Building an ADU in the Town of Islip, NY: Requirements & Guide

The Town of Islip sits at the heart of Suffolk County's south shore, covering communities from Bay Shore and Islip Village to Brentwood, Central Islip, Ronkonkoma, Hauppauge, and Bohemia. With its accessory apartment ordinance dating back to 1984, Islip has more than four decades of experience regulating ADUs, and the town's specific lot size and frontage requirements set it apart from its neighboring municipalities.

If you are an Islip homeowner considering an ADU, the single most important thing to know upfront is this: the Town of Islip requires a minimum lot size of 7,500 square feet and a minimum lot frontage of 75 feet for accessory apartment eligibility. These specific numerical thresholds determine whether your property qualifies before any other factor comes into play. This guide covers these requirements in detail, along with the full permit process, costs, and practical considerations for building an ADU in Islip.

Islip's Accessory Apartment Rules

The Town of Islip adopted its accessory apartment ordinance in 1984, five years after the Town of Babylon pioneered ADU legislation on Long Island. Islip's approach has been characterized by specific, measurable requirements that give homeowners a clear path to compliance. The ordinance has been refined over the years, but its core framework remains focused on ensuring that accessory apartments are compatible with the existing residential fabric of Islip's diverse communities.

Key provisions of Islip's current accessory apartment rules include:

• Owner-occupancy requirement. The property owner must reside in either the primary dwelling or the accessory apartment. This is enforced as a condition of the accessory apartment permit and must be maintained for as long as the ADU is occupied.
• Single-family appearance. The exterior of the property must retain the appearance of a single-family home. Any new entrances for the ADU should be on the side or rear of the building, not prominently visible from the street.
• One ADU per property. Only one accessory apartment is permitted per residential lot, regardless of the property's size.
• Separate utilities. The ADU must have separate utility metering for electricity, and its own bathroom, kitchen, and sleeping area to qualify as a self-contained dwelling unit.
• Fire and safety compliance. The accessory apartment must meet all New York State building code requirements for habitable space, including smoke and carbon monoxide detectors, egress windows, and adequate ventilation.

Lot Size and Frontage Requirements

Islip's most distinctive ADU requirement is its explicit lot size and frontage minimums. Unlike some towns that tie ADU eligibility to the zoning district's general lot requirements, Islip has set specific thresholds for accessory apartments:

• Minimum lot size: 7,500 square feet. Your property must have at least 7,500 sq ft of total lot area to be eligible for an accessory apartment. This is larger than the minimum lot requirement in some of Islip's residential zones, meaning that not every legal lot in Islip qualifies for an ADU.
• Minimum lot frontage: 75 feet. The property must have at least 75 feet of frontage on a public street. Corner lots and irregularly shaped lots may measure frontage differently, so this should be verified against your property survey.

What do these numbers mean in practice? A standard suburban lot in communities like Bay Shore, Ronkonkoma, or Bohemia is often 60 to 100 feet wide and 100 to 150 feet deep. Many lots in these areas meet the 7,500 sq ft minimum, but narrower lots (under 75 feet wide) may fall short of the frontage requirement. This is particularly relevant in older neighborhoods with smaller lot subdivisions where lots might be 50 or 60 feet wide.

Before investing time or money in ADU planning, the first step is to check your property survey against these two thresholds. If you do not have a recent survey, we can help you obtain one as part of our initial property assessment. During our free walkthrough, we measure and verify lot dimensions to confirm eligibility before any design work begins.

Zoning Considerations in Islip

Beyond the lot size and frontage minimums, accessory apartments in Islip must comply with the zoning requirements of the specific district the property is in. Islip's residential zones include Residence AA, Residence A, Residence B, and others, each with their own setback and lot coverage rules.

• Setbacks. Any new construction or addition for the ADU must meet front, side, and rear yard setback requirements for the zone. Interior conversions (basement or within-home ADUs) are generally unaffected by setback requirements since they do not change the building footprint.
• Lot coverage. The total building coverage, including any new ADU construction, cannot exceed the maximum lot coverage percentage for the zone. This is particularly relevant for detached ADU construction or garage additions.
• Height restrictions. New construction or additions must comply with the zone's maximum building height. Above-garage apartments, for example, must stay within the allowed height envelope.
• Parking. At least one additional off-street parking space must be provided for the accessory apartment, beyond the parking required for the primary home. The parking space must meet the town's dimensional standards.

We verify all zoning-specific requirements for your property before beginning the design phase. This prevents costly redesigns and ensures the application moves through review without zoning-related rejections.

Plus One ADU Program in Islip

The Town of Islip participates in New York's Plus One ADU Program, offering eligible homeowners grants of up to $125,000 toward ADU construction. For Islip homeowners, this grant represents a transformative opportunity to build an accessory apartment with significantly reduced out-of-pocket costs.

The Plus One Program is especially relevant in Islip because of the town's mix of moderate-income communities where the grant can make ADU construction financially accessible to homeowners who might not otherwise be able to afford it. Communities like Central Islip, Brentwood, and parts of Bay Shore have strong housing demand, and the combination of grant funding and rental income potential makes ADU construction a compelling investment.

To qualify, homeowners must meet income thresholds, agree to rent restrictions on the ADU for a specified period, and build in compliance with all town codes. We guide our clients through the eligibility determination and application process.

Communities in the Town of Islip

Islip is a geographically diverse town with communities that range from waterfront villages to inland suburban neighborhoods. Each area has its own character and ADU considerations:

• Bay Shore — one of Islip's most active communities with a revitalized downtown, LIRR access, and strong rental demand. Parts of Bay Shore are connected to the Southwest Sewer District, which simplifies ADU permitting by eliminating the need for Health Department septic approval.
• Islip & East Islip — waterfront communities along the Great South Bay with established residential neighborhoods and moderate lot sizes.
• Central Islip — a community with significant housing demand and many properties that meet the 7,500 sq ft lot minimum. The Plus One grant program is particularly impactful here.
• Brentwood — one of Long Island's largest hamlets with a diverse housing stock. Many Brentwood lots meet Islip's ADU requirements, and there is strong demand for rental housing.
• Ronkonkoma — a community near the geographic center of Long Island with LIRR access. ADUs here benefit from proximity to transportation and the Ronkonkoma Hub development.
• Hauppauge — a community known for its industrial park and office corridor, but also with established residential neighborhoods suitable for ADU construction.
• Bohemia — a residential community with a mix of home styles and lot sizes, many of which qualify for accessory apartments.

Sewer vs. Septic in Islip

The sewer-versus-septic question is a significant factor in Islip ADU projects. The town has a mixed infrastructure that varies by community:

• Sewer-connected areas. Parts of Bay Shore, Islip, and other communities near the south shore are connected to the Southwest Sewer District. Properties on municipal sewer have a simpler path to ADU approval because there is no need for Suffolk County Health Department septic review. This can save weeks of processing time and potentially thousands of dollars in system upgrades.
• Septic areas. Communities further inland, including parts of Brentwood, Central Islip, Ronkonkoma, Hauppauge, and Bohemia, are generally on private septic systems. For these properties, the Suffolk County Health Department must approve the septic system's capacity to handle the additional wastewater load from the ADU.

If your property is on septic, the existing system must demonstrate adequate capacity or be upgraded to handle the combined flow from the primary home and the ADU. We assess the septic situation during our initial walkthrough and can advise on whether an upgrade is likely to be needed and what it would cost.

Permit Process in Islip

The Town of Islip's Building Division handles accessory apartment permits. The process follows a structured sequence:

1. Lot eligibility verification. Confirm that your property meets the 7,500 sq ft minimum lot size and 75 ft minimum frontage requirements. This is the first and most critical step.
2. Zoning compliance check. Verify that accessory apartments are permitted in your zoning district and that the proposed ADU meets all setback, coverage, and height requirements.
3. Architectural plans. Prepare construction drawings stamped by a licensed architect or professional engineer, showing the ADU layout, dimensions, egress paths, utility connections, and parking.
4. Health Department application (if on septic). Submit septic capacity documentation to the Suffolk County Health Department for properties not connected to municipal sewer.
5. Building permit application. Submit the complete package to the Islip Building Division, including plans, survey, zoning compliance documentation, and the application form.
6. Plan review. The Building Division reviews the submission for code compliance. Review times in Islip typically run 4 to 8 weeks, depending on the current workload.
7. Construction and inspections. Once the permit is issued, construction proceeds with required inspections at each milestone: foundation, framing, rough mechanical, insulation, and final.
8. Certificate of occupancy. Passing the final inspection results in a CO for the accessory apartment, making it a legal dwelling unit.

We manage the entire permit process for our Islip ADU clients, from initial lot verification through certificate of occupancy. Our familiarity with the Islip Building Division's requirements means applications are complete and correct the first time, minimizing delays.

ADU Construction Costs in Islip

Construction costs for ADUs in the Town of Islip are consistent with central Suffolk County market rates in 2026:

• Basement conversion: $55,000 to $110,000. Many Islip homes have full basements with workable ceiling heights, making this a cost-effective option.
• Garage conversion: $50,000 to $100,000. The existing structure provides the shell, with costs focused on interior buildout, insulation, plumbing, and electrical.
• Above-garage apartment: $80,000 to $150,000. Structural reinforcement and stair access are the main additions beyond standard construction costs.
• Detached new construction: $150,000 to $290,000+. A ground-up build with foundation, framing, roofing, siding, and complete utility connections.

Additional cost factors specific to Islip include septic system upgrades ($15,000 to $35,000 if needed) and the requirement for professional survey verification of lot dimensions. The Plus One ADU Program's $125,000 grant can cover a substantial portion of conversion costs and significantly reduce the investment for detached construction. For broader cost information, see our Long Island ADU construction guide and our Suffolk County ADU laws overview.