I'm looking to build an ADU with a habitable basement and keep it classified as single story. Single story maximum height is 18' where I live (San Jose, CA). Additionally, basements must have an emergency exit in every room, which can be an emergency exit window, provided that the sill height of the window doesn't exceed 44" above finished floor and the window meets certain height and width and area requirements.

If the finished basement floor is 36" (3') below grade, and the sill height of the emergency windows are 44" above the finished basement floor (to meet emergency exit requirements), they will end up 8" above grade. With an 8' ceiling in the basement, this means the 1st floor finished height above grade will be 5' plus the thickness of the floor/ceiling separating the basement from the first floor. Depending on construction technique, I could imaging this being as little as 5" (post tension concrete slab) to as much as 14.125" (2"x12" floor joists, 5/8" rock below, 3/4" plywood above, 3/4" flooring above plywood). This would put the finished first floor level anywhere from 5' 5" to 6' 2.125" above grade. With a 8' ceiling and a flat roof, I think I can stay below the 18' overall height requirement, but I'm not sure about whether this finished first floor level height is too high or not. Obviously it would necessitate an entrance with some sort of exterior landing and stairs, but that seems solvable (a portion of my current home has a floor at 40" above grade with a landing and set of stairs), if a bit higher than your typical ground level with crawl space home.

So, how high is too high for the finished first floor level? Said another way, if you took a 2 story ADU structure with the first floor slightly above grade, how far do you have to transition it below grade before the first floor now is called a basement and the entire structure can be considered a "single story"?

  • There are other methods, a window well can be built and exterior stair wells , we did both , on the coast with different properties to provide more total square footage without exceeding the height restrictions.
    – Ed Beal
    Mar 16, 2020 at 6:25

2 Answers 2


Per the International Building Code (IBC):

"The portion of a building having its floor subgrade (below ground level) on all sides. This definition of “Basement” is limited in application to the provisions of Section 1612."

"BASEMENT. A story that is not a story above grade plane (see “Story above grade plane”). This definition of “Basement” does not apply to the provisions of Section 1612 for flood loads."

The definition is in the code that is used by your jurisdiction, which is almost surely the IBC. However, your city, county or state may have revised it to their desire as well. You need to determine the code that applies to your area to verify.

It would seem that a basement is any level whose floor that is not entirely at or above grade, and is not related at all to the wall heights


The Code defines a basement as “That portion of a building that is partially or completely below grade AND when the floor above is 1) more than 6’ above the grade plane, or 2) when any portion of the floor above is more than 12’ above any finish ground level at any point.” (See ICC R202)

“Grade Plane” is defined as the average height of grade on all walls.

Yes, an “egress window” from a sleeping room is required to have a maximum sill height of 44”. If the basement is lower than 44”, then you can create a “window well”, but it must always be accessible and a certain size.

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