Irondequoit · ADU cost
What it costs to build an ADU in Irondequoit, NY
An 800 sq ft accessory dwelling unit in Irondequoit 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 Irondequoit maps 4 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 Irondequoit
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 Irondequoit 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 Residential | 0.22 ac | 30 / 8 / 30 ft | 30 ft | Special permit |
| R-2 Residential | 0.22 ac | 30 / 8 / 30 ft | 30 ft | Special permit |
| R-3 Residential | 0.22 ac | 30 / 8 / 30 ft | 30 ft | Special permit |
| R-R Rural Residential | 1.03 ac | 50 / 15 / 50 ft | 30 ft | Special permit |
R-1 Residential - The side setback shown is a formula at the district's minimum width: 10% of lot width, never less than 5 ft and capped at 15 ft in R-1, so it grows on a wider lot. A corner lot owes the full front setback on every yard adjacent to a street.
- Minimum total floor area is 1,100 sq ft whether the dwelling is one storey or one and a half to two storeys, with a minimum ground floor area of 600 sq ft on the taller forms.
- Floor area counts only conditioned interior space along the main foundation walls; attached garages, three-season rooms, porches and breezeways are excluded.
- An approved tiny house is expressly exempt from the minimum building size.
- A through lot owes the full front setback on both streets for principal and accessory structures alike, with boundary fencing excepted.
- A deck attached to the principal structure may come within 20 ft of the rear lot line, where the building itself may not.
- Detached accessory structures may not sit in a front yard and, private garages excepted, must keep four feet to a rear or side property line.
R-2 Residential - The side yard is 10% of the lot width, never less than 5 ft and capped at 15 ft, so it grows on a wider lot until it reaches 15 ft. A corner lot owes the full front setback on every yard next to a street.
- A through lot owes the front setback on both streets for the house and accessory structures, boundary fencing excepted.
- The 900 sq ft minimum counts only conditioned interior space; attached garages, three-season rooms, porches and breezeways do not count.
- A dwelling of one and a half to two storeys also needs at least 550 sq ft of ground floor area.
- An entry porch, deck or stoop under 60 sq ft and 6 ft deep may project into the front yard, keeping 20 ft to the front lot line if attached; 20 sq ft and 4 ft if the house fails the front setback.
- A deck attached to the house may come within 20 ft of the rear lot line.
- Detached accessory structures may not stand in a front yard and, except private garages, must keep 4 ft from side and rear property lines.
R-3 Residential - The 8 ft side yard is a formula, not a fixed figure: 10% of lot width, never less than 5 ft and capped at 15 ft in R-3, so it grows on a wider lot. A corner lot owes the full 30 ft front setback on every yard next to a street.
- A through lot owes the front setback on both streets, for principal and accessory structures alike, with boundary fencing excepted.
- The 750 sq ft minimum counts only conditioned interior space along the main foundation walls; attached garages, three-season rooms, porches and breezeways are excluded.
- A dwelling of one and a half to two storeys also needs a ground floor area of at least 500 sq ft.
- An approved tiny house is exempt from the minimum building size.
- A deck attached to the principal structure may come within 20 ft of the rear lot line, closer than the building itself may sit.
- Detached accessory structures may not sit in a front yard and, private garages excepted, must keep four feet to a rear or side property line.
R-R Rural Residential - The front setback is a range, not just a floor: a house must sit at least 50 ft back and no more than 75 ft from the front lot line. The side yard is 10% of lot width, never less than 5 ft and capped at 25 ft, so it grows on a wider lot.
- A corner lot owes the full front setback on every yard adjacent to a street, and a through lot owes it on both streets for principal and accessory structures alike, boundary fencing excepted.
- On a lot inside a curve, the front yard is measured from the nearest point of the arc.
- A deck attached to the principal structure may come within 20 ft of the rear lot line, where the building itself may not.
- A porch, deck or stoop and its covering may project into the required front yard if it is under 60 sq ft and less than six feet deep, and an attached platform must keep 20 ft to the front lot line.
- Detached accessory structures may not sit in a front yard and, private garages excepted, must keep four feet to a rear or side property line.
- The 1,300 sq ft minimum floor area counts only conditioned interior space and expressly exempts an approved tiny house; one and a half to two storey forms also need a 700 sq ft ground floor.
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 local rules. Short term rentals are prohibited here except under a town permit. § 235-76.50 states the prohibition and § 235-76.52 requires the permit, both in Article XIIIB of Chapter 235, so the use is available only on the terms that article sets. Separate question from whether you can build the unit.
- Zoning codeecode360.com/6407632
- Building dept(585) 336-6020
Sizing up a specific parcel in Irondequoit?
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 Irondequoit runs $204–268/sq ft, about $370k–480k at 1,800 sq ft , plus roughly $11,886 for a conforming lot. See what it costs to build a house in Irondequoit.
ADU cost in nearby towns
All Monroe County towns: Rochester (City) · Brighton · Chili · Clarkson · Gates · Greece · Hamlin · Henrietta · Mendon · Ogden · Parma · Penfield · Perinton · Pittsford · Riga · Rush · Sweden · Webster · 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 Irondequoit: Can you build an ADU here? · Zoning for new construction · Monroe County build cost
ADU cost in Irondequoit: common questions
How much does it cost to build an ADU in Irondequoit, NY?
An 800 sq ft accessory dwelling unit in Town of Irondequoit 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 Irondequoit?
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 Irondequoit?
Town of Irondequoit has 4 districts. All districts allow ADUs with a special-use permit. Check the specific address with the free ADU checker or confirm with the building department at (585) 336-6020 before you spend money on design.
Can I rent an ADU in Irondequoit short term?
Short-term rentals allowed with local rules for Town of Irondequoit. 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 Irondequoit vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-08-07; verify with the building department at (585) 336-6020 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.