Webster · ADU cost
What it costs to build an ADU in Webster, NY
An 800 sq ft accessory dwelling unit in Webster 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 Webster maps 6 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 Webster
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 Webster 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-1 Single-Family Residential | 0.8 ac | 75 / 20 / 60 ft | 30 ft | Occupancy restricted |
| R-2 Single-Family Residential | 0.64 ac | 60 / 15 / 55 ft | 30 ft | Occupancy restricted |
| R-3 Single-Family Residential | 0.51 ac | 50 / 15 / 50 ft | 30 ft | Occupancy restricted |
| LL Large-Lot Single-Family Residential | 3 ac | 75 / 15 / 60 ft | 30 ft | Occupancy restricted |
| MHR Medium-High Residential | 0.26 ac | 35 / 10 / 35 ft | 30 ft | No |
| LMR Low-Medium Residential | 0.31 ac | 35 / 10 / 50 ft | 30 ft | No |
R-1 Single-Family Residential - The 75 ft front setback shown applies to an ordinary interior lot. It rises to 85 ft on a major highway, drops to 65 ft on a corner lot, and is 75 ft on a corner lot on a major highway.
- A nonresidential use takes a separate lot minimum of 5 acres, several times the residential figure; the width, setbacks, height and coverage are unchanged.
- A dwelling of more than one storey must have at least 2,160 sq ft of habitable floor area; the 1,440 sq ft figure is the one-storey case.
- On a flag lot, the minimum lot area must be held by the flag portion alone, behind the access strip; the pole is excluded.
- A lot on the exterior arc of a curve or cul-de-sac may narrow to 55 ft at the front lot line tangential to the arc.
- A driveway must stay 5 ft from the side lot line rather than meet the structure setback, except on a flag lot or a common driveway.
R-2 Single-Family Residential - The 60 ft front setback shown is for an ordinary interior lot. It becomes 70 ft on a major highway, 50 ft on a corner lot, and 60 ft on a corner lot on a major highway.
- A nonresidential use takes a separate lot minimum of 3 acres, several times the residential figure; the width, setbacks, height and coverage are unchanged.
- Minimum habitable floor area is 1,320 sq ft only for a one-storey dwelling; anything over one storey must have 1,800 sq ft.
- On a flag lot, the minimum lot area must be held by the flag portion alone, since the access pole is excluded, so the full minimum sits behind the strip.
- A lot on the exterior arc of a curve or cul-de-sac may narrow to 55 ft at the front lot line tangential to the arc.
- A driveway must stay 5 ft from the side line rather than meeting the structure setback, except on a flag lot or a common driveway.
R-3 Single-Family Residential - The 50 ft front setback shown applies to an ordinary interior lot. It becomes 60 ft on a major highway, 40 ft on a corner lot, and 50 ft on a corner lot that is also on a major highway.
- A nonresidential use takes a separate lot minimum of 2 acres, several times the residential figure; the width, setbacks, height and coverage are unchanged.
- A dwelling over one storey must have at least 1,560 sq ft of habitable floor area; the 1,200 sq ft figure is the one-storey case.
- On a flag lot, the minimum lot area must be met within the flag portion alone, since the access pole is excluded from the area.
- A lot on the exterior arc of a curve or cul-de-sac may narrow to 55 ft at the front lot line tangential to the arc.
- A driveway must be kept 5 ft from the side lot line rather than at the structure setback, except on a flag lot or a common driveway.
LL Large-Lot Single-Family Residential - Minimum livable floor area rises to 2,160 sq ft for a dwelling of more than one storey; the figure shown applies to a one-storey house.
- A nonresidential use takes a separate lot minimum of 5 acres, several times the residential figure; the width, setbacks, height and coverage are unchanged.
- On the exterior arc of a curve or cul-de-sac, a lot may narrow to 75 ft at the front lot line tangential to the arc.
- A driveway is held to 5 ft from the side line rather than the 15 ft structure setback, except on a flag lot or a common driveway.
- The district states no maximum lot coverage, and its stated intent is to keep both residential density and the amount of land under impervious surface low.
MHR Medium-High Residential - The figures shown cover single-family detached and semidetached homes only. Two-family, multiple-family, townhouse and garden home forms each carry their own lot minimum and dimensional standards. Total impervious area across the district is capped at 75%.
- A two-family dwelling takes 15,000 sq ft on the same 80 ft of width, with a 15 ft side yard, coverage raised to 25%, a 70 ft front setback on a major highway and 1,500 sq ft of floor area.
- A multiple-family development needs 5 acres at eight units per acre, a 75 ft front setback, three storeys, 40 ft between structures, and buildings to 250 ft with 25 ft paved access.
- Garden homes carry the largest threshold here: a ten-acre lot, eight units per acre, 1,000 sq ft per unit, 20 ft unit width, and a 50 ft buffer rising to 100 ft beside a single-family district.
- The front setback rises to 60 ft on a major highway, and a semidetached dwelling owes 15 ft total for both side yards where a detached dwelling owes 10 ft on each side.
- Minimum habitable floor area is 1,300 sq ft for a dwelling over one storey; the 1,000 sq ft figure shown applies to a one-storey dwelling.
- Site plan approval is required for multiple-family dwellings, townhouse buildings and garden homes under Chapter 269.
LMR Low-Medium Residential - Multiple-family dwellings were repealed from LMR on 7 January 2010, so the townhouse is the only multi-unit form left here. Total impervious area is capped at 75%, separate from the building coverage figure shown.
- A two-family dwelling takes 16,500 sq ft on the same 85 ft of width, with a 15 ft side yard, coverage raised to 25%, a 70 ft front setback on a major highway and 1,500 sq ft of floor area.
- A townhouse development needs 5 acres at 6 units per acre, units at least 20 ft wide, a 50 ft buffer rising to 150 ft beside a single-family district, 40 ft between structures, buildings to 165 ft.
- The front setback rises to 60 ft on a major highway, and a semidetached dwelling owes 15 ft at the side line where a detached dwelling owes 10 ft.
- Minimum habitable floor area is 1,300 sq ft for a dwelling over one storey; the 1,000 sq ft figure applies to a one-storey dwelling.
- A driveway must be held 5 ft from the side line, except on a flag lot or a common driveway.
- The figures shown are for single-family detached and semidetached dwellings; townhouse density is lower and buffers wider than in MHR, whose buffer is 100 ft.
Size limits on record in Webster
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 Webster:
- R-1 Single-Family ResidentialInside the house, the apartment is capped at 35% of the principal dwelling's gross floor area, not counting the apartment, or 750 sq ft, whichever is less.
- R-2 Single-Family ResidentialInside the house, the apartment is capped at 35% of the principal dwelling's gross floor area, not counting the apartment, or 750 sq ft, whichever is less.
- R-3 Single-Family ResidentialInside the house, the apartment is capped at 35% of the principal dwelling's gross floor area, not counting the apartment, or 750 sq ft, whichever is less.
- LL Large-Lot Single-Family ResidentialInside the house, the apartment is capped at 35% of the principal dwelling's gross floor area, not counting the apartment, or 750 sq ft, whichever is less.
- MHR Medium-High ResidentialInside the house, the apartment is capped at 35% of the principal dwelling's gross floor area, not counting the apartment, or 750 sq ft, whichever is less.
- LMR Low-Medium ResidentialInside the house, the apartment is capped at 35% of the principal dwelling's gross floor area, not counting the apartment, or 750 sq ft, whichever is less.
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 prohibited. Short-term rentals are prohibited in the Town of Webster. § 350-82A of zoning Chapter 350 specifically bars transient rentals, so short-term rental income should not be counted on for a property here. Separate question from whether you can build the unit.
- Zoning codeecode360.com/38728946
- Building dept(585) 872-7036
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Building a full house instead?
A standard-finish house in Webster runs $204–268/sq ft, about $370k–480k at 1,800 sq ft , plus roughly $6,382 for a conforming lot. See what it costs to build a house in Webster.
ADU cost in nearby towns
All Monroe County towns: Rochester (City) · Brighton · Chili · Clarkson · Gates · Greece · Hamlin · Henrietta · Irondequoit · Mendon · Ogden · Parma · Penfield · Perinton · Pittsford · Riga · Rush · Sweden · Wheatland · East Rochester (Village) · Brockport (Village) · Churchville (Village) · Fairport (Village) · Hilton (Village) · Honeoye Falls (Village) · Pittsford (Village) · Scottsville (Village) · Spencerport (Village) · Webster (Village)
More on Webster: Can you build an ADU here? · Zoning for new construction · Monroe County build cost
ADU cost in Webster: common questions
How much does it cost to build an ADU in Webster, NY?
An 800 sq ft accessory dwelling unit in Town of Webster 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 Webster?
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 Webster?
Town of Webster has 6 districts. 4 districts allow ADUs with occupancy restrictions, 2 do not. Check the specific address with the free ADU checker or confirm with the building department at (585) 872-7036 before you spend money on design.
Can I rent an ADU in Webster short term?
Short-term rentals prohibited for Town of Webster. 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 Webster vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-08-10; verify with the building department at (585) 872-7036 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.