Accessory Apartments on Long Island in 2026: Which Towns Allow Them, What They Require, and What Help Exists

“If our town and village government don’t allow enough housing to be built, legal, safe housing, the market will provide illegal and often unsafe housing.” That was Ian Wilder, executive director of Long Island Housing Services, speaking to News 12 in August 2026, and it describes the situation behind every question a homeowner has about adding a second unit to a house: the demand exists whether or not a town has written a rule for it.

The rules themselves vary so much from one municipality to the next that a house in Setauket and a house in Huntington Station can face opposite answers to the same question. Statewide law offers no uniform standard, grant programs open and close by town, and a lender’s view of rental income changed only this spring. For an owner considering an apartment, a buyer looking at a house that already has one, or a seller about to list it, the useful starting point is a map of who decides what.

This guide covers Long Island with attention to the North Shore towns. It reflects public sources available in early October 2026, several of which are older and are flagged where they appear, and a town’s building department has the last word on any specific property.

Accessory apartment, basement apartment, ADU: what the terms mean

Three labels circulate, and they overlap without matching. Towns such as Brookhaven regulate an “accessory apartment,” a second dwelling unit within or attached to a single-family house. “ADU,” short for accessory dwelling unit, is the label state and housing-agency programs prefer, and it can include detached units such as a converted garage or backyard cottage. A “basement apartment” is simply an accessory unit located below grade, and many towns regulate it separately because of the safety issues that come with it.

The distinction explains a pattern in the reporting. A town can ban new legal basement apartments and still permit accessory apartments elsewhere in the house, which is why the lists below should be read as a starting point, not a verdict on any property. Detached and converted outbuildings are a related subject, covered in What the Carriage House Knows.

Where New York State stands

Albany has tried, and backed away, once. In early 2022 Governor Kathy Hochul’s budget proposal would have required local governments to allow accessory units, including converted garages, basements, and backyard structures. Long Island officials objected, and the language came out of the budget. TBR News Media reported Hochul’s response as, “I have heard real concerns about the proposed approach on accessory dwelling units,” and quoted then-Brookhaven Town Supervisor Ed Romaine: “The call to remove this misguided proposal was finally heard by the governor.”

“Albany extremists will resurrect this terrible idea the moment bipartisan opposition gets distracted.”

Huntington Town Supervisor Ed Smyth, via TBR News Media, 2022

The state then shifted from mandate to money. In May 2024 the Governor’s office announced results from the $85 million ADU Plus One program, under which “homeowners outside of New York City may receive up to $125,000 per unit in the form of a grant.” Eligibility required households to “earn no more than 100 percent of the Area Median Income” and applied to “single-family homeowners who currently live in the primary unit.” Long Island was among the first regions to begin construction under the program.

A bill titled the Accessory Dwelling Unit Incentive Act, A6778, would create a forgivable loan program through the state’s housing agency. The State Senate’s legislation page shows it was referred to the Assembly Housing Committee on January 7, 2026, and the page showed no further action when it was checked for this guide. In practical terms, whether a Long Island homeowner can add or legalize a unit remains a local decision.

Which towns allow what

News 12’s August 2026 report, by Daniella Rodriguez, sorts Long Island’s thirteen towns by how they treat basement apartments specifically. Its wording: “Legal basement apartments are permitted in the towns of Hempstead, Brookhaven, Islip, Shelter Island and Southampton, provided they meet local code requirements and have the proper permits.” It adds that “several other towns — including Oyster Bay, North Hempstead, Babylon, East Hampton, Huntington, Riverhead, Smithtown and Southold — do not allow new legal basement apartments.”

Town	Basement apartments (News 12, Aug. 2026)	Other sources
Brookhaven	Allowed with permits	Accessory apartment license program; Plus One program listed as closed by the Long Island Housing Partnership
Huntington	No new legal units	2019 revisions reduced lot and frontage minimums and targeted unpermitted basement units; LIHP lists an active ADU program
Smithtown	No new legal units	No further detail found
Oyster Bay	No new legal units	No further detail found
North Hempstead	No new legal units	No further detail found
Hempstead	Allowed with permits	No further detail found
Islip	Allowed with permits	LIHP lists an ADU program as closed
Babylon	No new legal units	No further detail found
Riverhead	No new legal units	LIHP lists an active ADU program
Southold	No new legal units	LIHP lists an ADU program as closed; Plus One rollout reported in 2023
Southampton, Shelter Island	Allowed with permits	Plus One program reported in place in 2023
East Hampton	No new legal units	No further detail found

Two cautions come with the table. First, incorporated villages set their own zoning, and a Hofstra-affiliated study from 2016 counted 65 Long Island villages that prohibited accessory dwellings entirely, so a house inside a village such as Port Jefferson needs its own check with village hall. Second, the Huntington details are from 2019 reporting, the oldest material here. According to TBR News Media’s July 2019 coverage, the town was reducing minimum lot size for accessory apartments from 7,500 to 5,000 square feet, cutting frontage requirements from 75 to 50 feet, lowering the short-term rental allowance from 120 to 90 days a year, and banning basement apartments without valid existing permits. A CBS New York report that month described a related change letting owners live in the smaller unit and rent the larger one, with a cap of 10 percent of homes in any area. Anyone relying on those details should confirm what the code says now.

Brookhaven’s accessory apartment rules

Brookhaven, home to Setauket, Stony Brook, Port Jefferson Station, and Mount Sinai, publishes the most detailed framework among the North Shore towns. The figures below come from the town code as published on eCode360, Section 85-258, and they are quoted for orientation, not as a substitute for the current text.

• Owner occupancy: “The owner(s) of the lot upon which the accessory apartment is located shall reside within the dwelling that contains the accessory apartment.”
• Size: a minimum of 300 and a maximum of 650 square feet of habitable area, and “in no case shall it exceed 40% of the habitable area of the dwelling building.”
• Bedrooms: “In no event may there be more than one bedroom per accessory apartment.”
• Units per lot: “no more than one accessory apartment permitted per lot.”
• Appearance: the unit’s entry and design should keep the building looking like a one-family residence “to the degree reasonably feasible.”
• Parking: at least two off-street spaces for the resident owner, plus at least one designated space for the apartment’s occupants.
• Sewage: approval of the water supply and sewage disposal method from the town engineer before a building permit is issued.

The license is a separate matter. Brookhaven’s accessory apartment license page states that “Accessory Apartment Licenses are valid for 24 months from the date of issuance,” lists applications for both existing owners and new owner transfers, and names deeds, affidavits, photo ID, utility bills, property photos, and certificates of occupancy among the documents applicants should prepare. The code text on eCode360 describes provisional licenses for up to three years with renewals of up to five, which does not match the town’s own page. That mismatch is a sign details have changed, and a reason to confirm every figure above with Brookhaven’s building division before relying on it.

One consequence of the owner-occupancy rule deserves attention from investors: in Brookhaven the code, as published, contemplates the owner living in the house, not renting both units.

Safety and legal basics before renting

Legal status turns on paperwork and on physical safety, and the News 12 report covers both. Certificates of occupancy and permits are required, smoke detectors are required, and egress is a recurring problem for basement units. Annie Holdreith, a real estate broker interviewed for the story, put the window standard in concrete terms: “An egress window usually has to be the size to allow a 200-pound firefighter with oxygen tanks through.”

“If our town and village government don’t allow enough housing to be built, legal, safe housing, the market will provide illegal and often unsafe housing.”

Ian Wilder, Executive Director, Long Island Housing Services, via News 12

Three further points belong on any owner’s checklist. Short-term rentals are a separate legal category: the Plus One program guidelines for Brookhaven prohibit short-term, vacation, and seasonal rentals, and the Nassau side is covered in Nassau County’s Short-Term Rental Crackdown. Fair housing law applies to renting a unit, so advertising and tenant selection are worth reviewing with an attorney. And an unpermitted apartment can complicate a sale or financing later, a theme in The Seller’s Disclosure Form Is a Legal Document.

The PIX11 News video below, titled “The fight to safely legalize basement apartments,” covers the debate over bringing basement units up to code in the New York area; Long Island rules still differ town by town.

Septic and sewage approvals

For most Suffolk homes the limiting factor is not zoning but the pipe in the yard. Brookhaven’s code makes sewage disposal approval a precondition for the building permit, and the East End Beacon reported in 2023 that outdated or undersized septic systems were proving to be one stumbling block for ADU construction, with septic upgrades likely to push a project above the maximum state grant award.

The county side has its own thresholds. Suffolk County’s septic grant program lists “single-family with one accessory apartment” among its eligible residential property types, and the county’s 2020 law on nitrogen-reducing systems lists a bedroom addition that takes a home past five bedrooms as a trigger, according to Long Island Land Use and Zoning’s summary. An owner planning to add bedrooms for an apartment should ask the county early whether the project crosses that line. The full rules, costs, and grants for Suffolk systems are in Suffolk County Septic Systems in 2026, and the sequencing advice in How to Sequence a North Shore Fixer-Upper Renovation to Avoid the Permit Trap applies here too.

Grants for building or legalizing a unit

The grant landscape has two layers. At the state level, ADU Plus One funded construction for owners at or below 100 percent of area median income, with the Governor’s office describing grants of up to $125,000 per unit outside New York City. Locally, the Long Island Housing Partnership and Community Development Corporation of Long Island administer the money town by town, and the lists change. LIHP’s programs page currently shows the Huntington and Riverhead programs as active and the Brookhaven, Islip, and Southold programs as closed.

The strings attached are significant. The Brookhaven Plus One guidelines, dated 2023, require applicants to be the owner and to have lived in the home as a principal residence for at least a year, bar short-term rentals, and impose a 10-year compliance period with annual recertification. For the East End programs, the East End Beacon reported a 10-year commitment to occupy either the unit or the main house as a primary residence while renting the other to a permanent resident, with income limits near $109,000 for a household of one and $156,000 for a household of four at the time.

Community Development Long Island’s president, Gwen O’Shea, framed the case in the state’s announcement: “Accessory dwelling units provide multiple benefits to regions like Long Island with high costs and a shortage of rental housing.” Whether a particular town has funds open now is a question for LIHP at 631-435-4710 or the town itself, since the programs described in older coverage may have closed.

Financing and taxes

Lending policy shifted this year. Fannie Mae’s updated rental income policy, implemented in its underwriting software in March 2026, allows income from an ADU to be considered toward qualifying income, with conditions: the property must be a one-unit principal residence, the transaction must be a purchase or limited cash-out refinance, rental income may come from only one ADU even if several exist, and the amount counted “cannot exceed 30% of the borrower’s total qualifying income.” Whether a particular unit counts as an ADU under a lender’s definition, and whether an unpermitted unit can be counted at all, are questions for the loan officer, and Beyond the 30-Year Fixed lays out other products that first-time buyers should compare.

Owners building a unit with existing equity can weigh the tradeoffs in Renovating Without Refinancing. On taxes, the IRS notes on its rental income page that real estate rentals are generally reported on Schedule E and points to a separate topic for “special rules relating to the rental of real property that you also use as your main home.” A CPA can say how those rules apply to a specific house.

Buying a house with an apartment

A buyer’s first job is to learn whether the apartment is legal, because the answer decides what the house is. The list below applies in most towns.

• Ask for the certificate of occupancy and any accessory apartment license. In Brookhaven the license application includes a track for new owner transfers, so ask the town how a license passes to a buyer.
• Check egress, smoke detection, and permits with an inspector; the approach in The Inspection Report as a Negotiating Document fits here.
• Ask about sewage approval and the septic system and how many bedrooms the system was designed for.
• Read the owner-occupancy rule before counting on two rent streams, since Brookhaven’s code expects the owner to live in the dwelling.
• Talk to the lender early about whether rental income can be counted, given Fannie Mae’s 30 percent cap and the unit’s permit status.

Selling a house with an apartment

For sellers, legal status shapes both the marketing and the liability. A permitted, licensed apartment can be described accurately as a feature. An unpermitted one cannot be presented as a legal rental, and the line between presentation and misrepresentation is the subject of Staging Is a Material Representation. Sellers with a licensed unit should confirm how the town handles a license when ownership changes, since Brookhaven’s page distinguishes existing owners from new owner transfers, and sellers who suspect their unit lacks permits should speak with an attorney before listing. Older homeowners considering an apartment as part of a move can find related planning in The North Shore Downsizing Playbook.

What the older data says about scale

Reliable current counts are hard to find. A 2016 Hofstra University study of Long Island accessory dwellings, by Christopher Niedt and Katrin B. Anacker, estimated roughly 14,500 to 16,000 legal units, about 1.85 to 2.35 percent of single-family homes, and noted that older estimates of illegal units ran from 90,000 to 100,000 island-wide while being difficult to verify. The authors also wrote that “the devil is in the regulatory details like parking requirements, which have significant effects on ADUs’ incentives.” The report is a decade old, so it describes the shape of the issue, not today’s inventory.

Frequently asked questions

Can a homeowner add an accessory apartment anywhere on Long Island?

No. Rules are set by towns and by incorporated villages, and they differ widely. News 12’s August 2026 report lists five towns that allow legal basement apartments and eight that do not allow new ones.

Does New York State require towns to allow accessory units?

No. A 2022 proposal to require it was removed from the state budget, and a 2026 Assembly bill on incentives was in committee at last check.

Does the owner have to live in the house in Brookhaven?

According to the town code as published on eCode360, yes: the owner must reside within the dwelling that contains the accessory apartment.

How large can a Brookhaven accessory apartment be?

The code as published sets a range of 300 to 650 square feet, capped at 40 percent of the dwelling’s habitable area, with no more than one bedroom.

Are there grants for building an accessory unit?

The state’s ADU Plus One program offered up to $125,000 per unit outside New York City to owners at or below 100 percent of area median income, and local programs open and close by town. The Long Island Housing Partnership lists Huntington and Riverhead as active and Brookhaven, Islip, and Southold as closed.

Can rental income help a buyer qualify for a mortgage?

Under Fannie Mae’s policy updated in 2026, income from one ADU can count toward qualifying income, up to 30 percent of the borrower’s total qualifying income, on a one-unit principal residence purchase or limited cash-out refinance. Individual lenders determine how it applies.

Is a basement apartment automatically illegal in a town that bans new ones?

Not necessarily. The reporting refers to new legal basement apartments, and existing units with valid permits are a separate case. Confirm a unit’s status with the town’s building department.

Does a second unit affect septic requirements?

It can. Brookhaven requires sewage disposal approval before a building permit, and Suffolk’s nitrogen-reducing system rules list adding bedrooms beyond five as a trigger.

This is for informational purposes only — consult a licensed attorney or financial advisor. Zoning, grants, and lending rules change, and nothing here predicts the value of any property.

Real estate markets change. For current listings and market data, contact Maison Pawli at maisonpawli.com/about/.

Sources

• News 12 Long Island: Think the basement apartment is legal on LI? It depends on your town (Aug. 2026)
• Town of Brookhaven: Accessory Apartment License
• Town of Brookhaven Code, Article XIV, Section 85-258 (eCode360)
• TBR News Media: Hochul drops accessory apartment plans from proposed budget (Feb. 24, 2022)
• TBR News Media: Huntington revising accessory apartment rules (July 12, 2019)
• CBS New York: Long Island town changes accessory apartment law in response to high housing costs (2019)
• Governor Hochul: Nearly $60 Million Awarded to Create New Housing in 500 Accessory Dwelling Units (May 2, 2024)
• New York State Senate: Assembly Bill A6778, Accessory Dwelling Unit Incentive Act
• Long Island Housing Partnership: ADU Programs
• Long Island Housing Partnership: Plus One Accessory Apartment for Brookhaven Program guidelines (PDF)
• East End Beacon: Help for Accessory Apartments (Oct. 11, 2023)
• Hofstra University National Center for Suburban Studies: Accessory Dwellings on Long Island: An Overview (PDF, 2016)
• Fannie Mae Selling Guide Announcement SEL-2025-08
• Pennymac Correspondent: Fannie Mae Updates to ADU Rental Income (Announcement 26-29)
• IRS: Topic No. 414, Rental Income and Expenses
• Suffolk County: Septic Improvement Grants
• Long Island Land Use and Zoning: New Standards for Nitrogen-Reducing Septic Systems in Suffolk County Starting July 1st (2021)
• PIX11 News: The fight to safely legalize basement apartments (YouTube)

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