Thompson · ADU cost
What it costs to build an ADU in Thompson, NY
An 800 sq ft accessory dwelling unit in Thompson runs roughly $190k–250k, or about $238–312/sq ft, and that per-square-foot figure sits deliberately above the $195–256/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 Thompson maps 8 districts that treat accessory units differently from one another, so read the table below as a starting point rather than a verdict.
Check a specific parcel for an ADU in Aldo, free →
ADU cost by size in Thompson
Standard finish, slab on grade, on a flat and clear part of the lot, general-contractor margin included. These are structure costs at Sullivan County pricing, before the service and septic work a second dwelling can trigger.
| Size | Type | $ / sq ft | Estimated total |
|---|---|---|---|
| 400 sq ft | Studio | $316–$414 | $125k–165k |
| 600 sq ft | One bedroom | $264–$346 | $160k–210k |
| 800 sq ft | One bedroom, larger | $238–$312 | $190k–250k |
| 1,000 sq ft | Two bedroom | $223–$292 | $225k–290k |
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–$10 | $7 | — |
| Foundation | $16–$22 | $16 | — |
| Framing | $24–$30 | $24 | — |
| Envelope - roof, siding, windows | $30–$39 | $30 | — |
| Plumbing | $26–$32 | $16 | 55% |
| Electrical | $18–$23 | $12 | 35% |
| HVAC | $22–$27 | $14 | 45% |
| Insulation | $7–$9 | $7 | — |
| Interior finishes | $31–$41 | $26 | 15% |
| Millwork & cabinetry | $18–$26 | $10 | 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 Thompson 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 |
|---|---|---|---|---|
| Suburban Residential District | 0.92 ac | 50 / 20 / 50 ft | 30 ft | As of right |
| Rural Residential-1 District | 0.92 ac | 50 / 20 / 50 ft | 30 ft | As of right |
| Rural Residential-2 District | 2 ac | 50 / 20 / 50 ft | 30 ft | As of right |
| Highway Commercial-1 District | 0.46 ac | 40 / 25 / 50 ft | 35 ft | No |
| Highway Commercial-2 District | 0.46 ac | 40 / 25 / 50 ft | 35 ft | No |
| Commercial Industrial District | 3 ac | 40 / 25 / 50 ft | 35 ft | No |
| Extractive Industry District | 50 ac | 200 / 50 / 50 ft | 35 ft | No |
| East Broadway Gateway District | 0.92 ac | 40 / 25 / 50 ft | 35 ft | No |
Suburban Residential District - A home here needs 40,000 square feet, 0.9183 acre, on a lot 150 feet wide and 150 feet deep, set back 50 feet at the front, 50 feet at the rear and 20 feet on one side, with 50 feet for both sides together. Buildings cover up to 10% of the lot and reach 30 feet.
- With central water or sewer the dwelling takes 20,000 square feet, 0.4591 acre, on 100 feet of width and 125 feet of depth, with 40 foot front and rear yards, 15 feet one side and 40 feet both sides.
- That smaller lot carries 20% coverage, 2.0 units per acre and 1,000 square feet of habitable floor area per unit, at the same 30 feet of height.
- A cluster development for one-family detached dwellings under § 250-26 needs site plan review and takes 10 acres plus 7,500 square feet per unit, on lots 65 feet wide and 90 feet deep.
- Those cluster lots take 30 foot front and rear yards, 10 feet one side, 25 feet both sides, 2.0 units per acre and 25% coverage.
- Multiple dwellings under § 250-28 need site plan review on 10 acres, 150 feet of width and depth, 40 feet front, 50 feet rear, 30 feet one side, 50 feet both sides and 35 feet of height.
- Row houses and attached dwellings under § 250-28 need a special use permit on 10 acres, lots 22 feet wide and 100 feet deep, 30 feet front, 35 feet rear, 4 feet one side and 25 feet both sides.
Rural Residential-1 District - A home here needs 40,000 square feet, 0.9183 acre, on a lot 150 feet wide and 150 feet deep, set back 50 feet at the front, 50 feet at the rear and 20 feet on one side, with 50 feet for both sides together. Buildings cover up to 10% of the lot and reach 30 feet.
- A dwelling must hold at least 1,000 square feet of habitable floor area per unit and be at least 20 feet wide, at 1.0 dwelling unit per acre.
- A two-family dwelling needs site plan review and takes the same lot area, width, depth, yards, height and coverage as a one-family dwelling.
- A mobile home park under § 250-25 needs site plan review on 10 acres, lots 400 feet wide and 400 feet deep, 80 feet front, 50 feet rear, 50 feet one side and 100 feet both sides.
- That park takes 4.0 units per acre, 20% coverage and 20 feet of height, with 720 square feet per unit at 12 feet of width.
- A planned unit development under § 250-27 needs site plan review on 30 acres, lots 500 feet wide and 500 feet deep, 100 feet front, 50 feet rear, 50 feet one side and 100 feet both sides.
- That development takes 1,000 square feet of habitable floor area, 2.0 units per acre, 15% coverage and 30 feet of height.
Rural Residential-2 District - A home here needs 2 acres on a lot 150 feet wide and 300 feet deep, set back 50 feet at the front, 50 feet at the rear and 20 feet on one side, with 50 feet for both sides together. Buildings cover up to 10% of the lot and reach 30 feet.
- A dwelling must hold at least 1,000 square feet of habitable dwelling area and be at least 20 feet wide, at 1.0 dwelling unit per acre.
- A two-family dwelling needs site plan review and takes the same lot area, width, depth, yards, height and coverage as a one-family dwelling.
- A cluster development under § 250-26 needs site plan review on 10 acres plus 7,500 square feet per dwelling unit, with lots 65 feet wide and 90 feet deep.
- Those cluster lots take 30 foot front and rear yards, 10 feet one side, 25 feet both sides, 0.5 dwelling unit per acre, 30% coverage and 35 feet of height.
- Summer camps and campgrounds under §§ 250-31 and 250-34 take 10 acres, lots 400 feet wide and deep, 100 feet front, 50 feet rear, 50 feet one side, 100 feet both sides and 10% coverage.
- An airport takes 100 acres, lots 1,000 feet wide and 4,000 feet deep, 50 foot front, rear and one side yards, 100 feet both sides, 15% coverage and 45 feet of height.
Highway Commercial-1 District - A shop, restaurant or service building here needs 20,000 square feet, 0.4591 acre, on a lot 100 feet wide and 125 feet deep, set back 40 feet at the front, 50 feet at the rear and 25 feet on one side, with 50 feet for both sides, at 30% coverage and 35 feet.
- A one-family dwelling with central water and sewer takes 20,000 square feet, 0.4591 acre, 100 feet of width, 125 feet of depth, 40 foot front and rear yards, 15 feet one side and 40 feet both sides.
- Without central water and sewer that dwelling takes 40,000 square feet, 0.9183 acre, 150 feet of width and depth, 50 foot front and rear yards, 20 feet one side, 50 feet both sides and 10% coverage.
- A dwelling holds at least 1,000 square feet of habitable floor area and reaches 30 feet, at 2.0 dwelling units per acre on the smaller lot and 1.0 on the larger, with 20% coverage on the smaller.
- Multiple dwellings under § 250-28 need a special use permit on 10 acres, 150 feet of width and depth, 40 feet front, 50 feet rear, 30 feet one side, 50 feet both sides and 20% coverage.
- Row houses and attached dwellings under § 250-28 need a special use permit on 10 acres, lots 22 feet wide and 100 feet deep, 30 feet front, 35 feet rear, 4 feet one side and 25 feet both sides.
- A mobile home park under § 250-25 needs a special use permit on 10 acres, lots 400 feet wide and deep, 80 feet front, 50 feet rear, 50 feet one side, 100 feet both sides and 7 units per acre.
Highway Commercial-2 District - A shop, restaurant or service building here needs 20,000 square feet, 0.4591 acre, on a lot 100 feet wide and 125 feet deep, set back 40 feet at the front, 50 feet at the rear and 25 feet on one side, with 50 feet for both sides, at 30% coverage and 35 feet.
- A one-family dwelling with central water and sewer takes 20,000 square feet, 0.4591 acre, 100 feet of width, 125 feet of depth, 40 foot front and rear yards, 15 feet one side and 40 feet both sides.
- Without central water and sewer that dwelling takes 40,000 square feet, 0.9183 acre, 150 feet of width and depth, 50 foot front and rear yards, 20 feet one side, 50 feet both sides and 10% coverage.
- A dwelling holds at least 1,000 square feet of habitable floor area and reaches 30 feet, at 2.0 dwelling units per acre on the smaller lot and 1.0 on the larger, with 20% coverage on the smaller.
- Multiple dwellings under § 250-28 need a special use permit on 10 acres, 150 feet of width and depth, 40 feet front, 50 feet rear, 30 feet one side, 50 feet both sides and 25% coverage.
- Row houses and attached dwellings under § 250-28 need a special use permit on 10 acres, lots 22 feet wide and 100 feet deep, 30 feet front, 35 feet rear, 4 feet one side and 25 feet both sides.
- A mobile home park under § 250-25 needs a special use permit on 10 acres, lots 400 feet wide and deep, 80 feet front, 50 feet rear, 50 feet one side, 100 feet both sides and 7 units per acre.
Commercial Industrial District - A manufacturing or processing building here needs 3 acres on a lot 250 feet wide and 300 feet deep, set back 40 feet at the front, 50 feet at the rear and 25 feet on one side, with 50 feet for both sides, at 30% coverage and 35 feet.
- No use is permitted outright in the district; every use on the schedule needs site plan review and a special use permit.
- Retail stores, eating places, funeral homes and motor vehicle service stations take 40,000 square feet on 150 feet of width and depth, 50 feet front and rear, 35 feet one side and 70 feet both sides.
- A bed-and-breakfast or inn takes 20,000 square feet with central sewer and 40,000 without, on 100 or 150 feet of width and 125 or 150 feet of depth, at 30% coverage and 35 feet.
- A hotel or motel with central sewer takes 1 acre plus 2,000 square feet per unit over 50 units, and without central sewer 10 acres plus 10,000 square feet per unit over 12 units.
- Hotels and motels sit on 600 feet of width and 300 feet of depth, 100 feet front, 50 feet rear, 50 feet one side and 100 feet both sides, with 250 square feet per unit and 30% coverage.
- Summer camps, bungalow colonies and campgrounds under §§ 250-31 and 250-34 take 10 acres, 400 feet of width and depth, 150 feet front and rear, 100 feet one side and 200 feet both sides.
Extractive Industry District - Quarrying, processing and the stockpiling and distribution of quarry products here need 50 acres on a lot 800 feet wide and 1,000 feet deep, set back 200 feet at the front, 50 feet at the rear and 50 feet on one side, with 100 feet for both sides, and reach 35 feet.
- No use is permitted outright in the district; every use on the schedule needs site plan review by the Planning Board.
- Every row of the schedule reads N/A for habitable dwelling area, density per acre and percentage of lot coverage.
- Storage and maintenance facilities for trucks and equipment, warehouses for supplies, petroleum storage and manufacturing facilities using aggregates are accessory uses here.
East Broadway Gateway District - A retail store, office, eating place or motor vehicle repair shop here needs 40,000 square feet, 0.9183 acre, on a lot 200 feet wide and 200 feet deep, set back 40 feet at the front, 50 feet at the rear and 25 feet on one side, at 30% coverage and 35 feet.
- Those business uses need site plan review and a special use permit, and keep 50 feet for both side yards together.
- A one-family dwelling with central water and sewer takes 20,000 square feet, 0.4591 acre, 100 feet of width, 125 feet of depth, 40 foot front and rear yards, 15 feet one side and 40 feet both sides.
- Without central water and sewer that dwelling takes 40,000 square feet, 150 feet of width and depth, 50 foot front and rear yards, 20 feet one side, 50 feet both sides and 10% coverage.
- A dwelling holds at least 1,000 square feet of habitable floor area, is at least 20 feet wide and reaches 30 feet, at 2.0 dwelling units per acre with central service and 1.0 without.
- A mixed-use development needs a special use permit on 10 acres, 150 feet of width, 300 feet of depth, 50 foot front and rear yards, 35 feet one side, 70 feet both sides and 4 units per acre.
- A hotel and motel takes 7,500 square feet plus 1,500 square feet per rental unit, on 200 feet of width and depth, 40 feet front, 50 feet rear, 25 feet one side and 50 feet both sides.
Size limits on record in Thompson
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 Thompson:
- Suburban Residential DistrictLand needed per house: SR 20,000 sq ft with central water or sewer or 40,000 sq ft without, RR-1 40,000 sq ft, RR-2 2 acres.
- Rural Residential-1 DistrictLand needed per house: SR 20,000 sq ft with central water or sewer or 40,000 sq ft without, RR-1 40,000 sq ft, RR-2 2 acres.
- Rural Residential-2 DistrictLand needed per house: SR 20,000 sq ft with central water or sewer or 40,000 sq ft without, RR-1 40,000 sq ft, RR-2 2 acres.
- Highway Commercial-1 DistrictLand needed per house: SR 20,000 sq ft with central water or sewer or 40,000 sq ft without, RR-1 40,000 sq ft, RR-2 2 acres.
- Highway Commercial-2 DistrictLand needed per house: SR 20,000 sq ft with central water or sewer or 40,000 sq ft without, RR-1 40,000 sq ft, RR-2 2 acres.
- Commercial Industrial DistrictLand needed per house: SR 20,000 sq ft with central water or sewer or 40,000 sq ft without, RR-1 40,000 sq ft, RR-2 2 acres.
- Extractive Industry DistrictLand needed per house: SR 20,000 sq ft with central water or sewer or 40,000 sq ft without, RR-1 40,000 sq ft, RR-2 2 acres.
- East Broadway Gateway DistrictLand needed per house: SR 20,000 sq ft with central water or sewer or 40,000 sq ft without, RR-1 40,000 sq ft, RR-2 2 acres.
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 NYSEG 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. As of June 2026 the Town of Thompson has no local law specific to short-term rentals. That is not the same as permission, so confirm with the building department before counting on renting short term. Separate question from whether you can build the unit.
- Zoning codeecode360.com/TH0750
- Building dept(845) 794-2500
Sizing up a specific parcel in Thompson?
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 Thompson runs $195–256/sq ft, about $350k–460k at 1,800 sq ft , plus roughly $13,572 for a conforming lot. See what it costs to build a house in Thompson.
ADU cost in nearby towns
All Sullivan County towns: Bethel · Callicoon · Cochecton · Delaware · Fallsburg · Forestburgh · Fremont · Highland · Liberty · Lumberland · Mamakating · Neversink · Rockland · Tusten · Bloomingburg (Village) · Jeffersonville (Village) · Liberty (Village) · Monticello (Village) · Woodridge (Village)
More on Thompson: Can you build an ADU here? · Zoning for new construction · Sullivan County build cost
ADU cost in Thompson: common questions
How much does it cost to build an ADU in Thompson, NY?
An 800 sq ft accessory dwelling unit in Town of Thompson runs roughly $190k–250k at a standard finish, general-contractor margin included. A 400 sq ft studio runs about $125k–165k. 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 Thompson?
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 $238–312/sq ft against $195–256/sq ft for a full house, even though it is the same trades doing the same work.
Are ADUs allowed in Thompson?
Town of Thompson has 8 districts. 5 districts prohibit ADUs, 3 allow them. Check the specific address with the free ADU checker or confirm with the building department at (845) 794-2500 before you spend money on design.
Can I rent an ADU in Thompson short term?
No local short-term-rental law on record - which is not the same as permission for Town of Thompson. 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 Sullivan 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 Thompson vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-10-07; verify with the building department at (845) 794-2500 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.