Hillburn · ADU cost
What it costs to build an ADU in Hillburn, NY
An 800 sq ft accessory dwelling unit in Hillburn runs roughly $225k–295k, or about $280–366/sq ft, and that per-square-foot figure sits deliberately above the $229–300/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 Hillburn maps 8 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 Hillburn
Standard finish, slab on grade, on a flat and clear part of the lot, general-contractor margin included. These are structure costs at Rockland County pricing, before the service and septic work a second dwelling can trigger.
| Size | Type | $ / sq ft | Estimated total |
|---|---|---|---|
| 400 sq ft | Studio | $371–$485 | $150k–195k |
| 600 sq ft | One bedroom | $310–$406 | $185k–245k |
| 800 sq ft | One bedroom, larger | $280–$366 | $225k–295k |
| 1,000 sq ft | Two bedroom | $261–$342 | $260k–340k |
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 | $8–$12 | $8 | — |
| Foundation | $18–$26 | $18 | — |
| Framing | $29–$36 | $29 | — |
| Envelope - roof, siding, windows | $36–$46 | $36 | — |
| Plumbing | $31–$38 | $18 | 55% |
| Electrical | $21–$26 | $14 | 35% |
| HVAC | $26–$32 | $16 | 45% |
| Insulation | $8–$10 | $8 | — |
| Interior finishes | $36–$48 | $31 | 15% |
| Millwork & cabinetry | $21–$31 | $12 | 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 Hillburn 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 |
|---|---|---|---|---|
| Open Space | 5.51 ac | 75 / 50 / 100 ft | 35 ft | No |
| Residence | 1.38 ac | 50 / 40 / 75 ft | 35 ft | No |
| Residence | 0.21 ac | 25 / 10 / 30 ft | 35 ft | No |
| Residence | 0.14 ac | 25 / 10 / 30 ft | 35 ft | No |
| Local Shopping | 0.23 ac | 25 / 20 / 25 ft | 25 ft | No |
| General Commercial | 0.23 ac | 25 / 20 / 25 ft | 25 ft | No |
| Regional Economic Development | 0.46 ac | 25 / 20 / 25 ft | 25 ft | No |
| Light Industry | 0.92 ac | 35 / 50 / 50 ft | 25 ft | No |
Open Space - Hillburn sets bulk per use, not per district, and the figures shown are the row for a park or playground. Public utility lines and rights-of-way serving the immediate neighbourhood are permitted by right on the same figures.
- Utility lines and rights-of-way that do not serve the immediate neighbourhood need a special use permit and site plan approval, on the same figures.
- Accessory buildings have their own required distances, measured to the principal building, the front street, the side street, the exterior side lot line and the rear lot line.
Residence - The side setbacks must add to at least 100 ft, so 40 ft on each side falls twenty feet short. Hillburn sets bulk per use, so the figures shown are one row among several.
- A place of worship, a community facility, a fire, police or ambulance building or a public utility substation shares one row, by special use permit with site plan approval.
- That row's yards are deeper than a house's on every side: 200 ft of width, 75 ft in front, 50 ft to each side with 125 ft for the pair and 100 ft to the rear.
- An animal sanctuary is possible by special use permit, on three acres with a two-hundred-foot width.
- Accessory buildings have their own required distances, measured to the principal building, the front street, the side street, the exterior side lot line and the rear lot line.
Residence - The side setbacks must add to at least 30 ft, so 10 ft on each side falls ten feet short. Hillburn sets bulk per use, so the figures shown are one row among several.
- A two-family house needs 0.2755 acres on 120 ft of width, with 15 ft to each side and 40 ft for the pair, by special use permit with site plan approval.
- An assisted living facility is the one thing in this village allowed three storeys and 40 ft; everything else in a residence district stops at two and a half storeys and 35 ft.
- It needs 2 acres on 125 ft of width with 50 ft to every line and 100 ft for the pair of sides, and may cover 50% of the lot at a 0.5 floor area ratio, by special use permit.
- A place of worship, a community facility, a fire, police or ambulance building or a public utility substation shares a further row, by special use permit.
- That row needs 0.5051 acres on 125 ft of width, with 35 ft in front, 25 ft to each side, 60 ft for the pair and 50 ft to the rear.
- Accessory buildings have their own required distances, measured to the principal building, the front street, the side street, the exterior side lot line and the rear lot line.
Residence - On a lot of the 50 ft minimum width, the 10 ft side setbacks on each side leave thirty feet for the building. Hillburn sets bulk per use, so the figures shown are one row among several.
- A two-family house needs only two thousand square feet more than a one-family house, 0.1837 acres, with the same 50 ft of width, yards, height and coverage, by special use permit.
- A place of worship, a community facility, a fire, police or ambulance building or a public utility substation shares a further row, by special use permit.
- That row needs 0.5051 acres on 125 ft of width, with 35 ft in front, 8 ft to each side, 20 ft for the pair and 30 ft to the rear.
- Accessory buildings have their own required distances, measured to the principal building, the front street, the side street, the exterior side lot line and the rear lot line.
Local Shopping - The figures shown are the smallest of six tiers and cover only small shopfronts, such as a retail shop, bar, delicatessen, take-out restaurant, laundromat or personal service shop. Other uses have their own rows.
- A sit-down restaurant needs fifteen thousand square feet on a hundred feet of width, with forty percent coverage.
- A museum or a professional or medical office needs twenty thousand square feet on a hundred and twenty-five feet of width, at fifty percent coverage and a floor area ratio of 0.25.
- A commercial recreation use, grocery store, health and fitness facility or designed shopping centre needs forty thousand square feet on two hundred feet of width, 35 ft front and 50 ft each side.
- A heavier group, including automobile sales, a building contractor, a hotel, a cinema, a pharmacy and a warehouse, needs twenty thousand square feet on a hundred and fifty feet of width.
- Only that heavier group may reach three storeys and 35 ft, at a floor area ratio of 0.5.
- Where a use in that heavier group abuts a residence district, its 25 ft side and rear setbacks double to 50 ft and the combined side requirement rises by half.
General Commercial - The figures shown are the smallest of six tiers and only a delicatessen sits on them; every other use in the district has its own row.
- An automotive garage, building contractor, retail landscape materials yard, microbrewery or woodworking shop needs fifteen thousand square feet on a hundred feet of width, at forty percent coverage.
- A business, professional or medical office needs twenty thousand square feet on a hundred and twenty-five feet of width.
- Public utility distribution and substations need twenty-two thousand square feet, with an eight-foot side setback.
- A dry-cleaning depot, health and fitness facility, laboratory, light industry, commercial recreation use or wholesale warehouse needs forty thousand square feet on two hundred feet of width.
- A concrete mixing facility needs a hundred and twenty thousand square feet, with 75 ft to each side and 75 ft for the pair.
- Accessory buildings have their own required distances, measured to the principal building, the front street, the side street, the exterior side lot line and the rear lot line.
Regional Economic Development - The figures shown cover a day spa, a medical or professional office, tourism-related retail and a tourism-related winery. An antique shop or art gallery needs only ten thousand square feet on 75 ft of width, at thirty-five percent coverage and a ratio of 0.15.
- A sit-down restaurant needs fifteen thousand square feet.
- A commercial recreation use, health and fitness facility or designed shopping centre needs forty thousand square feet on two hundred feet of width.
- A bus depot, conference centre, cultural and performing arts centre, resort hotel or museum needs eighty thousand square feet on two hundred feet of width, with 50 ft to every line.
- Those uses need a hundred feet for the pair of side setbacks and may reach three storeys and 45 ft.
- Accessory buildings have their own required distances, measured to the principal building, the front street, the side street, the exterior side lot line and the rear lot line.
Light Industry - The figures shown cover light industry, a laboratory or research facility, commercial recreation, a health and fitness facility and a wholesale warehouse; all but the fitness facility need a special use permit as well as site plan approval.
- A building contractor or retail landscape materials yard needs fifteen thousand square feet on a hundred feet of width, 25 ft to the front, rear and each side, at forty percent coverage.
- A business, professional or medical office is permitted by right on twenty thousand square feet and a hundred and twenty-five feet of width, the least land any use needs here without a special permit.
- Public utility distribution and substations need twenty-two thousand square feet with an eight-foot side setback, and are the only rows here allowed two and a half storeys and 35 ft.
- Accessory buildings have their own required distances, measured to the principal building, the front street, the side street, the exterior side lot line and the rear lot line.
Size limits on record in Hillburn
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 Hillburn:
- Open SpaceLimits there include noncommercial use, no more than two operable vehicles parked, no more than 500 square feet of impervious paving, and no accessory building exceeding one story or 15 feet.
- ResidenceLimits there include noncommercial use, no more than two operable vehicles parked, no more than 500 square feet of impervious paving, and no accessory building exceeding one story or 15 feet.
- ResidenceLimits there include noncommercial use, no more than two operable vehicles parked, no more than 500 square feet of impervious paving, and no accessory building exceeding one story or 15 feet.
- ResidenceLimits there include noncommercial use, no more than two operable vehicles parked, no more than 500 square feet of impervious paving, and no accessory building exceeding one story or 15 feet.
- Local ShoppingLimits there include noncommercial use, no more than two operable vehicles parked, no more than 500 square feet of impervious paving, and no accessory building exceeding one story or 15 feet.
- General CommercialLimits there include noncommercial use, no more than two operable vehicles parked, no more than 500 square feet of impervious paving, and no accessory building exceeding one story or 15 feet.
- Regional Economic DevelopmentLimits there include noncommercial use, no more than two operable vehicles parked, no more than 500 square feet of impervious paving, and no accessory building exceeding one story or 15 feet.
- Light IndustryLimits there include noncommercial use, no more than two operable vehicles parked, no more than 500 square feet of impervious paving, and no accessory building exceeding one story or 15 feet.
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 Orange & Rockland 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 regulated in the Village of Hillburn. The zoning chapter defines the use at § 250-7, and Chapter 188, Rental Properties, carries a rental permit requirement at § 188-8. Separate question from whether you can build the unit.
- Zoning codeecode360.com/15367578
- Building dept(845) 357-2036
Sizing up a specific parcel in Hillburn?
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Building a full house instead?
A standard-finish house in Hillburn runs $229–300/sq ft, about $410k–540k at 1,800 sq ft , plus roughly $905 for a conforming lot. See what it costs to build a house in Hillburn.
ADU cost in nearby towns
All Rockland County towns: Clarkstown · Haverstraw · Orangetown · Ramapo · Stony Point · Airmont (Village) · Chestnut Ridge (Village) · Grand View-on-Hudson (Village) · Haverstraw (Village) · Montebello (Village) · New Hempstead (Village) · Nyack (Village) · Piermont (Village) · Pomona (Village) · Sloatsburg (Village) · Spring Valley (Village) · Suffern (Village) · Upper Nyack (Village) · Wesley Hills (Village) · West Haverstraw (Village)
More on Hillburn: Can you build an ADU here? · Zoning for new construction · Rockland County build cost
ADU cost in Hillburn: common questions
How much does it cost to build an ADU in Hillburn, NY?
An 800 sq ft accessory dwelling unit in Village of Hillburn runs roughly $225k–295k at a standard finish, general-contractor margin included. A 400 sq ft studio runs about $150k–195k. 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 Hillburn?
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 $280–366/sq ft against $229–300/sq ft for a full house, even though it is the same trades doing the same work.
Are ADUs allowed in Hillburn?
Village of Hillburn has 8 districts. All districts prohibit ADUs. Check the specific address with the free ADU checker or confirm with the building department at (845) 357-2036 before you spend money on design.
Can I rent an ADU in Hillburn short term?
Short-term rentals allowed with local rules for Village of Hillburn. 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 Rockland 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 Hillburn vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-07-21; verify with the building department at (845) 357-2036 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.