Can you build an ADU in Williamsville, NY?

R-1 Single-Family Residential District - The front setback is a band, not a floor: at least 25 ft and no more than 50 ft, or the average of the existing principal buildings within 150 ft either way. The rear yard is 25% of the lot depth and never less than 25 ft, so a deep lot needs more than that floor.

• A side yard used for vehicle ingress or egress must be at least 12 ft.
• On a corner lot the exterior side yard must be at least 15% of the lot width, never less than 10 ft, and need not exceed the front yard required along the street it faces.
• The 25% coverage cap is for the principal building alone, and it rises to 35% with accessory buildings counted.
• No principal building may stand within 10 ft of another unless it is built to one hour of fire protection, and never closer than 8 ft.
• A two-storey house needs at least 1,500 sq ft of floor area, against 1,200 for a single storey.
• On a through lot a front yard equivalent takes the place of the rear yard.

R-2 Single-Family Residential District - The front setback is a band, not a floor: at least 25 ft and no more than 50 ft, or the average of the existing principal buildings within 150 ft either way. The rear yard is 25% of the lot depth and never less than 25 ft, so a deep lot needs more than that floor.

• A side yard used for vehicle ingress or egress must be at least 12 ft.
• On a corner lot the exterior side yard must be at least 15% of the lot width, never less than 10 ft, and need not exceed the front yard required along the street it faces.
• The 25% coverage cap is for the principal building alone, and it rises to 35% with accessory buildings counted.
• No principal building may stand within 10 ft of another unless it is built to one hour of fire protection, and never closer than 8 ft.
• A two-storey house needs at least 1,250 sq ft of floor area, against 1,000 for a single storey.
• On a through lot a front yard equivalent takes the place of the rear yard.

R-3 Single-Family or Two-Family Residential District - A duplex needs more land here than a house: 7,500 sq ft against 6,250. The front setback is a band, not a floor: at least 25 ft and no more than 50 ft, or the average of the existing principal buildings within 150 ft either way.

• The rear yard is 25% of the lot depth and never less than 25 ft, so a deep lot needs more than that floor.
• A side yard used for vehicle ingress or egress must be at least 12 ft.
• On a corner lot the exterior side yard must be at least 15% of the lot width, never less than 10 ft, and need not exceed the front yard required along the street it faces.
• The 25% coverage cap is for the principal building alone, and it rises to 35% with accessory buildings counted.
• No principal building may stand within 10 ft of another unless it is built to one hour of fire protection, and never closer than 8 ft.
• A two-storey house needs at least 1,000 sq ft of floor area, against 800 for a single storey.

R-3M Multiple-Dwelling District - This district runs on form-based design standards. The front setback is a build-to band of 0 to 10 ft, or the average of the principal buildings within 200 ft, and the rear yard of 20 to 40 ft applies only where the lot abuts a district boundary.

• The two side yards together run 0 to 20 ft, rising to a 40 ft maximum where a side yard carries a driveway and falling to 0 to 10 ft where the lot abuts open space or parkland.
• The rear yard drops to 10 ft against open space or parkland.
• Three usable storeys and 36 ft, or four usable storeys and 40 ft measured to the midpoint of the roof ridge where the top storey is built into the attic slope and dormered.
• Coverage is capped by floor area rather than by percentage, at 10,000 gross sq ft per storey.
• The standards set no minimum lot area, no minimum lot width and no coverage percentage.
• Parking is one space per unit plus a tenth of the unit count, set back five feet from the side and rear lines.

Mixed Use District - The front setback is a build-to band that depends on the street: 0 to 10 ft on main street and 0 to 20 ft on any other street, or 11 to 20 ft on main street and 21 to 30 ft elsewhere where approved public open space is provided.

• The 30 ft rear setback applies only where the lot abuts a district boundary, and a further five feet is added for every ten feet of building height above 30.
• Height has a floor as well as a ceiling: at least two usable storeys with a 15 ft minimum first floor, 50 ft overall, but 30 ft at the rear setback where the lot abuts a district boundary.
• Coverage is capped at 15,000 gross sq ft per storey, and 100 percent lot coverage is expressly permitted unless the lot abuts a district boundary.
• The two side yards together run 0 to 20 ft, rising to a 40 ft maximum where a side yard carries a driveway.
• The standards set no minimum lot area, no minimum lot width and no coverage percentage.
• Off-street parking is 1.5 spaces per 1,000 sq ft of leasable area and 1.5 per residential unit.

Neighborhood Mixed Use District - The front setback is a build-to band of 0 to 10 ft, or 11 to 20 ft where approved public open space is provided. The 20 ft rear setback applies only where the lot abuts a district boundary.

• Height has a floor as well as a ceiling: at least two usable storeys, capped at 36 ft, and falling to 20 ft at a setback abutting a residential district.
• The two side yards together run 0 to 20 ft, rising to a 40 ft maximum where a side yard carries a driveway.
• Coverage is capped at 10,000 gross sq ft per storey, and 100 percent lot coverage is expressly permitted unless the lot abuts a district boundary.
• The standards set no minimum lot area, no minimum lot width and no coverage percentage.
• Parking and driveways stand five feet off the side and rear lines and ten feet from a right-of-way in a side yard.

Light Manufacturing District - The 10 ft side yard rises to 20 ft where the side lot line abuts a lot in a residential district. Where the building is residential the lot minimum is 6,000 sq ft for a single-family dwelling plus 2,500 for each additional dwelling unit.

• The front yard, the height and the lot coverage are set by the landscaping and off-street parking requirements rather than by figures of their own.
• No minimum lot area is set for a non-residential or mixed-occupancy building.
• An accessory structure stops at 18 ft and must stand back from any property line on a residential district boundary by at least its own height.
• The district states no restriction on minimum floor area.