Suffolk County ADU Guide: Permits, Costs & Contractors

Suffolk County is Long Island's larger county by area, and its ten towns — Babylon, Brookhaven, East Hampton, Huntington, Islip, Riverhead, Shelter Island, Smithtown, Southampton, and Southold — each administer their own zoning and building codes. ADU opportunities in Suffolk County are generally stronger than in Nassau because lots are larger and many towns have active accessory apartment programs. Here's what homeowners in the major Suffolk County towns need to know.

Huntington, Brookhaven, and Islip: The Most Active ADU Markets

The Town of Huntington has one of the most established accessory apartment programs in all of Long Island. The program has been running for decades and has a defined application process, clear eligibility requirements, and relatively predictable permit timelines. Key requirements include owner occupancy (the property owner must live on the premises), minimum lot size of typically 10,000–15,000 square feet, a maximum ADU size of approximately 900–1,200 square feet, separate entrance access, and one additional off-street parking space. Permit applications in Huntington for qualifying properties often move through the process in 3–5 months — faster than many other Long Island municipalities.

The Town of Brookhaven, covering communities from Port Jefferson to Patchogue to Coram, also has a formal accessory apartment program. Brookhaven's requirements are broadly similar to Huntington's, though lot size minimums and ADU size limits vary by zoning district. Brookhaven's large geographic footprint means property characteristics vary significantly — a property in the dense village areas near the North Shore has different lot sizes and zoning contexts than one in rural central Suffolk.

The Town of Islip, which includes Bay Shore, Brentwood, and East Islip, also permits accessory apartments through a formal program. Islip's program has historically been active, and the town's proximity to the LIRR corridor makes ADU rental units particularly viable.

Costs and What to Budget in Suffolk County

ADU construction costs in Suffolk County are broadly similar to Nassau County. Basement conversions run $45,000–$90,000; attached additions $90,000–$180,000; detached ADUs $130,000–$260,000. Permit and design fees add $7,000–$15,000. Suffolk County's somewhat lower labor costs compared to Nassau can reduce total project costs by 5–10% on average.

One consideration unique to eastern Suffolk County: many properties rely on private septic systems rather than municipal sewer. Adding an ADU means adding sanitary load, and you may need to upgrade or expand your septic system to accommodate the additional dwelling unit. A septic expansion can add $15,000–$40,000 to the project cost and requires its own permit through the Suffolk County Department of Health Services.

ADU Guides by Town

Accessory dwelling rules change from town to town, and inside Nassau County they change again from village to village. These town-by-town guides cover the zoning, permit process and costs where you actually live.

Suffolk County: Amagansett · Babylon · Bay Shore · Bridgehampton · Brookhaven · Cold Spring Harbor · East Hampton · Huntington · Islip · Medford · Montauk · Northport · Patchogue · Port Jefferson · Quogue · Riverhead · Sag Harbor · Sayville · Shelter Island · Smithtown · Southampton · Southold · Water Mill · Westhampton Beach

Nassau County: Hempstead · North Hempstead · Oyster Bay