ADU rules in Ithaca

Detached ADUs are a permitted (as-of-right) accessory use, shown with a solid dot rather than "SP" on the District Regulations Chart, in every conventional residential district except R-U, plus Collegetown Residential CR-1, CR-2 and CR-3 and the B-1 and B-2 business districts. Ordinance No. 2025-04, adopted 5-7-2025, added "accessory dwelling unit" to Column 3 (Permitted Accessory Uses) of § 325-8A for R-1, R-2, R-3, CR-1, CR-2, CR-3, B-1 and B-2. The code defines a detached ADU as one "located within an existing or newly constructed accessory structure such as a detached garage, carriage house, or other freestanding building," so both garage conversions and new backyard cottages qualify. There is no minimum lot size, no lot-area-per-unit gate and no discretionary hearing; one ADU is allowed per parcel and it may not sit in the primary front yard.

Attached and internal ADUs are permitted in the same districts and by the same as-of-right accessory-use listing as detached ones; the chart has a single "Accessory Dwelling Unit" row covering both types. § 325-3 defines an attached ADU as one "connected to or contained within the primary structure, such as a basement, building addition, or attached garage," which covers basement, attic and addition conversions. § 325-28C(7)(a) caps an attached ADU at 40% of the floor area of the primary structure, and § 325-28C(8)(a) applies the primary structure's setbacks. A purely interior conversion is not "development" under Chapter 276, so it avoids site plan review entirely and needs only a building permit and certificate of occupancy.

§ 325-28C(7) splits the size rules three ways. A newly built detached ADU is limited to an 800 square foot maximum building footprint and a maximum height of two stories and 20 feet. An attached ADU may not exceed 40% of the floor area of the primary structure, so the cap floats with the size of the house. Converting an existing accessory building carries no size limit at all, and such a building may be altered or expanded so long as it stays within the new-construction limits. Separately, § 325-28C(3) caps every ADU at two bedrooms regardless of type, and § 325-28C(1) allows only one ADU per parcel.

§ 325-28 imposes no owner-occupancy condition of any kind, at application or afterwards, and the City's own ADU FAQ answers the question flatly: "No, there is no owner occupancy requirement for parcels with ADUs." This was a deliberate change; before Ord. No. 2025-04 an owner had to go through a special permit for an accessory apartment or build a second primary structure on a lot meeting the full minimum lot size. Two adjacent rules are worth knowing but are not owner-occupancy mandates: renting the ADU as a short-term rental under § 325-27D(2) requires the property to be the host's primary residence, and the § 325-3 definition of "family" allows one extra unrelated occupant per dwelling unit in R-1 zones only when the dwelling is owner-occupied.

§ 325-28C(2) is explicit: "There shall be no off-street parking requirement for an ADU." That overrides the otherwise applicable residential minimum in the § 325-20 parking charts, which would call for one space per dwelling unit for the first three bedrooms. The new Southside Form Districts adopted in November 2025 set "MIN OFF-STREET PARKING: None" district-wide, so the outcome is the same there. Note that adding parking areas for three or more cars would pull an otherwise exempt project into site plan review under § 276-3C(2), which is one more reason not to add spaces you do not need.

ADUs appear on the District Regulations Chart with a solid dot (permitted accessory use), not the "SP" that flags a Board of Zoning Appeals special permit, so no discretionary hearing is involved in the qualifying districts. The approvals are ministerial: a building permit from the Director of Planning and Development under § 325-38 and a certificate of occupancy under § 325-39. A new detached ADU does draw site plan review, because § 276-3C(2)'s exemption for residential exterior work expressly excludes "accessory structures other than those permitted for occupancy," but at well under the 4,000 square foot affected-area threshold it is a "project of limited scope" that the Director reviews and acts on with no public hearing and no advisory commission review. Design review under Chapter 160 applies in B-1b, B-2c, B-2d and on parcels in the 2009 Collegetown Planning Area, and its recommendations are non-binding unless folded into a BZA decision or a site plan condition; in a locally designated historic district, Ithaca Landmarks Preservation Commission approval is binding instead.

§ 325-28C(5) says an ADU "may be leased separately but may not be sold separately from the primary structure," and C(6) bars subdividing a parcel to split the ADU off, so the unit is a permanent rental rather than a for-sale product. Any rented dwelling in Ithaca needs a valid certificate of compliance from the Building Division under § 210-43A, good for up to five years for one and two-family rentals and up to three years where there are three or more units or five or more unrelated occupants. Short-term renting is allowed but tightly drawn: § 325-27D(3)(a) makes accessory dwelling units eligible, yet D(2) issues permits only for the host's primary residence, D(5) limits permits to natural persons (no LLCs or trusts), each unit needs its own annual operating permit, and there is no cap on nights so long as the host keeps their primary residence on the property. Occupancy of the unit itself is bounded by the § 325-3 definition of "family" and by the two-bedroom cap in § 325-28C(3), which matters in this student-heavy market.