Bathrooms in Basements and Cellars

Owners of homes with underground floors frequently ask if it’s possible to build another bathroom there.  The answer entirely depends on whether it’s a basement or cellar.

Although the two terms basement and cellar are largely interchangeable in everyday English, they’re distinctly different terms according to the Department of Buildings (DOB).  A basement is at least 50% above grade and is considered a “habitable space” by the DOB.  If it meets certain code requirements, it can serve as a bedroom or an entire apartment unit.  As such, it can accommodate a bathroom.

A cellar, which is at least 50% below grade, may have a half-bathroom, but in most cases, it may not have a full, three-fixture bathroom.  This is because a cellar is not considered habitable space.  It can be renovated to accommodate certain household functions—for instance, a rec or storage room—but its official utility is fairly limited.  Since it’s illegal for people to live in the cellar anyway, there’s generally no need for them to have a full bathroom.

However, note above we wrote “most cases.”  In some cases, one- and two-family houses can have a full bathroom in the cellar.  However, it must meet the following criteria given by the DOB:

  1. The cellar is open without partitions except for enclosures for:
    1. a boiler room
    2. a laundry room
    3. windowless storage and/or utility rooms limited to less than 60 square feet
  2. The cellar is connected via an unenclosed stair to the residence above, as well as direct access to the outdoors via stairs and an operable door
  3. The cellar is equipped with smoke and carbon monoxide detectors
  4. The boiler room is enclosed with one-hour fire-rated construction and properly ventilated
  5. If the cellar also contains a garage, the garage must be separated from the cellar by one-hour fire-rated construction.