Mendon · ADU cost
What it costs to build an ADU in Mendon, NY
An 800 sq ft accessory dwelling unit in Mendon 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 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 Mendon maps 4 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 Mendon
Standard finish, slab on grade, on a flat and clear part of the lot, general-contractor margin included. These are structure costs at Monroe 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 Mendon 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 |
|---|---|---|---|---|
| Residential Agricultural 5 (RA-5) — the town's largest-lot district, stated as a DENSITY of 1 LOT PER 5 ACRES rather than as a flat minimum lot area, which is how every Mendon residential district is written. Front setback 60 ft on town and county roads, RISING TO 80 FT ON A STATE ROAD; side 20 ft; rear 20 ft; height 2 1/2 stories or 35 feet. THE LOT WIDTH IS TWO SEPARATE NUMBERS AND THE GAP IS THE LARGEST IN THE TOWN: 100 ft of width AT THE RIGHT-OF-WAY but 300 FT AT THE BUILDING LINE. The recorded 300 is the building-line figure because that is what governs where a house can actually sit; the 100 ft ROW figure is a frontage minimum and is far smaller, which means RA-5 expressly contemplates a lot that necks down to the road and opens out behind — a flag-lot geometry. Sizing a subdivision from the ROW width alone would badly overstate how many lots a road frontage can carry. MENDON STATES NO LOT COVERAGE IN ANY RA DISTRICT AND maxLotCoverage IS NULL BY DESIGN — the schedule's third column is headed 'Habitable Floor Area (square feet) or Lot Coverage' and for RA-5 it carries only floor area: a MINIMUM of 1,200 sf for a one-storey dwelling and 1,500 sf for anything else. That is a house-size FLOOR, the opposite of a coverage ceiling, and the two must not be confused because they share a column. RA-5 is one of only two districts permitting TOURIST HOMES AND BED-AND-BREAKFAST ESTABLISHMENTS by special use permit; it has no residential-conversion route and no multi-unit route. | 5 ac | 60 / 20 / 20 ft | 35 ft | No |
| Residential Agricultural 2 (RA-2) — a density of 1 LOT PER 2 ACRES, not a flat lot minimum. Lot width is 50 ft at the right-of-way and 200 FT AT THE BUILDING LINE; the recorded figure is the building-line width, and note the ROW frontage drops to just 50 ft here against RA-5's 100 ft, so the flag-lot geometry is even more pronounced — a two-acre RA-2 lot may present only 50 feet to the road. Front setback 60 ft on town and county roads and 80 ft on a state road; side 20 ft; rear 20 ft; height 2 1/2 stories or 35 feet. Setbacks are identical to RA-5 and RA-1 — the three agricultural districts differ ONLY in density and in building-line width, not in envelope. NO LOT COVERAGE IS STATED, so maxLotCoverage is null by design; the schedule gives only a MINIMUM habitable floor area of 1,200 sf for a one-storey dwelling and 1,500 sf for anything else, which is a house-size floor rather than a coverage cap. RA-2 IS ONE OF THE TWO MOST RESTRICTED DISTRICTS IN MENDON FOR ANYTHING BEYOND A SINGLE HOUSE: unlike RA-5 it does not permit tourist homes or bed-and-breakfasts, and unlike RS-30 it has no residential-conversion or multi-unit route. One dwelling per lot is the whole programme. | 2 ac | 60 / 20 / 20 ft | 35 ft | No |
| Residential Agricultural 1 (RA-1) — a density of 1 LOT PER ACRE, not a flat lot minimum. Lot width 50 ft at the right-of-way and 150 FT AT THE BUILDING LINE (the recorded figure). Front setback 60 ft on town and county roads and 80 ft on a state road; side 20 ft; rear 20 ft; height 2 1/2 stories or 35 feet — dimensionally identical to RA-5 and RA-2 in every respect except density and building-line width, which is the clean pattern in this town: Mendon varies how much land a lot needs and how wide it must be where the house sits, and holds the envelope constant. NO LOT COVERAGE IS STATED so maxLotCoverage is null by design; the schedule carries only a MINIMUM habitable floor area of 1,200 sf one-storey and 1,500 sf otherwise, a house-size floor rather than a ceiling. NOTE THAT THE 1,500 SF FLOOR IS UNCHANGED FROM RA-5 EVEN THOUGH THE LAND REQUIREMENT HAS FALLEN BY FOUR FIFTHS — a one-acre RA-1 lot must deliver exactly the same house as a five-acre RA-5 lot. Like RA-2, RA-1 permits no tourist home, no bed-and-breakfast, no residential conversion and no multi-unit form; it is single-dwelling-per-lot with no exceptions. | 1 ac | 60 / 20 / 20 ft | 35 ft | No |
| Residential Suburban 30 (RS-30) — a density of 1 LOT PER 30,000 SQUARE FEET (0.69 acres), the town's smallest-lot district and the ONLY district in Mendon where any additional density is available. Lot width 50 ft at the right-of-way and 100 FT AT THE BUILDING LINE (the recorded figure). Front setback 60 ft on town and county roads and 80 ft on a state road; SIDE 15 FT — the only district that departs from the town-wide 20 ft side yard; rear 20 ft; height 2 1/2 stories or 35 feet. RS-30 IS THE ONLY MENDON DISTRICT THAT STATES A LOT COVERAGE AND IT IS RECORDED: 25% of the lot. The schedule's shared column reads '1 story: 1,000 ~ Other: 1,200 ~ 25% coverage of lot', so RS-30 carries BOTH a minimum habitable floor area (1,000 sf one-storey, 1,200 sf otherwise — lower than the RA districts' 1,200/1,500) AND a genuine 25% coverage ceiling. Every other district in the town gives floor area only, which is why their coverage is null; do not read those nulls as a missing 25%. EVERY ROUTE TO A SECOND UNIT IN MENDON IS CONCENTRATED HERE, all by Planning Board special use permit under Article VII: RESIDENTIAL CONVERSIONS (the town's de facto ADU, available in no other district), apartment houses, townhouse clusters, multiple-dwelling units and dwelling groups, condominiums and cooperatives, plus tourist homes and bed-and-breakfast establishments. | 0.6887 ac | 60 / 15 / 20 ft | 35 ft | Special permit |
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 RG&E 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 registration. MENDON HAS A FULL STANDALONE SHORT-TERM RENTAL LAW WITH AN OPERATING PERMIT, AND IT IS NOT IN THE ZONING CHAPTER — Ch. 208 Short-Term Rentals, §§ 208-1 to 208-16 (guid 42871418, ~32,000 characters, 14 sections). THIS IS THE SECOND CONFIRMED INSTANCE OF THE GATES Ch. 148 PATTERN IN MONROE and it is the reason the whole-code sweep is mandatory: a search of Mendon's Ch. 260 Zoning alone returns nothing for short-term rental and would have produced a false 'no regime' finding. § 208-5 REQUIRES A REVOCABLE SHORT-TERM RENTAL OPERATING PERMIT before a property may be used as an STR at all, granted 'in accordance with the Schedule of Zoning Districts' at § 208-6 — so WHICH DISTRICTS PERMIT AN STR IS SET BY THAT SCHEDULE. Permit mechanics: a NEW permit is valid for a maximum of ONE year and a RENEWAL for a maximum of TWO; all permits expire 31 December of the relevant calendar year regardless of when issued; renewals are filed at the Town of Mendon Building Department and must be received by 1 DECEMBER or the application is treated as new; and THE PERMIT IS NONTRANSFERABLE, so a purchaser of an operating STR must apply afresh and cannot rely on the seller's permit — a material diligence point on any Mendon acquisition premised on rental income. § 208-7 sets property standards that are life-safety rather than dimensional: full compliance with the NYS Uniform Fire Prevention and Building Code and the Town Code with any pre-existing deficiencies corrected BEFORE issuance, a working smoke detector in every sleeping room plus one per floor, carbon monoxide detectors, evacuation procedures posted in each sleeping room and in the kitchen and main living room, and portable fire extinguishers on each floor and in the kitchen under NY Fire Code § 906 with an inspection record produced at application. §§ 208-10 to 208-13 carry compliance, penalties, suspension and revocation, renewal and appeals. SEPARATELY AND NOT THE SAME THING, the zoning chapter permits TOURIST HOMES AND BED-AND-BREAKFAST ESTABLISHMENTS by SPECIAL USE PERMIT from the Planning Board, but only in RA-5 and RS-30 — not in RA-2 or RA-1. A bed-and-breakfast is an owner-present lodging use on a different track from the Ch. 208 STR permit and the two should not be conflated. New York State's 2025 statewide short-term rental registry (Ch. 99 of the Laws of 2025) obliges every host to register in addition to the Town permit. Confirm with the Building Department at (585) 624-6066. Separate question from whether you can build the unit.
- Building dept(585) 624-6066
Sizing up a specific parcel in Mendon?
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 Mendon runs $204–268/sq ft, about $370k–480k at 1,800 sq ft , plus roughly $8,486 for a conforming lot. See what it costs to build a house in Mendon.
ADU cost in nearby towns
All Monroe County towns: Rochester (City) · Brighton · Chili · Clarkson · Gates · Greece · Hamlin · Henrietta · Irondequoit · Ogden · Parma · Penfield · Perinton · Pittsford · Riga · Rush · Sweden · Webster · Wheatland · Town/Village of East Rochester (Village) · Brockport (Village) · Churchville (Village) · Fairport (Village) · Hilton (Village) · Honeoye Falls (Village) · Pittsford (Village) · Scottsville (Village) · Spencerport (Village) · Webster (Village)
More on Mendon: Can you build an ADU here? · Zoning for new construction · Monroe County build cost
ADU cost in Mendon: common questions
How much does it cost to build an ADU in Mendon, NY?
An 800 sq ft accessory dwelling unit in Town of Mendon 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 Mendon?
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 Mendon?
Town of Mendon has 4 districts. 3 districts prohibit ADUs, 1 requires a special-use permit. Check the specific address with the free ADU checker or confirm with the building department at (585) 624-6066 before you spend money on design.
Can I rent an ADU in Mendon short term?
Short-term rentals allowed with registration for Town of Mendon. 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 Monroe 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 Mendon vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-08-13; verify with the building department at (585) 624-6066 before relying on any answer. Sources: Offsite / Aldo cost engine (costEngine.js + constants.js); zoning dataset (municipalData.js).