Montgomery · ADU cost
What it costs to build an ADU in Montgomery, NY
An 800 sq ft accessory dwelling unit in Montgomery runs roughly $205k–270k, or about $258–337/sq ft, and that per-square-foot figure sits deliberately above the $211–276/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 Montgomery maps 11 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 Montgomery
Standard finish, slab on grade, on a flat and clear part of the lot, general-contractor margin included. These are structure costs at Orange County pricing, before the service and septic work a second dwelling can trigger.
| Size | Type | $ / sq ft | Estimated total |
|---|---|---|---|
| 400 sq ft | Studio | $342–$447 | $135k–180k |
| 600 sq ft | One bedroom | $286–$374 | $170k–225k |
| 800 sq ft | One bedroom, larger | $258–$337 | $205k–270k |
| 1,000 sq ft | Two bedroom | $241–$315 | $240k–315k |
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–$11 | $8 | — |
| Foundation | $17–$24 | $17 | — |
| Framing | $26–$33 | $26 | — |
| Envelope - roof, siding, windows | $33–$42 | $33 | — |
| Plumbing | $29–$35 | $17 | 55% |
| Electrical | $19–$24 | $13 | 35% |
| HVAC | $24–$29 | $15 | 45% |
| Insulation | $8–$9 | $8 | — |
| Interior finishes | $33–$45 | $28 | 15% |
| Millwork & cabinetry | $19–$29 | $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 Montgomery 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 |
|---|---|---|---|---|
| Residential One-Family | 0.41 ac | 40 / 25 / 35 ft | 35 ft | No |
| Residential Agriculture One-Family | 0.28 ac | 35 / 15 / 35 ft | 35 ft | No |
| Residential One-Family | 0.18 ac | 30 / 10 / 30 ft | 35 ft | No |
| Residential One-Family | 0.23 ac | 30 / 10 / 20 ft | 35 ft | No |
| Residential One-Family | 0.11 ac | 25 / 10 / 25 ft | 35 ft | No |
| Residential One- and Two-Family | 0.11 ac | 25 / 10 / 25 ft | 35 ft | No |
| Multifamily Residential | 0.11 ac | 25 / 10 / 25 ft | 35 ft | No |
| Neighborhood Business | 0.23 ac | 50 / 20 / 30 ft | 35 ft | No |
| Village Business | — | 0 / 12 / 20 ft | 35 ft | No |
| Industrial Park | 0.92 ac | 50 / 30 / 50 ft | 35 ft | No |
| Village Industry | 0.11 ac | 0 / 12 / 20 ft | 35 ft | No |
Residential One-Family - The lot size shown is for land with both public sewer and public water. With only one of them a lot needs at least twenty thousand square feet, and with neither, building here is not permitted.
- The pair of side yards must total at least forty feet, so a house needs at least twenty-five feet on one side and at least fifteen on the other.
- A side yard on the street side of a corner lot must be at least thirty-five feet.
- Lot width is measured at the required minimum front yard depth, not at the street.
- Coverage counts main and accessory buildings together.
- An accessory building may cover up to 20% of the required rear yard, rise no more than 18 feet there, and must stand at least 5 feet from any lot line.
- Only a single-family detached dwelling is permitted; a two-family house, a multiple dwelling and a planned residential development are not.
Residential Agriculture One-Family - The lot size shown is for land with both public sewer and public water. With only one of them a lot needs at least eighteen thousand square feet, and with neither at least twenty thousand; RA-3 is the only district in the Village buildable without any public service.
- A side yard on the street side of a corner lot must be at least thirty-five feet, the full front-yard figure.
- A planned residential development, and a golf course or country club on at least fifty acres, are available by special exception.
- Lot width is measured at the required minimum front yard depth.
- Coverage counts main and accessory buildings together.
- An accessory building may cover up to 20% of the required rear yard, rise no more than 18 feet there, and must stand at least 5 feet from any lot line.
Residential One-Family - At least one public service is required: the lot size shown holds with public sewer and public water or with only one of them, but with neither, building here is not permitted.
- Because the pair of side yards must total at least twenty-five feet, one side yard must be at least fifteen feet.
- A side yard on the street side of a corner lot must be at least thirty feet.
- Lot width is measured at the required minimum front yard depth.
- Coverage counts main and accessory buildings together.
- A planned residential development is available by special exception.
- An accessory building may cover up to 25% of the required rear yard, rise no more than 18 feet there, and must stand at least 3 feet from any lot line.
Residential One-Family - At least one public service is required: the lot size shown holds with public sewer and public water or with only one of them, but with neither, building here is not permitted.
- A side yard on the street side of a corner lot must be at least thirty feet.
- Lot width is measured at the required minimum front yard depth.
- Coverage counts main and accessory buildings together.
- A planned residential development is available by special exception.
- An accessory building may cover up to 25% of the required rear yard, rise no more than 18 feet there, and must stand at least 3 feet from any lot line.
Residential One-Family - At least one public service is required: the lot size shown holds with public sewer and public water or with only one of them, but with neither, building here is not permitted.
- The pair of side yards must total at least eighteen feet, with at least ten feet on one side and at least eight on the other.
- A side yard on the street side of a corner lot must be at least twenty-five feet.
- Only a single-family detached dwelling is permitted.
- Lot width is measured at the required minimum front yard depth.
- Coverage counts main and accessory buildings together.
- An accessory building may cover up to 25% of the required rear yard, rise no more than 18 feet there, and must stand at least 3 feet from any lot line.
Residential One- and Two-Family - Nothing may be built here without both public sewer and public water. Each dwelling unit needs at least twenty-five hundred square feet of land, so the minimum lot carries a two-family house exactly and the per-unit figure is what binds.
- A two-family detached dwelling and a one-family semidetached dwelling are permitted by right; converting an existing single-family detached house for two families needs a special exception.
- A side yard on the street side of a corner lot must be at least twenty-five feet.
- Lot width is measured at the required minimum front yard depth.
- Coverage counts main and accessory buildings together.
- An accessory building may cover up to 35% of the required rear yard, rise no more than 20 feet there, and must stand at least 3 feet from any lot line.
Multifamily Residential - Land per unit rises with bedroom count: at least five thousand square feet for an efficiency, one- or two-bedroom unit, at least five thousand five hundred for three bedrooms, and at least eight thousand seven hundred for four or more. That ladder sizes a building here.
- Nothing may be built here without both public sewer and public water.
- The pair of side yards must total at least fifteen feet, with at least ten feet on one side and at least five on the other.
- A side yard on the street side of a corner lot must be at least twenty-five feet.
- RM-1 is the only district in the Village where a multiple dwelling is permitted.
- Coverage counts main and accessory buildings together.
- An accessory building may cover up to 35% of the required rear yard, rise no more than 20 feet there, and must stand at least 3 feet from any lot line.
Neighborhood Business - No dwelling of any kind is permitted in B-1. A side yard on the street side of a corner lot must be at least fifty feet, the full front-yard figure.
- Lot width is measured at the building line.
- Coverage counts main and accessory buildings together.
- No separate rear-yard coverage or height limit applies to accessory buildings.
Village Business - No side yard is required, but one that is provided must be at least twelve feet. The rear yard is the only yard that always binds.
- The district requires no lot area, no lot width and no front yard, which lets buildings stand on the street line.
- No combined side yard and no corner-lot side yard apply.
- A dwelling is possible only above a first-floor nonresidential use, and only by special exception.
- Coverage counts main and accessory buildings together.
- No separate rear-yard coverage or height limit applies to accessory buildings.
Industrial Park - No dwelling of any kind is permitted in I-1. A side yard on the street side of a corner lot must be at least fifty feet.
- Lot width is measured at the building line.
- Coverage counts main and accessory buildings together.
- No separate rear-yard coverage or height limit applies to accessory buildings.
Village Industry - No front yard and no corner-lot side yard are required. The pair of side yards must total at least twenty feet, with at least twelve feet on one side and at least eight on the other.
- No dwelling of any kind is permitted.
- Lot width is measured at the building line.
- Coverage counts main and accessory buildings together.
- No separate rear-yard coverage or height limit applies to accessory buildings.
Size limits on record in Montgomery
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 Montgomery:
- Village BusinessDwelling units in B-2 need at least 2,500 square feet of lot area each.
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. Short-term rentals in the Village of Montgomery need a short-term rental permit under the Village's Short-Term Rental Properties chapter, with an annual inspection. Separate question from whether you can build the unit.
- Zoning codeecode360.com/13326521
- Building dept(845) 457-9621
Sizing up a specific parcel in Montgomery?
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Building a full house instead?
A standard-finish house in Montgomery runs $211–276/sq ft, about $380k–495k at 1,800 sq ft . See what it costs to build a house in Montgomery.
ADU cost in nearby towns
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More on Montgomery: Can you build an ADU here? · Zoning for new construction · Orange County build cost
ADU cost in Montgomery: common questions
How much does it cost to build an ADU in Montgomery, NY?
An 800 sq ft accessory dwelling unit in Village of Montgomery runs roughly $205k–270k at a standard finish, general-contractor margin included. A 400 sq ft studio runs about $135k–180k. 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 Montgomery?
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 $258–337/sq ft against $211–276/sq ft for a full house, even though it is the same trades doing the same work.
Are ADUs allowed in Montgomery?
Village of Montgomery has 11 districts. All districts prohibit ADUs. Check the specific address with the free ADU checker or confirm with the building department at (845) 457-9621 before you spend money on design.
Can I rent an ADU in Montgomery short term?
Short-term rentals allowed with local rules for Village of Montgomery. 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 Orange 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 Montgomery vary by district and often carry exceptions this page does not show; zoning last reviewed 2026-09-15; verify with the building department at (845) 457-9621 before relying on any answer. Sources: the Offsite / Aldo cost engine and zoning dataset.