Farnham · ADU cost
What it costs to build an ADU in Farnham, NY
An 800 sq ft accessory dwelling unit in Farnham runs roughly $200k–260k, or about $249–327/sq ft, and that per-square-foot figure sits deliberately above the $204–268/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 Farnham maps 1 district 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 Farnham
Standard finish, slab on grade, on a flat and clear part of the lot, general-contractor margin included. These are structure costs at Erie County pricing, before the service and septic work a second dwelling can trigger.
| Size | Type | $ / sq ft | Estimated total |
|---|---|---|---|
| 400 sq ft | Studio | $331–$433 | $130k–175k |
| 600 sq ft | One bedroom | $276–$362 | $165k–215k |
| 800 sq ft | One bedroom, larger | $249–$327 | $200k–260k |
| 1,000 sq ft | Two bedroom | $233–$305 | $235k–305k |
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 | $7–$11 | $7 | — |
| Foundation | $16–$23 | $16 | — |
| Framing | $25–$32 | $25 | — |
| Envelope - roof, siding, windows | $32–$41 | $32 | — |
| Plumbing | $28–$34 | $16 | 55% |
| Electrical | $18–$24 | $13 | 35% |
| HVAC | $23–$28 | $15 | 45% |
| Insulation | $7–$9 | $7 | — |
| Interior finishes | $32–$43 | $27 | 15% |
| Millwork & cabinetry | $18–$28 | $11 | 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 Farnham 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 |
|---|---|---|---|---|
| R Residential District | — | 50 / 20 / 50 ft | 35 ft | No |
Size limits on record in Farnham
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 Farnham:
- R Residential DistrictThe average floor area of a unit must be at least 920 sq ft, exclusive of garage and basement
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 National Grid 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 short-term-rental law found for the Village of Farnham. The Village's code IS reachable, contrary to what was previously assumed here — villageoffarnham.com publishes the complete Village of Farnham Code Book as a single document (26 chapters, ~570,000 characters, adopted 1-15-89 with amendments). Read in full 2026-08-27. Nothing in it regulates short-term rental or transient occupancy, and Chapter A108, the code's own 'Table of Required Licenses and Permits' — which states that it 'enumerates all licenses and permits required by the Village of Farnham' — lists bingo, fire prevention and building construction, games of chance, hawker, peddler, solicitor, circus, carnival, theater, sound truck, tree cutting, sidewalk construction and use, trailer park, tractors on streets, building, zoning, special use and certificate of occupancy. No rental permit and no rental registration. BUT THE FEE SCHEDULE APPENDED TO THE CODE BOOK CHARGES A 'RENTAL PERMIT FEE — $50 PER UNIT PER YEAR', alongside a separate '$50 per year' Operating Permit. So the Village does administer a rental permit that no codified chapter creates, and an owner should establish with the code enforcement officer what it covers and whether it reaches a short-term let — a rental permit regulates without authorising in any case, and holding one is not evidence the use is allowed. THE CODE DEFINES A TOURIST HOME AND NEVER LISTS IT AS A USE. § 101-5: 'TOURIST HOME — A dwelling in which overnight accommodations are provided or offered for transient guests for compensation' — the language that captures whole-house short-term renting — but the code's own index shows Tourist Homes with a 'Defined' entry only, and the use appears in no district's list. THE RESIDENTIAL DISTRICT LIST IS SHORT AND HAS NO LODGING IN IT. § 101-10 permits one- and two-family dwellings, places of worship, schools, municipal and government parks and buildings, county/state/federal parks, nursing homes, and professional offices and home occupations; its accessory uses are garages, storage structures, animal shelters, pools and home occupations; and ITS ONLY SPECIAL PERMIT USE IS PUBLIC UTILITY USES. 'House, Lodging or Boarding' — defined as a multiple dwelling furnishing lodging to fifteen or fewer transient occupants for compensation — is a permitted principal use in the Commercial District (§ 101-11B(1)(h)) and in the Industrial District (§ 101-12), and nowhere in the Residential District. The code states conformance generally (§ 101-13) rather than saying in terms that an unlisted use is prohibited, so the closure is by implication from the enumerated lists rather than by an express clause. That reading has not been confirmed with the Village. New York State's 2025 STR registry separately requires every host statewide to register through the Erie County registry. Confirm with the code enforcement officer at (716) 954-0159 before listing. Separate question from whether you can build the unit.
- Building dept(716) 954-0159
Sizing up a specific parcel in Farnham?
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 Farnham runs $204–268/sq ft, about $370k–480k at 1,800 sq ft . See what it costs to build a house in Farnham.
ADU cost in nearby towns
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More on Farnham: Can you build an ADU here? · Zoning for new construction · Erie County build cost
ADU cost in Farnham: common questions
How much does it cost to build an ADU in Farnham, NY?
An 800 sq ft accessory dwelling unit in Village of Farnham runs roughly $200k–260k at a standard finish, general-contractor margin included. A 400 sq ft studio runs about $130k–175k. 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 Farnham?
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 $249–327/sq ft against $204–268/sq ft for a full house, even though it is the same trades doing the same work.
Are ADUs allowed in Farnham?
Village of Farnham has 1 district. The only district prohibits ADUs. Check the specific address with the free ADU checker or confirm with the building department at (716) 954-0159 before you spend money on design.
Can I rent an ADU in Farnham short term?
No local short-term-rental law on record - which is not the same as permission for Village of Farnham. 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 Erie 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 Farnham vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-08-06; verify with the building department at (716) 954-0159 before relying on any answer. Sources: Offsite / Aldo cost engine (costEngine.js + constants.js); zoning dataset (municipalData.js).