may not stand in the front, side or side-front yard (§ 381-32).
- The 1,000 sq ft minimum is first-story floor area for a house of 2 to 2 1/2 stories; at one story the first story must be 1,500 sq ft.
One-Family Residence District - The 30 ft front yard is a maximum as well as a minimum: for a one-family dwelling the front and side-front yard may be no deeper than the average of the nearest four occupied lots in the same block on the same side of the street, where that average is less (§ 381-33A).
- A side-front yard of 30 ft is also required.
- A second lot coverage limit keyed to the lot's actual area applies on top of the building coverage shown, from 55% at 3,000 sq ft down to 35% at 40,001 sq ft and above (§ 381-38A).
- A one-family residence may not exceed 30 ft at any point around its perimeter, measured from the lower of existing or proposed grade (§ 381-34A).
- A sky-plane applies as well: at the minimum required side and rear setback line the height may not exceed 20 ft, rising 2 ft for each extra 1 ft of setback (§ 381-34C).
- An accessory building must be at least 5 ft from the side and rear lot lines, may cover 6% of the lot, and may not stand in the front, side or side-front yard (§ 381-32).
- The 1,000 sq ft minimum is first-story floor area for a house of 2 to 2 1/2 stories; at one story the first story must be 1,500 sq ft.
One-Family Residence District - The 30 ft front yard is a maximum as well as a minimum: for a one-family dwelling the front and side-front yard may be no deeper than the average of the nearest four occupied lots in the same block on the same side of the street, where that average is less (§ 381-33A).
- A side-front yard of 30 ft is also required.
- A second lot coverage limit keyed to the lot's actual area applies on top of the building coverage shown, from 55% at 3,000 sq ft down to 35% at 40,001 sq ft and above (§ 381-38A).
- A one-family residence may not exceed 30 ft at any point around its perimeter. A sky-plane caps height at 20 ft at the minimum side and rear setback line, rising 3 ft for each extra 1 ft of setback.
- An accessory building must be at least 5 ft from the side and rear lot lines, may cover 6% of the lot, and may not stand in the front, side or side-front yard (§ 381-32).
- The 800 sq ft minimum is first-story floor area for a house of 2 to 2 1/2 stories; at one story the first story must be 1,100 sq ft.
- A townhouse development takes a Board of Trustees special permit under § 381-51B, and the district publishes no by-right townhouse standards.
One-Family Residence District - The 30 ft front yard is a maximum as well as a minimum: for a one-family dwelling the front and side-front yard may be no deeper than the average of the nearest four occupied lots in the same block on the same side of the street, where that average is less (§ 381-33A).
- A side-front yard of 30 ft is also required.
- A second lot coverage limit keyed to the lot's actual area applies on top of the building coverage shown, from 55% at 3,000 sq ft down to 35% at 40,001 sq ft and above (§ 381-38A).
- A one-family residence may not exceed 30 ft at any point around its perimeter. A sky-plane caps height at 20 ft at the minimum side and rear setback line, rising 3 ft for each extra 1 ft of setback.
- An accessory building must be at least 5 ft from the side and rear lot lines, may cover 8% of the lot, and may not stand in the front, side or side-front yard (§ 381-32).
- The 700 sq ft minimum is first-story floor area for a house of 2 to 2 1/2 stories; at one story the first story must be 1,000 sq ft.
Multifamily Residence District - The 1,500 sq ft lot area is land per family, not a conforming lot for a multi-unit building: the lot has to carry 1,500 sq ft for every family in it. A one-family dwelling in MF instead needs a lot of at least 5,000 sq ft and 50 ft of width.
- A one-family dwelling's yards are 15 ft front, 6 ft on one side and 14 ft combined, and 35 ft rear, with a 12 ft side-front yard.
- Accessory buildings keep 5 ft from the side and rear lot lines, may not stand in a front, side or side-front yard, and are capped at 16% coverage against the principal building's 35%.
- The 700 sq ft minimum first-story area applies at every story count here.
- A sliding lot coverage limit keyed to the lot's actual area applies on top of the 35% building coverage (§ 381-38A).
- Height is also subject to the building height and shape rules of § 381-34.
Retail Business Commercial District - The 1,000 sq ft lot area is land per family, not a conforming lot for a multi-unit building: the lot has to carry 1,000 sq ft for every family in it. The figures shown are otherwise the nonresidential case.
- A multifamily building here follows its own standards: a 40 ft rear yard, 30 ft and 2 1/2 stories of height, 35% coverage and a 15 ft side-front yard.
- The livable area for a unit in that multifamily case is at least 700 sq ft as an average across the units, not a floor for each one.
- A side yard on any side abutting a residence district must be at least 15 ft; the schedule prints only the 30 ft both-sides figure otherwise.
- In Block 614 on Chatsworth Avenue the front yard is 8 ft instead of 10 ft.
- Accessory buildings in the multifamily case are capped at 16% coverage.
- Height is also subject to the building height and shape rules of § 381-34.
Retail Center Commercial District - The 1,000 sq ft lot area is land per family, not a conforming lot for a multi-unit building: the lot has to carry 1,000 sq ft for every family in it. The figures shown are otherwise the nonresidential case.
- A public-amenity-supplemented mixed-use development follows its own schedule under a Village Board special permit: a lot of 10,000 sq ft, 40 ft wide, and zero front, side and rear yards (§ 381-51N).
- Within that permit the 50 ft height, four stories and 100% coverage are maximums the Village Board sets case by case, measured from existing grade along the Chatsworth Avenue frontage.
- A side yard on any side abutting a residence district must be at least 15 ft, and in Block 614 on Chatsworth Avenue the front yard is 8 ft.
- No multifamily standards are stated for RC: the footnote that carries them names only the RB district, though multifamily is a permitted use here (§ 381-26A(7)).
- Height is also subject to the building height and shape rules of § 381-34.
Waterfront Coastal Zone District - The 30 ft front yard is a maximum as well as a minimum: for a one-family dwelling the front and side-front yard may be no deeper than the average of the nearest four occupied lots in the same block on the same side of the street, where that average is less (§ 381-33A).
- A side-front yard of 30 ft is also required.
- A second lot coverage limit keyed to the lot's actual area applies on top of the building coverage shown, from 55% at 3,000 sq ft down to 35% at 40,001 sq ft and above (§ 381-38A).
- A one-family residence may not exceed 30 ft at any point around its perimeter. A sky-plane caps height at 20 ft at the minimum side and rear setback line, rising 2 ft for each extra 1 ft of setback.
- An accessory building must be at least 5 ft from the side and rear lot lines, may cover 5% of the lot, and may not stand in the front, side or side-front yard (§ 381-32).
- On the Yacht Club, Shore Club and Manor Park parcels the principal uses are parks, beaches and private membership clubs; a one-family dwelling needs a Board of Trustees special permit (§ 381-51E).
- The 1,500 sq ft minimum is first-story floor area for a house of 2 to 2 1/2 stories; at one story the first story must be 2,100 sq ft.
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 Larchmont 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/LA0851
- Building dept(914) 834-4349
Sizing up a specific parcel in Larchmont?
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Building a full house instead?
A standard-finish house in Larchmont runs $242–318/sq ft, about $435k–570k at 1,800 sq ft , plus roughly $25,800 for a conforming lot. See what it costs to build a house in Larchmont.
ADU cost in nearby towns
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More on Larchmont: Can you build an ADU here? · Zoning for new construction · Westchester County build cost
ADU cost in Larchmont: common questions
How much does it cost to build an ADU in Larchmont, NY?
An 800 sq ft accessory dwelling unit in Village of Larchmont 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 Larchmont?
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 Larchmont?
Village of Larchmont has 11 districts. All districts prohibit ADUs. Check the specific address with the free ADU checker or confirm with the building department at (914) 834-4349 before you spend money on design.
Can I rent an ADU in Larchmont short term?
No local short-term-rental law on record - which is not the same as permission for Village of Larchmont. 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 Larchmont vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-09-28; verify with the building department at (914) 834-4349 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.