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Guide to Accessory Apartments in the Town of Smithtown

What Homeowners and Buyers Need to Know

Important Note on Incorporated Villages: The Town of Smithtown includes incorporated villages such as the Village of the Branch, Head of the Harbor, and Nissequogue, which have their own independent zoning and building codes. If your property is located within one of these villages, accessory apartment rules may differ significantly or be prohibited. This guide applies primarily to unincorporated areas of the Town of Smithtown. Always check with your village’s building or zoning department for village-specific regulations. For example, villages may have stricter limits on secondary units or require separate applications. Contact information for villages is available on their respective websites (e.g., Village of the Branch).

Welcome to the Jones Hollow Realty Group resource center! If you’re a homeowner in the Town of Smithtown considering adding an accessory apartment (also known as an accessory dwelling unit or ADU, often referred to as a “mother/daughter” unit) 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 housing for family members, but they are strictly regulated and typically limited to family occupancy in Smithtown. Non-family rentals may not be permitted, and compliance is essential to avoid penalties.

This guide is based on the Town of Smithtown’s Building Zone Ordinance (Chapter 322) and related FAQs. Note that Smithtown does not have a dedicated code section for general accessory apartments; they are handled as “mother/daughter” permits for family-related use. 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?

In Smithtown, accessory apartments are commonly known as “mother/daughter” units—a secondary dwelling unit within an owner-occupied single-family home, typically intended for close family members (e.g., parents, children, or siblings). They must be integrated into the main structure and cannot be in detached buildings like garages or sheds. The purpose is to provide temporary family housing while preserving the single-family character of neighborhoods. General rental apartments for non-family members are not typically allowed; permits are restricted to family occupancy. Unlike neighboring towns, Smithtown emphasizes family-only use and does not broadly encourage ADUs for rental income.

Key Eligibility and Requirements

Accessory apartments require a special “mother/daughter” permit and must comply with building codes for two-family dwellings. Detailed size and design limits are not explicitly outlined in the zoning code but are reviewed case-by-case by the Building Department. Common requirements based on similar Long Island towns include:

Property and Lot Requirements

  • Zoning Districts: Allowed in single-family residence districts (e.g., R-43, R-21, etc.), subject to approval. The home must meet general zoning standards for the district.
  • Location: Must be within the principal dwelling; basement or detached units may be restricted.
  • Limit: Only one per property.
  • Health and Safety: Must meet New York State Uniform Fire Prevention and Building Code for two-family dwellings, Suffolk County health codes (e.g., septic systems), and Town standards.

Design and Size Limits

  • Size and Bedrooms: Typically limited to ensure the unit remains accessory (e.g., no more than 35-40% of the main home’s area, up to 2 bedrooms), but specific limits are determined during permit review. Separate kitchens and entrances are allowed but must not alter the single-family appearance.
  • Exterior Appearance: Changes must maintain the single-family look; no visible alterations that suggest a multi-family structure.
  • Expansions: Additions for the unit require building permits and may trigger variances or reassessments.

Owner and Occupancy Rules

  • Owner Occupancy: The property must be owner-occupied, and the accessory unit is for family members only (e.g., mother/daughter or similar relations). Non-family rentals are generally prohibited.
  • Rental Restrictions: Not intended for short-term or non-family rentals; discrimination in family housing is prohibited under fair housing laws.
  • Prohibitions: Permits may be denied if the setup violates zoning or if used commercially.

Parking and Maintenance

  • Off-Street Parking: Additional spaces required for the unit (typically at least 1-2), arranged to maintain residential appearance.
  • Upkeep: Property must be maintained per Town codes; violations can lead to permit denial or revocation.

Violations can result in fines, orders to remove the unit, or legal action.

The Permitting Process

Accessory apartments require a “mother/daughter” permit from the Building Department:

  1. Application: Submit to the Building Department with plans, proof of ownership, affidavits of family occupancy, and fees. May involve a review for compliance with zoning and building codes.
  2. Review and Approvals: Building Department reviews; may require inspections or variances from the Zoning Board of Appeals if needed.
  3. Timeline: Varies; construction must comply with permit timelines.
  4. Renewal: Every 3 years; submit renewal application with updated affidavits confirming family occupancy.
  5. Revocation: For violations or non-family use, after notice.
  6. Fees: Application and renewal fees apply; contact the department for current rates.

For forms, visit the Town’s Applications page: https://www.smithtownny.gov/266/Applications-Construction-Information. Schedule inspections at (631) 360-7522.

Buying a Home with an Existing Accessory Apartment

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

If It’s Already Legal

  • Permits are tied to the property and occupancy; upon sale, the new owner must apply for renewal or transfer within a reasonable time.
  • Renew every 3 years with proof of family occupancy.
  • Check Town records: Contact the Building Department for permit status.

If It’s Illegal or Unpermitted

  • Common issue; purchase does not legalize it—you’re liable for violations.
  • To legalize: Apply for a mother/daughter permit, obtain building approvals, and update the certificate of occupancy.
  • Costs can be significant; negotiate seller concessions. A home inspection can identify illegal features (e.g., unauthorized kitchens).

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

As of the latest information, the Town of Smithtown does not participate in New York State’s Plus One Accessory Apartment Program, which offers grants up to $125,000 for creating or legalizing ADUs in other Suffolk County towns. Smithtown has expressed concerns about broader ADU initiatives. However, check for updates or alternative programs like the Senior Citizen Home Maintenance Program for eligible repairs. For general Plus One info, visit https://hcr.ny.gov/adu.

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 Smithtown, 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.

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