ADU rules in Islip

Detached backyard ADUs are flatly prohibited. Islip's only ADU pathway is the accessory apartment permit in Chapter 68, Article XLVII, and § 68-601 defines an accessory apartment as a dwelling unit that "is located within the principal structure," while § 68-605 adds a one-sentence ban: "An accessory apartment shall not be located within an accessory structure." Converting a detached garage, cottage, pool house or shed into a dwelling unit is therefore not available, and § 68-609 further limits accessory apartments to structures that already carry a certificate of occupancy or compliance for a single-family dwelling. The Town's other second-unit route, the "two-family, family use only" temporary special exception in § 68-419.1, likewise works by converting the existing house rather than by adding a separate building.

An attached or internal accessory apartment (a basement, attic or interior conversion, or an addition that keeps the unit inside the principal structure) is the only form Islip allows, and it needs a discretionary accessory apartment permit from the Zoning Board of Appeals plus a certificate of occupancy from the Department of Building (§ 68-602). Several gates commonly bite: the lot must be at least 7,500 square feet (§ 68-606) and at least 75 feet wide (§ 68-607); a dwelling whose certificate of occupancy postdates September 1, 1992 must be at least seven years old (§ 68-613); and the house must already hold a certificate of occupancy or compliance as a single-family dwelling, with only one accessory apartment per lot (§ 68-609). Exterior changes may not alter the single-family appearance of the house, only one visible front entrance is allowed (a second front door is permitted only if the Board finds it existed on or before July 7, 1992 under a two-family, family-use-only permit), and meters serving the apartment must not be visible from any roadway (§ 68-610B). Before occupancy the unit has to meet all New York State Uniform Fire Prevention and Building Code requirements for two-family dwellings, within six months of the building and zoning permit (§ 68-604).

§ 68-608 sets the cap: the accessory apartment's gross floor area "shall be at least 300 square feet and shall not exceed 50% or 800 square feet, whichever is less, of the gross floor area of the principal dwelling, including the area of the accessory apartment in which it is located." The same section allows a maximum of two bedrooms and caps occupancy at one person per 150 square feet of gross floor area. Basement conversions count against the number: § 68-3's gross floor area definition excludes residential basements "except those that are 50% or more above grade or utilized as an accessory apartment." The lot has its own floor, 7,500 square feet of area and 75 feet of width (§§ 68-606, 68-607), measured at the front building line of the principal dwelling on curvilinear roads and culs-de-sac.

An owner must live in the main house both to get the permit and to keep it: § 68-603 provides that no accessory apartment permit "shall be granted nor shall any said permit remain valid unless an owner of the lot upon which the accessory apartment is located resides within the principal dwelling unit," and defines an owner as a title holder with no less than a 50% interest. Tenants are confined to the accessory apartment, so renting out the main house is not an option. The permit terminates automatically on the applicant's death or that of the survivor, on transfer of title, or when the applicant stops occupying the premises as their principal residence (§ 68-615B), and each three-year renewal requires a fresh affidavit of residency (§ 68-616A). The Town's current application packet makes that affidavit swear the property "is my primary residence, where I reside for at least nine (9) months of the year," and § 68-665 lets the Town presume the owner does not live there if a voter registration, motor vehicle registration, driver's license or any other document filed with a public agency shows a different address. New York has no statewide ADU statute displacing this requirement; zoning here is entirely the Town's.

This is a whole-property requirement, not one extra space for the unit: § 68-612 requires that "there shall be located on-site not fewer than four off-street parking spaces," provided so that at least two of them can freely exit the property at any time. The Town Engineer directs how the parking is arranged, in a manner consistent with the residential appearance of the property, and any required driveway improvements must be finished before the certificate of occupancy or compliance for the apartment issues. The application packet includes a mandatory Accessory Apartment Driveway Review form that the Engineering Division completes before the application is filed; it can find the driveway compliant, legal nonconforming (predating current code), modifiable, or denied for excessive width or for more than one driveway existing. On a narrow lot with a single-width driveway this is frequently the binding constraint. The section has been amended repeatedly, most recently on May 14, 2024.

Approval is discretionary, not ministerial. § 68-602 requires an accessory apartment permit from the Zoning Board of Appeals plus a certificate of occupancy for the use from the Department of Building, and § 68-615D requires "a public hearing before the Town of Islip Zoning Board of Appeals ... for all applications for an accessory apartment permit, except for renewals by the same owner and transfers of said permit." The applicant must file a notarized acknowledgment that the use and permit end on death, transfer of title, the owner moving out, or conviction for a violation, and the grant lapses if a building and change-of-use permit is not obtained within 90 days (§ 68-615B, E). The code and the Town's forms treat it as a temporary special permit; any variance needed alongside it is heard by the same Board under § 68-621.2. The ZBA fee schedule effective May 1, 2024 charges $350 for a new accessory apartment application and $200 for the three-year renewal.

Renting the unit to an unrelated long-term tenant is the point of the permit, and no separate rental registration is needed: § 68-650D exempts units established under § 68-602 from Article L's rental occupancy permit regime. The permission is time-limited and personal, though. Permits "must be renewed every three years or upon transfer of title," with an affidavit of residency and written notice to the owners of all dwellings within 100 feet, and the Board may require a hearing on renewal if neighbor responses or an inspection suggest the conditions are not being met (§ 68-616). Occupancy is capped at two bedrooms and one person per 150 square feet, tenants may occupy only the accessory apartment (§§ 68-603, 68-608), and discriminatory practices in renting it are prohibited under Chapter 26 (§ 68-621). On short-term rentals, § 68-650E makes it unlawful to operate any "transient rental property," which § 68-649 defines as renting for a period of less than 14 nights, with a rebuttable presumption triggered by an Airbnb, VRBO or similar listing; but that same definition expressly excludes "a dwelling unit lawfully and validly permitted as an accessory apartment in accordance with § 68-602." The exclusion is definitional rather than an affirmative permission, so confirm any intended short-term use with the Zoning Division before relying on it.