Basement finishing on Long Island

In the late 1940s, Abraham Levitt and his company on Long Island combined a contemporary plan and materials with a traditional Cape Cod form and built thousands of copies of it in Levittown, making the Cape Cod a standard postwar suburban house.¹¹ Levitt houses were built on a concrete slab with embedded heating pipes instead of over a costlier excavated basement.¹² Those heating pipes are still in many of the slabs, so drilling or cutting the floor of a Levitt house takes care.

Houses by other builders, and many from later decades, do have a basement or a lower level partly below grade. Those are the houses the rest of this guide is about.

The EPA recommends testing for radon on the lowest lived-in level, so a basement that hasn't been tested should be tested before it is remodeled; fixing a radon problem is usually easier and cheaper before the basement is finished.¹³ Nassau and Suffolk are not among the 40 counties on the New York State Department of Health's list of high radon area counties, but the state still recommends testing all homes for radon.¹⁴

The New York State Department of Health advises testing for radon every five years, every two years if the home has a mitigation system, and also when buying a home and after major renovations.¹⁵ A state test kit costs $12¹⁵. The EPA's guidance is to fix your home if radon tests at 4 picocuries per liter or higher, and to consider fixing it between 2 and 4.¹⁶

The state's Long Island groundwater study found that more recharge and higher sea levels can raise water levels, raising concerns about groundwater flooding of below-ground spaces in low-lying areas where the soil above the water table is already thin.¹⁷ Ask the company whether houses on your street have had high water in their basements before you plan a finished floor below grade.

Suffolk County's wastewater study puts about 209,000 of its septic systems in priority areas, which include places where groundwater is less than 10 feet below grade.¹⁸ Before turning basement space into living space, the EPA advises checking it for leaks and other signs of moisture and fixing any moisture problem first, because damp basements promote mold and other biological growth.¹³ The EPA warns that carpet on a basement slab can stay damp from water or vapor coming up through the concrete, or from room moisture condensing on a slab that is colder than the air, and either can lead to mold.¹³

Building America advises keeping fiberglass batts and cellulose out of contact with foundation walls, and not putting polyethylene sheeting, vinyl or foil wallpaper, or epoxy, oil or alkyd paint on the inside of basement walls, because they trap moisture.¹⁹ ENERGY STAR's guidance for existing homes, based on the 2021 IECC, is R5 insulating sheathing or R13 batts on basement walls in climate zone 3, R10 sheathing or R13 batts in zones 4A and 4B, and R15 sheathing or R19 batts in zones 4C and 5 through 8.²⁰ Ask which climate zone and wall assembly the quote assumes, and check that the permit drawings show the same.

In a mapped A flood zone with a base flood elevation, New York requires the lowest floor of a new or substantially improved building, including any basement, to sit at or above the base flood elevation plus two feet.²¹ A basement counts as the lowest floor under that rule, so in a substantially improved house in that zone, finished space below grade is generally off the table.

In New York, a building that is substantially improved, meaning the work is valued at more than half of the structure's market value, falls under the same floodplain development rules as new construction.²¹ Floodplain management in New York is run locally: work in a FEMA special flood hazard area needs a floodplain development permit from the municipality, whose designated floodplain administrator is usually the building inspector.²¹

The figures below come from the national model code, which New York adapts for its own residential code, so confirm them with your building department. The 2021 IRC requires habitable space and hallways, including the parts of a basement that contain them, to have a ceiling at least 7 feet high, and bathrooms and laundry rooms at least 6 feet 8 inches.²² In a basement with habitable space, beams, girders, ducts and other obstructions may hang down to within 6 feet 4 inches of the finished floor.²²

The 2021 International Residential Code requires at least one operable emergency escape and rescue opening in a basement, plus one in each sleeping room, including every bedroom in a basement.²² Under that code, an escape opening needs a net clear opening of at least 5.7 square feet, at least 24 inches high and 20 inches wide, with its bottom no more than 44 inches above the floor.²² An escape window whose bottom sits below the ground outside needs an area well of at least 9 square feet, at least 36 inches out from the wall and 36 inches wide, big enough for the window to open fully.²²

In Suffolk a basement bedroom raises a separate question. Suffolk's Sanitary Code counts as an expanded single-family residence any addition or change that requires new or modified sewage disposal, or that adds bedrooms beyond the number the Department's Office of Wastewater Management previously approved.²³

More than one in four adults 65 and older fall each year.²⁴ An open-sided cellar stair is a likely spot for a fall, so finishing the basement is the time to add a second rail, a light switch at the top and bottom and contrasting edges on the treads. NAHB's Certified Aging-in-Place Specialist (CAPS) credential trains remodelers in home modifications for aging in place.²⁵

The number on this page reaches one independent company that handles basement finishing in the Long Island area, and the call is recorded. What gets built, when and for how much is between you and that company.

The company pays us for the call. You pay us nothing, and we don't call or text you later. How we make money

New York State does not license home improvement contractors, but some local governments do, including New York City, the City of Buffalo, and Nassau, Suffolk, Westchester, Putnam and Rockland counties; where a local license is required, the Department of State says to hire only a licensed contractor.²⁶ Nassau County's Office of Consumer Affairs licenses home improvement contractors, and its daily-updated lookup shows each license's status and what work the contractor's insurance does and doesn't cover.²⁷ Suffolk County's License and Complaint Violation Look Up searches county license records, including home improvement, master plumbing and master electrical licenses, by license number, name, business name, phone or address, along with complaint and violation information.²⁸ Check a New York license

A New York homeowner may cancel a home improvement contract until midnight of the third business day after signing, by written notice that shows they don't intend to be bound; the right doesn't apply when the owner called the contractor for a bona fide emergency and signed a handwritten waiver.²⁹