Washington · ADU cost
What it costs to build an ADU in Washington, NY
An 800 sq ft accessory dwelling unit in Washington runs roughly $210k–275k, or about $263–345/sq ft, and that per-square-foot figure sits deliberately above the $215–282/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 Washington maps 9 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 Washington
Standard finish, slab on grade, on a flat and clear part of the lot, general-contractor margin included. These are structure costs at Dutchess County pricing, before the service and septic work a second dwelling can trigger.
| Size | Type | $ / sq ft | Estimated total |
|---|---|---|---|
| 400 sq ft | Studio | $349–$457 | $140k–185k |
| 600 sq ft | One bedroom | $292–$382 | $175k–230k |
| 800 sq ft | One bedroom, larger | $263–$345 | $210k–275k |
| 1,000 sq ft | Two bedroom | $246–$322 | $245k–320k |
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 | $17–$24 | $17 | — |
| Framing | $27–$34 | $27 | — |
| Envelope - roof, siding, windows | $34–$43 | $34 | — |
| Plumbing | $29–$36 | $17 | 55% |
| Electrical | $19–$25 | $13 | 35% |
| HVAC | $24–$30 | $15 | 45% |
| Insulation | $8–$10 | $8 | — |
| Interior finishes | $34–$46 | $29 | 15% |
| Millwork & cabinetry | $19–$29 | $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 Washington 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 |
|---|---|---|---|---|
| Rural Residential 10-Acre | 10 ac | 100 / 100 / 75 ft | 35 ft | Special permit |
| Rural Residential Special 10-Acre | 10 ac | 100 / 100 / 75 ft | 35 ft | Special permit |
| Low Density Residential 5-Acre | 5 ac | 100 / 75 / 75 ft | 35 ft | Special permit |
| Low Density Residential Special | 5 ac | 100 / 75 / 75 ft | 35 ft | Special permit |
| Medium Density Residential 2-Acre | 2 ac | 75 / 50 / 50 ft | 35 ft | Special permit |
| High Density Residential 1-Acre | 1 ac | 50 / 30 / 25 ft | 35 ft | Special permit |
| Hamlet Mixed-Use | 1 ac | 50 / 30 / 25 ft | 35 ft | Special permit |
| Land Conservation | 10 ac | 100 / 100 / 75 ft | 15 ft | No |
| Hamlet (floating district) | 0.25 ac | 20 / 15 / 25 ft | 35 ft | No |
Rural Residential 10-Acre, Rural Residential Special 10-Acre - Height is capped at 2 1/2 stories as well as 35 ft, and the lot needs at least 400 ft of road frontage, a separate requirement from lot width.
- Rear lot frontage is governed by § 165-68.
Low Density Residential 5-Acre, Low Density Residential Special - Height is capped at 2 1/2 stories as well as 35 ft, and the lot needs at least 300 ft of road frontage, a separate requirement from lot width.
- Rear lot frontage is governed by § 165-68.
Medium Density Residential 2-Acre, High Density Residential 1-Acre - Height is capped at 2 1/2 stories as well as 35 ft, and the lot needs at least 200 ft of road frontage, a separate requirement from lot width.
- Rear lot frontage is governed by § 165-68.
Hamlet Mixed-Use - The figures shown are for residential uses. A non-residential use needs half an acre, 100 ft of lot width, an 80 ft front yard, a 15 ft side yard and a 50 ft rear yard, with coverage up to 60%.
- A residential building is also capped at 2 1/2 stories; the non-residential cap is 35 ft.
- Minimum road frontage is 100 ft.
- Rear lot frontage is governed by § 165-68.
Land Conservation - Height is capped at 1 story as well as 15 ft, the lowest in the Town, and the lot needs at least 400 ft of road frontage, a separate requirement from lot width.
- Rear lot frontage is governed by § 165-68.
Hamlet (floating district) - H is a floating district under § 165-13 that applies only once the Town Board maps it, replacing the district previously in place. The quarter-acre minimum is a floor, and the lot must have any greater area the Dutchess County Department of Health requires.
- On an unsewered lot, the Health Department's requirement is usually a great deal more.
- The front and side yards also have maximums of 50 ft each, build-to limits on top of the minimums shown.
- Height is capped at 2 1/2 stories as well as 35 ft.
- Minimum road frontage is 50 ft.
- Rear lot frontage is governed by § 165-68.
Size limits on record in Washington
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 Washington:
- Rural Residential 10-AcreA converted unit must be at least 800 sq ft, with one converted building per lot, though the Planning Board may allow up to three.
- Rural Residential Special 10-AcreA converted unit must be at least 800 sq ft, with one converted building per lot, though the Planning Board may allow up to three.
- Low Density Residential 5-AcreA converted unit must be at least 800 sq ft, with one converted building per lot, though the Planning Board may allow up to three.
- Low Density Residential SpecialA converted unit must be at least 800 sq ft, with one converted building per lot, though the Planning Board may allow up to three.
- Medium Density Residential 2-AcreA converted unit must be at least 800 sq ft, with one converted building per lot, though the Planning Board may allow up to three.
- High Density Residential 1-AcreA converted unit must be at least 800 sq ft, with one converted building per lot, though the Planning Board may allow up to three.
- Hamlet Mixed-UseA converted unit must be at least 800 sq ft, with one converted building per lot, though the Planning Board may allow up to three.
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 Central Hudson 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. A short-term rental needs a Town permit under Chapter 115, adopted 1-31-2024, and § 115-3 caps how many permits may be issued. Advertising the property is itself treated as evidence of short-term rental use (§ 115-2). Separate question from whether you can build the unit.
- Zoning codeecode360.com/37495610
- Building dept(845) 677-3419
Sizing up a specific parcel in Washington?
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 Washington runs $215–282/sq ft, about $390k–510k at 1,800 sq ft , plus roughly $8,033 for a conforming lot. See what it costs to build a house in Washington.
ADU cost in nearby towns
All Dutchess County towns: Beacon (City) · Poughkeepsie (City) · Amenia · Beekman · Clinton · Dover · East Fishkill · Fishkill · Hyde Park · La Grange · Milan · North East · Pawling · Pine Plains · Pleasant Valley · Poughkeepsie · Red Hook · Rhinebeck · Stanford · Union Vale · Wappinger · Fishkill (Village) · Millbrook (Village) · Millerton (Village) · Pawling (Village) · Red Hook (Village) · Rhinebeck (Village) · Tivoli (Village) · Wappingers Falls (Village)
More on Washington: Can you build an ADU here? · Zoning for new construction · Dutchess County build cost
ADU cost in Washington: common questions
How much does it cost to build an ADU in Washington, NY?
An 800 sq ft accessory dwelling unit in Town of Washington runs roughly $210k–275k at a standard finish, general-contractor margin included. A 400 sq ft studio runs about $140k–185k. 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 Washington?
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 $263–345/sq ft against $215–282/sq ft for a full house, even though it is the same trades doing the same work.
Are ADUs allowed in Washington?
Town of Washington has 9 districts. 7 districts allow ADUs with a special-use permit, 2 do not. Check the specific address with the free ADU checker or confirm with the building department at (845) 677-3419 before you spend money on design.
Can I rent an ADU in Washington short term?
Short-term rentals allowed with local rules for Town of Washington. 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 Dutchess 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 Washington vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-08-16; verify with the building department at (845) 677-3419 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.