Somers · ADU cost

What it costs to build an ADU in Somers, NY

An 800 sq ft accessory dwelling unit in Somers 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 Somers maps 15 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 Somers

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.

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.

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 Somers 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.

Size limits on record in Somers

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 Somers:

• Residence R120R120 is a residence district under § 170-4, so § 170-70 (Art. XII, Special Exception Uses in Residence Districts) reaches it. Zoning Board of Appeals special exception use permit. The owner must occupy and maintain as his legal residence at least one of the dwelling units on the premises (§ 170-70D). THE STRUCTURE MUST PREDATE 1 APRIL 1992 — the apartment may be in the principal dwelling or an accessory building only where that building was constructed or permitted before that date (B), so there is no route to an accessory apartment in new construction. Minimum one-family lot of 40,000 sq ft, and never in an accessory building on a smaller lot (C). 300–800 sq ft; ONE bedroom, with a second only where the ZBA so finds and the building had a CO before 4-1-1992 (E). One per one-family building lot (F).
• Residence R80R40 is a residence district under § 170-4, so § 170-70 reaches it. ZBA special exception use permit with owner-occupancy of one of the units as the owner's legal residence (§ 170-70D). The building must have been constructed or permitted before 1 April 1992 (B). The 40,000 sq ft minimum lot of § 170-70C bites hardest in this district — an R40 lot at the district minimum is 40,000 sq ft, and anything below it qualifies only if the ZBA finds the apartment fits within the existing principal dwelling, with an accessory-building unit ruled out entirely. 300–800 sq ft; one bedroom, with a second only on a ZBA finding and a pre-4-1-1992 CO.
• Residence R80R80 is a residence district under § 170-4, so § 170-70 reaches it. ZBA special exception use permit with owner-occupancy of one of the dwelling units as the owner's legal residence required (§ 170-70D). The principal or accessory building must have been constructed or permitted before 1 April 1992 (B). Minimum one-family lot of 40,000 sq ft; never in an accessory building on a smaller lot (C). 300–800 sq ft; one bedroom, with a second only on a ZBA finding and a pre-4-1-1992 certificate of occupancy (E). One per one-family building lot.
• Residence R40R40 is a residence district under § 170-4, so § 170-70 reaches it. ZBA special exception use permit with owner-occupancy of one of the units as the owner's legal residence (§ 170-70D). The building must have been constructed or permitted before 1 April 1992 (B). The 40,000 sq ft minimum lot of § 170-70C bites hardest in this district — an R40 lot at the district minimum is 40,000 sq ft, and anything below it qualifies only if the ZBA finds the apartment fits within the existing principal dwelling, with an accessory-building unit ruled out entirely. 300–800 sq ft; one bedroom, with a second only on a ZBA finding and a pre-4-1-1992 CO.
• Multi-Family Residence HamletMFR-H is the Multifamily Residence Hamlet district, a residence district under § 170-4, so § 170-70 (Art. XII, Special Exception Uses in Residence Districts) reaches it. The section keys off a “one-family building lot” throughout (C, D, F), so it attaches where a one-family house stands rather than to a multifamily building. ZBA special exception use permit; the owner must occupy one of the dwelling units as his legal residence (D); the building must have been constructed or permitted before 1 April 1992 (B); minimum one-family lot of 40,000 sq ft, with smaller lots qualifying only for a unit inside the existing principal dwelling (C); 300–800 sq ft; one bedroom by default (E).
• Residence R10R10 is a residence district under § 170-4, so § 170-70 reaches it — but the land test does most of the work here. § 170-70C sets a 40,000 sq ft minimum one-family lot, which is four times the R10 district minimum, so on a typical R10 lot an accessory apartment is available only where the ZBA finds it can be adequately accommodated WITHIN THE EXISTING principal dwelling without overburdening the property; an apartment in an accessory building is expressly barred on any lot under 40,000 sq ft. ZBA special exception permit; owner must occupy one of the units as his legal residence (D); the building must predate 1 April 1992 (B); 300–800 sq ft; one bedroom by default (E).

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. No local STR ordinance found as of Jun 2026; standard town zoning and building codes apply. Westchester County hotel occupancy tax (3%) applies to short-term stays. Separate question from whether you can build the unit.
• Building dept(914) 277-3539

Building a full house instead?

A standard-finish house in Somers runs $242–318/sq ft, about $435k–570k at 1,800 sq ft , plus roughly $1,412 for a conforming lot. See what it costs to build a house in Somers.

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 · Lewisboro · Mamaroneck · Mount Pleasant · New Castle · North Castle · North Salem · Ossining · Pound Ridge · 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 Somers: Can you build an ADU here? · Zoning for new construction · Westchester County build cost

ADU cost in Somers: common questions

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 Somers 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) 277-3539 before relying on any answer. Sources: Offsite / Aldo cost engine (costEngine.js + constants.js); zoning dataset (municipalData.js).