Buchanan · ADU cost

What it costs to build an ADU in Buchanan, NY

An 800 sq ft accessory dwelling unit in Buchanan 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 Buchanan maps 10 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 Buchanan

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 Buchanan 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 Buchanan

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 Buchanan:

• ResidentialAccessory apartment by Planning Board special permit; R-20 is named in § 211-22B(1). Owner-occupancy as primary residence plus three years' prior occupancy required. Maximum two occupants, one bedroom, 300–600 sq ft and ≤ 33% of the building, building at least 10 years old. Detached units are excluded (B(3)(c)).
• ResidentialAccessory apartment by Planning Board special permit; R-40 is named in § 211-22B(1). Owner-occupancy as primary residence plus three years' prior occupancy required. Maximum two occupants, one bedroom, 300–600 sq ft and ≤ 33% of the building, building at least 10 years old. Detached units are excluded (B(3)(c)).
• ResidentialAccessory apartment by Planning Board special permit; R-10 is named in § 211-22B(1). Owner-occupancy as primary residence plus three years' prior occupancy required (B(2)(a), C(1)). Maximum two occupants (B(2)(c)), one bedroom (C(2)(a)), 300–600 sq ft and ≤ 33% of the building (C(2)(b)), building at least 10 years old. No accessory apartment in an accessory building (B(3)(c)).
• ResidentialAccessory apartment by Planning Board special permit; R-15 is named in § 211-22B(1). Owner-occupancy as primary residence plus three years' prior occupancy required. Maximum two occupants, one bedroom, 300–600 sq ft and ≤ 33% of the building, building at least 10 years old. Detached units are excluded (B(3)(c)).
• ResidentialAccessory apartment by Planning Board special permit; R-7.5 is named in § 211-22B(1). Owner must occupy the principal or accessory unit as their primary residence and must have occupied the dwelling for three years before applying (§ 211-22B(2)(a), C(1)). Occupancy of the apartment is capped at two persons (B(2)(c)) and it may have only one bedroom (C(2)(a)). Building must be at least 10 years old; unit 300–600 sq ft and ≤ 33% of the building (C(2)(b)). Not permitted in an accessory building (B(3)(c)). These conditions cannot be varied (B(5)(b)).
• Neighborhood CommercialUnusually, § 211-22B(1) extends the accessory-apartment special permit to the C-1 Commercial District as well as the residential districts. The apartment still has to be accessory to a single-family DETACHED residence (B(3)(b)), so it only arises where such a house exists on a C-1 lot. Owner-occupancy as primary residence plus three years' prior occupancy required; maximum two occupants; one bedroom; 300–600 sq ft and ≤ 33% of the building; building at least 10 years old; not permitted in an accessory building.
• Neighborhood / General Commercial OverlayThis GIS label denotes land carrying both the C-1 and C-2 commercial designations, and § 211-22B(1) names both C-1 and C-2 among the districts where the Planning Board may grant an accessory-apartment special permit, so the answer is the same either way. The apartment must be accessory to a single-family detached residence (§ 211-22B(3)(b)); owner-occupancy as primary residence plus three years' prior occupancy is required; occupancy is capped at two persons; one bedroom; 300–600 sq ft and ≤ 33% of the building; not permitted in an accessory building. Confirm the underlying designation with the Building Department for a specific parcel.
• General Commercial§ 211-22B(1) extends the accessory-apartment special permit to the C-2 Commercial District. The apartment must still be accessory to a single-family detached residence (B(3)(b)). Owner-occupancy as primary residence plus three years' prior occupancy required; maximum two occupants; one bedroom; 300–600 sq ft and ≤ 33% of the building; building at least 10 years old; not permitted in an accessory 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. No local STR ordinance found as of Jun 2026; standard village 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) 293-8395

Building a full house instead?

A standard-finish house in Buchanan runs $242–318/sq ft, about $435k–570k at 1,800 sq ft . See what it costs to build a house in Buchanan.

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 · Somers · Yorktown · Ardsley (Village) · Briarcliff Manor (Village) · Bronxville (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 Buchanan: Can you build an ADU here? · Zoning for new construction · Westchester County build cost

ADU cost in Buchanan: 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 Village of Buchanan 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) 293-8395 before relying on any answer. Sources: Offsite / Aldo cost engine (costEngine.js + constants.js); zoning dataset (municipalData.js).