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Guide to Accessory Apartments in the Town of Islip

What Homeowners and Buyers Need to Know

Welcome to the Jones Hollow Realty Group resource center! If you’re a homeowner in the Town of Islip considering adding an accessory apartment (also known as an accessory dwelling unit or ADU) to your single-family home, or if you’re buying a property that already has one, this guide is for you. Accessory apartments can provide affordable housing options, extra income, or space for family members, but they must comply with strict local regulations to remain legal and safe.

This guide is based on the Town of Islip’s Zoning Code (Chapter 68, Article XLVII) and related programs. Always verify details with Town officials, as rules can change. At Jones Hollow Realty Group, our experienced agents can help you navigate these processes during your home search or sale—contact us at [email protected] or (631) 650-3977 for personalized advice.

What is an Accessory Apartment?

An accessory apartment is a secondary dwelling unit located within the main structure of an owner-occupied single-family detached home. It must be subordinate in size, location, and appearance to the primary residence. The goal is to address housing needs, promote efficient use of existing homes, and maintain neighborhood stability without altering the single-family character of the area. Detached structures like garages or pool houses are not allowed for ADUs.

Key Eligibility and Requirements

To create or maintain a legal accessory apartment, your property must meet these criteria:

Property and Lot Requirements

  • Minimum Lot Size: 7,500 square feet.
  • Minimum Lot Width: 75 feet (measured at the front building line for curvilinear roads or cul-de-sacs).
  • Home Age: The structure must have had its certificate of occupancy for at least 7 years if issued after September 1, 1992.
  • Limit: Only one accessory apartment per lot, and it must be in a home originally certified as a single-family dwelling.
  • Location: Must be fully integrated into the main house—no detached buildings.

Design and Size Limits

  • Bedrooms and Occupancy: Up to 2 bedrooms; occupancy limited to 1 person per 150 square feet of gross floor area.
  • Floor Area: Minimum 300 square feet; maximum 50% of the main dwelling’s gross floor area or 800 square feet (whichever is smaller).
  • Exterior Appearance: No alterations that change the single-family look. Only one visible front entrance (with limited exceptions for pre-1992 doors). Utility meters must not be visible from the street.
  • Expansions: Any additions for the apartment will trigger a property tax reassessment.

Owner and Occupancy Rules

  • Owner Occupancy: The property owner (with at least 50% interest) must reside in the main dwelling as their primary home. The apartment is for tenants only.
  • Rental Restrictions: No short-term rentals (e.g., Airbnb stays under 14 nights). Discrimination in renting is prohibited under Town Code Chapter 26.
  • Health and Safety: Must comply with New York State Uniform Fire Prevention and Building Code for two-family dwellings, plus Suffolk County health codes (e.g., septic systems).

Parking and Maintenance

  • Off-Street Parking: At least 4 spaces, arranged to maintain a residential appearance and allow easy access/exit.
  • Upkeep: The property must be well-maintained; the Town can inspect and enforce, billing owners for any required work.

Violations can lead to fines, permit revocation, and legal action under Town Code § 68-666.

The Permitting Process

Obtaining a legal accessory apartment requires approvals from the Zoning Board of Appeals (ZBA) and Building Department:

  1. Application: Submit to the ZBA with a notarized statement that the permit ends upon the owner’s death, sale, or if they move out. A public hearing is required for new permits.
  2. Approvals: ZBA special permit first, then Building Department permits for any construction. Variances (e.g., for lot size) go through the ZBA.
  3. Timeline: Start construction within 90 days of approval; complete within 6 months. Full code compliance needed before occupancy.
  4. Renewal: Every 3 years or upon property transfer. Includes an affidavit of residency and neighbor notification; hearings if complaints arise.
  5. Revocation: Possible for violations; refusing inspections (with 24-hour notice) leads to automatic revocation.
  6. Fees: Vary; contact the ZBA for current rates.

For electronic submissions, use the Town’s Planning and Development Application Portal: https://planning.islipny.gov/.

Buying a Home with an Existing Accessory Apartment

If you’re purchasing a property in Islip with an already-set-up apartment, check its legal status during due diligence:

If It’s Already Legal

  • The permit isn’t automatically transferred—it ties to the owner-occupant.
  • Apply to the ZBA for a transfer (no public hearing typically needed). Provide proof of ownership, residency affidavit, and confirm compliance.
  • Renew every 3 years or with title transfer.
  • Search Town records for prior approvals: Use the Accessory Apartment Permits Search tool at https://islipny.gov/community-and-services/acc-apartments-permits/apartments-search.

If It’s Illegal or Unpermitted

  • Common on Long Island; buying doesn’t legalize it—you’re responsible for violations.
  • To legalize: Apply to the ZBA for a new permit (with hearing), get Building Department approvals, and update the certificate of occupancy.
  • Costs can add up; negotiate seller credits. A home inspection can spot illegal setups (e.g., separate kitchens or meters).

At Jones Hollow Realty Group, we recommend a real estate attorney and thorough title search to avoid surprises.

Incentives: The Plus One Accessory Apartment Program

Launched in December 2024, this state-funded program (administered by the Long Island Housing Partnership—LIHP) offers grants up to $125,000 ($112,500 for construction) to create new ADUs or legalize/upgrade existing ones. Eligibility includes:

  • Owner-occupants of single-family homes in Islip.
  • Income limits (low- to middle-income households).
  • Compliance with Town zoning and codes.
  • Priority for properties near transit or with affordability commitments.

Applications are first-come, first-served via LIHP. Review guidelines and apply at https://www.lihp.org/adu.html. For assistance, call LIHP at (631) 435-4710.

Resources and Contacts

Disclaimer

This guide is for informational purposes only and is not legal advice. Rules are subject to change; consult the Town of Islip, a qualified attorney, or a licensed professional for your specific situation. Jones Hollow Realty Group is not responsible for any actions taken based on this information. Updated as of January 2026.

*For more guides on accessory apartments in Suffolk County townships like Islip, Brookhaven, Babylon, Smithtown, and Huntington, visit our main blog post: [Link to Accessory Apartments: Your Roadmap to Legal ADUs in Long Island’s Key Townships].

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