Building an ADU in Southampton, NY: Zoning, 30,000 Sq Ft Lots & Permits

The Town of Southampton is the largest town on Long Island's South Fork, stretching from Eastport and Remsenburg in the west through Hampton Bays, Southampton Village, Water Mill, Bridgehampton, and out to the edges of Sag Harbor. It is an area of extraordinary wealth, natural beauty, and — like much of the East End — a critical shortage of housing for the people who keep the community running.

Southampton has responded to this challenge with specific ADU guidelines, a Community Housing Fund, and participation in the Plus One ADU Program. But building an ADU in Southampton comes with requirements that differ from other Suffolk County towns, most notably a 30,000 square foot minimum lot size in most residential zones. This guide explains what Southampton homeowners need to know before breaking ground.

Southampton's 30,000 Sq Ft Lot Requirement

The most important thing to understand about ADUs in Southampton is the lot size threshold. In most residential zoning districts, your property must be at least 30,000 square feet — roughly three-quarters of an acre — to be eligible for an accessory apartment. This requirement filters out smaller parcels and ensures that ADUs are built on properties with adequate space for both the primary residence and the secondary unit.

• Why 30,000 square feet? The threshold reflects Southampton's commitment to maintaining its rural and low-density character. Larger lots provide sufficient room for setbacks, septic systems, parking, and the ADU itself without overcrowding the property.
• Which zones apply? The 30,000 square foot minimum applies to most standard residential zones in the town. Some zones have even larger minimum lot requirements. Properties in agricultural-residential zones or hamlet center zones may have different standards.
• What if my lot is smaller? Properties below 30,000 square feet may still have options, but they are more limited. Internal conversions (basement or portion of the main house) may be feasible even on smaller lots, depending on the zoning district. A variance may also be possible in certain circumstances.
• How to check your lot size. Your property survey or tax assessment records will show your lot's square footage. We pull this information during our initial feasibility assessment and confirm it against the town's GIS records.

Many Southampton properties easily clear the 30,000 square foot threshold. Homes in Hampton Bays, Water Mill, Bridgehampton, and the unincorporated areas of the town commonly sit on one-acre or larger parcels. If your lot qualifies, the path to an ADU is straightforward.

Community Housing Fund

Southampton has established a Community Housing Fund dedicated to addressing the affordable and workforce housing shortage in the town. The fund is supported by a portion of the Community Preservation Fund (CPF) revenue generated from real estate transfer taxes on property sales within the town.

The Community Housing Fund supports a range of housing initiatives, and ADUs are a key component. The fund can provide financial support for ADU projects that meet workforce housing criteria, and it works in coordination with the Plus One program and other state and county resources. Homeowners building ADUs for workforce housing purposes may benefit from fund resources in addition to or in combination with Plus One grants.

Plus One ADU Program

Southampton participates in the Plus One Accessory Dwelling Unit Program, which offers grants of up to $125,000 to homeowners who build ADUs for workforce housing. The program requirements are consistent with what applies in other participating East End towns:

• Grant amount: Up to $125,000 toward construction, renovation, or conversion costs.
• Tenant income limits: The ADU must be rented to tenants earning at or below 80% of the Area Median Income.
• Rental commitment: The unit must serve as a long-term rental at affordable rates for a specified period, typically 10 years.
• No short-term rentals: ADUs funded through Plus One cannot be used as vacation or short-term rentals.

For Southampton homeowners on qualifying lots, the combination of the Plus One grant and the Community Housing Fund can significantly reduce the net cost of ADU construction. A project that might cost $200,000 could have $125,000 or more covered through grants, leaving the homeowner with a modest investment that generates long-term rental income.

Southampton's ADU Design Guidelines

Southampton has published ADU guidelines that outline the town's expectations for accessory dwelling unit design and construction. These guidelines cover:

• Architectural compatibility. The ADU should complement the primary residence and surrounding neighborhood in terms of materials, style, roof pitch, and scale. Southampton does not want ADUs that look like afterthoughts.
• Size limits. ADUs are generally limited in size relative to the primary dwelling. Maximum square footage varies by zone but typically caps between 800 and 1,200 square feet.
• Entrance placement. The ADU's entrance should be positioned to minimize visual impact from the street. Side or rear entrances are generally preferred over front-facing entries.
• Parking. At least one additional off-street parking space must be provided for the ADU, and it should be screened or positioned to maintain the property's aesthetic.
• Landscaping and screening. The town may require landscape buffers between the ADU and neighboring properties, particularly for detached units.

These guidelines ensure that ADUs integrate seamlessly into Southampton's neighborhoods. We design every ADU to meet or exceed these standards, so the approval process runs smoothly.

Types of ADUs for Southampton Properties

Southampton's large lots and architectural heritage open up a range of ADU options, many with a character unique to the South Fork:

• Guest house / backyard cottage. A standalone 600 to 1,000 square foot unit positioned behind the main house. On one-acre-plus lots, these cottages can be set far enough from both the primary home and property lines to provide genuine privacy.
• Carriage house conversion. Many older Southampton properties include existing carriage houses or outbuildings. Converting these structures into modern living space preserves historic character while creating a functional apartment.
• Garage conversion. Detached garages are common on Southampton properties, and converting them into studio or one-bedroom apartments is one of the most cost-effective ADU approaches.
• Above-garage apartment. A second-story addition over an existing garage adds living space without increasing the building footprint. This option works well on properties where ground-level space is constrained by setbacks or septic placement.
• Basement or interior conversion. Converting a walk-out basement or unused portion of the main house is the least visible option and often the fastest to permit since no new structure is added to the site.

Permit Process in Southampton

The permit process in Southampton involves the town's Building and Zoning Department and, for most properties, the Suffolk County Health Department for septic review. Here is the typical sequence:

1. Property evaluation. We visit your property to confirm lot size, zoning district, existing structures, septic capacity, and the best ADU configuration for your situation.
2. Architectural plans. Licensed plans are prepared in compliance with Southampton's ADU guidelines, including site plan, floor plan, elevations, and utility connections.
3. Zoning review. Plans are submitted to the Southampton Building Department. The department reviews for compliance with lot size requirements, setbacks, lot coverage, height limits, and the published ADU guidelines.
4. Health Department review. The Suffolk County Health Department reviews septic capacity for the additional dwelling. An upgraded or new system may be required.
5. Building permit issuance. With zoning and health approvals secured, the building permit is issued.
6. Construction and inspections. Construction proceeds with inspections at foundation, framing, mechanical, and final stages. A certificate of occupancy is issued upon completion.

We handle every step of this process for our Southampton clients. From the initial lot size confirmation through the final CO inspection, you have a single point of contact managing the entire project.

ADU Construction Costs in Southampton

Like East Hampton, Southampton construction costs carry a Hamptons premium. Labor, materials, and the higher design standards all contribute to costs that exceed central Suffolk County by 20 to 30 percent. Here are realistic ranges for 2026:

• Garage conversion: $65,000 to $130,000. The most affordable option when an existing detached garage is available.
• Basement conversion: $75,000 to $145,000. Walk-out basements with existing access points are the most efficient to convert.
• Above-garage apartment: $100,000 to $180,000. Structural engineering and design review add to the cost beyond what central Suffolk projects require.
• Detached new construction: $185,000 to $360,000+. A standalone guest cottage with foundation, full utilities, and Hamptons-appropriate finishes.

The Plus One grant offsets up to $125,000 of these costs. A $130,000 garage conversion with a Plus One grant becomes a $5,000 out-of-pocket project. That is an exceptional return for a unit that generates rental income for years to come.

Southampton Communities We Serve

Alec's Construction serves homeowners across the Town of Southampton, including:

• Southampton Village — historic village center with affluent residential streets and strong ADU interest for staff and caretaker housing
• Hampton Bays — year-round community with a mix of lot sizes and active workforce housing demand
• Westhampton Beach — coastal community where seasonal property ADUs serve year-round workers
• Water Mill — large estate properties with ample space for detached guest cottages and carriage house conversions
• Bridgehampton — agricultural and residential area with growing interest in ADU construction
• Quogue — quiet residential hamlet with properties that can accommodate accessory units
• Remsenburg — western gateway to the Hamptons with spacious lots and ADU potential
• Flanders — more affordable area within Southampton with growing housing needs
• Sag Harbor (shared with East Hampton) — historic whaling village where carriage house and outbuilding conversions fit the architectural character