Building an ADU in Brookhaven, NY: The Complete Homeowner Guide

The Town of Brookhaven is the largest town in Suffolk County, stretching from the North Shore to the South Shore and encompassing dozens of communities that more than 500,000 people call home. It is also where Alec's Construction is based. Our office at 354 Barton Ave in East Patchogue sits squarely in Brookhaven, and we have been building, renovating, and constructing ADUs across this town since 2018. No contractor knows Brookhaven's building codes, zoning districts, and permit process better than we do.

If you are a homeowner in Brookhaven considering an accessory dwelling unit, this guide covers everything you need to know: the town's accessory apartment ordinance, zoning requirements by district, septic considerations, the permit process, and realistic costs.

Brookhaven's Accessory Apartment Ordinance

The Town of Brookhaven adopted its accessory apartment ordinance in 1983, establishing a legal framework for homeowners to create secondary dwelling units on their single-family properties. The ordinance has been updated over the years to reflect changes in state law, building codes, and community needs, but the core intent remains the same: allowing homeowners to create a self-contained apartment within or attached to their existing home.

Brookhaven's ordinance was an important step for Suffolk County because of the town's sheer size. With communities ranging from suburban Centereach and Selden to more rural areas in the eastern reaches of the town, the ordinance had to be flexible enough to work across very different types of neighborhoods. Today, the rules accommodate that diversity through zoning-district-specific requirements.

Under the current ordinance, accessory apartments are permitted in most residential zoning districts, but the specific requirements differ based on which zone your property falls in. This means that a homeowner in Medford may face different lot size or setback requirements than a homeowner in Port Jefferson Station, even though both are in the Town of Brookhaven. Understanding your specific zoning district is the first step in any ADU project here.

Zoning Districts and ADU Rules

Brookhaven's residential zoning districts include A1, A2, A5, and several other designations, each with its own minimum lot sizes, setbacks, and building coverage allowances. Here is how the key districts affect ADU construction:

• A1 Residence (1 acre minimum). Found in more rural and semi-rural parts of Brookhaven, these lots offer the most space for ADU construction. Detached ADUs are more feasible here due to generous lot sizes and setback clearance.
• A2 Residence (half-acre minimum). Common in communities like East Patchogue, Medford, and Holbrook. Many properties in A2 zones have enough space for garage conversions, basement ADUs, or above-garage apartments.
• A5 Residence (quarter-acre minimum). Found in denser areas like Centereach and parts of Selden. Smaller lots make interior conversions (basement or within-home ADUs) the most practical option.

Regardless of zoning district, several universal requirements apply across Brookhaven:

• Owner-occupancy. The property owner must reside in either the primary dwelling or the accessory apartment.
• Single-family appearance. The property must maintain the appearance of a single-family home from the street. Exterior modifications should be minimal and architecturally consistent.
• Parking. One additional off-street parking space is required for the accessory apartment.
• Maximum size. The accessory apartment cannot exceed a specified percentage of the primary dwelling's floor area, and must have its own kitchen, bathroom, and sleeping area.
• Separate entrance. A dedicated entrance is required, but it should ideally be located on the side or rear of the home rather than facing the street.

Septic Considerations in Brookhaven

This is one of the most critical factors for ADU construction in Brookhaven: the vast majority of the town is on private septic systems, not municipal sewer. Unlike the Town of Babylon, which has significant sewer coverage through the Southwest Sewer District, Brookhaven homeowners building ADUs almost always need Suffolk County Health Department approval for their septic system.

The Health Department evaluates whether your existing septic system can handle the additional wastewater load from a second dwelling unit. Key considerations include:

• System capacity. The existing septic system must have enough capacity for the combined flow from the primary home and the ADU. The Health Department calculates this based on the number of bedrooms in both units.
• System condition. An older or failing septic system may need to be replaced or upgraded before an ADU permit will be issued.
• Memorandum #19. This Suffolk County Health Department guideline governs the approval of accessory apartments on properties with septic systems. It outlines the flow calculations, system requirements, and documentation needed for approval. Understanding Memo 19 is essential for any Brookhaven ADU project.
• Nitrogen reduction. In environmentally sensitive areas near waterways, advanced nitrogen-reducing septic systems may be required. This adds cost but is necessary for environmental protection.

We have navigated the Health Department approval process on numerous Brookhaven projects and know exactly what documentation to prepare, what system specifications to meet, and how to avoid delays. This is one of the areas where working with a local contractor makes the biggest difference.

Communities We Serve in Brookhaven

Because Brookhaven covers such a vast area, it helps to understand the specific communities where we regularly build. We are active in all of the following Brookhaven communities:

• East Patchogue — our home base. We know this community inside and out, from the building department to the local suppliers.
• Patchogue — the vibrant village adjacent to East Patchogue, with a mix of older homes and revitalized downtown properties.
• Medford — a large residential community with many ranch-style homes that are well-suited for basement and garage ADU conversions.
• Bellport — a historic village with specific architectural considerations for any additions or modifications.
• Centereach & Selden — densely developed communities where interior ADU conversions are often the best fit.
• Holbrook — residential area with a mix of lot sizes, many in the A2 zone.
• Farmingville — community with generous lot sizes that can accommodate various ADU types.
• Mastic Beach & Shirley — south shore communities where waterfront proximity may trigger additional environmental review requirements.
• Stony Brook, Port Jefferson & Setauket — north shore communities with larger lots, hillier terrain, and higher property values.

Permit Process in Brookhaven

The Town of Brookhaven Building Division, located at One Independence Hill in Farmingville, processes all ADU permit applications. Here is the step-by-step process:

1. Zoning verification. Confirm your property's zoning district and that accessory apartments are permitted. We pull this information during the initial property assessment.
2. Design and plans. Prepare architectural drawings showing the ADU layout, dimensions, egress, utility connections, parking, and compliance with all setback and coverage requirements. Plans must be stamped by a licensed architect or professional engineer.
3. Health Department application. For properties on septic (which is most of Brookhaven), submit a concurrent application to the Suffolk County Health Department for septic approval under Memorandum #19.
4. Building permit application. Submit the complete package to the Brookhaven Building Division, including architectural plans, survey, Health Department approval (or concurrent application), and the completed application form.
5. Plan review. The Building Division reviews the plans for code compliance. This typically takes 4 to 10 weeks in Brookhaven, depending on the backlog and complexity.
6. Permit issuance and construction. Once approved, the permit is issued and construction can begin. Inspections are required at each major phase.
7. Final inspection and CO. After construction is complete, a final inspection leads to the issuance of a certificate of occupancy for the accessory apartment.

We have built strong relationships with the Brookhaven Building Division over the years. We know what the plan reviewers look for, what documentation to include upfront to avoid revision requests, and how to keep the timeline moving. This is the advantage of working with a contractor who is headquartered in the same town where you are building.

ADU Construction Costs in Brookhaven

Construction costs in Brookhaven reflect Suffolk County's 2026 market rates. Here are realistic ranges based on our direct experience building in this town:

• Basement conversion: $55,000 to $115,000. Most Brookhaven homes have full basements, and many have the 7-foot-plus ceiling height needed for habitable space. The main cost drivers are waterproofing, egress windows, and finish level.
• Garage conversion: $50,000 to $105,000. Detached garages are common throughout Brookhaven, especially in A1 and A2 zones, making this a popular option.
• Above-garage apartment: $85,000 to $155,000. Structural engineering for the existing garage and stair access are the primary cost additions.
• Detached new construction: $155,000 to $300,000+. Ground-up builds with foundation, framing, roofing, siding, and full utility connections.

An important cost factor unique to Brookhaven is septic system work. If your existing system needs to be replaced or upgraded to accommodate the ADU, that can add $15,000 to $40,000 to the project. We assess this during the initial walkthrough so there are no surprises. For a broader view of ADU costs across the region, see our Long Island ADU construction guide.

Why We Know Brookhaven Best

Alec's Construction has been based in Brookhaven since the company was founded. Our office is in East Patchogue. Our team lives in Brookhaven communities. We have completed projects in Medford, Patchogue, Centereach, Holbrook, Farmingville, and across the town. This is not just a service area for us; it is our home.

That local presence translates into real advantages for our ADU clients:

• Building department relationships. We interact with the Brookhaven Building Division regularly and understand their review process, preferences, and timelines.
• Subcontractor network. Our electricians, plumbers, and HVAC contractors are all local to Brookhaven and familiar with the town's code enforcement standards.
• Material sourcing. We source materials from local suppliers, reducing lead times and delivery costs.
• Site knowledge. We understand Brookhaven's soil conditions, water table levels, and drainage patterns, all of which affect foundation work and septic system design.

If you are a Brookhaven homeowner considering an ADU, you will not find a contractor with deeper roots in your town. Learn more about the regulatory landscape in our Suffolk County ADU laws guide, or visit our ADU service page for details on our process.