Amherst, NY Accessory Structures: ADU Rules (2026)

L. No. 13-2023) lets a homeowner build one within a single-family detached dwelling only after approval of a Special Use Permit under § 8-6. Only one ADU is allowed per lot, and it must be within or substantially attached to the principal structure, except on R-R, R-1, SA and AG lots that meet the district's minimum dimensional standards, where a detached ADU is allowed. Size is capped at a minimum of 500 square feet and cannot exceed 49% of the principal dwelling's gross floor area, up to a hard ceiling of 900 square feet, whichever limit is more permissive.

Height cannot exceed the principal structure's maximum for that zoning district, and the unit is limited to two bedrooms, with a third permitted only if built within the roof. The ADU must have its own entrance (which may share a common area with the principal dwelling), maintain a complementary exterior finish, and meet the Ordinance's § 7-3 exterior lighting standards. Parking requires a minimum of one off-street space and a maximum of two, and driveways cannot be widened between the front wall of the house and the right-of-way to fit that parking.

A detached ADU in the RR, R-1, SA or AG districts must sit at least 10 feet from the principal structure, match its roof style, cap out at two stories or 25 feet, and cannot sit between the principal structure's front and the street right-of-way. The Special Use Permit is valid for up to two years and is renewable by the Building Commissioner on proof of continued compliance; it is not transferable to a new owner, who must seek fresh Zoning Board of Appeals approval.