# Basements and allowable area in the 2012 IBC



## Markmax33 (Sep 29, 2013)

I'm trying to summarize the requirements for basements in the 2012 IBC when it comes to allowable area in a building.  Can somebody please confirm my logic or correct me?

506.4 Single occupancy buildings with more than one story.

The total allowable building area of a single occupancy building with more than one story above grade plane shall be determined in accordance with this section. The actual aggregate building area at all stories in the building shall not exceed the total allowable building area.

Exception: A single basement need not be included in the total allowable building area, provided such basement does not exceed the area permitted for a building with no more than one story above grade plane.

506.5 Mixed occupancy area determination.

The total allowable building area for buildings containing mixed occupancies shall be determined in accordance with the applicable provisions of this section. A single basement need not be included in the total allowable building area, provided such basement does not exceed the area permitted for a building with no more than one story above grade plane.

1.  If you have a single basement it does not count toward the allowable area of the building under any circumstances.

2.  The single basement must not exceed the area permitted for a building with no more than one story above grade plane.  I take this to mean that you pretend the basement is a one story building and apply the building code to it and figure out the allowable area.

Here's where it get a little murkier for me:

3.  If you have more than one basement - those basements most be included in the allowable are for the building calculation.  You do not need to include the first basement though.  It seems to say that in the 506.4 commentary.

4.  I do not see a maximum number of basements.  I guess once you get too many basements you end up at an underground building?

5.  The fire areas for the basements could theoretically span from the basement vertically up to the 1st floor and maybe beyond?  Generally you would see rated construction but maybe you would see a Type IIB building where that fire area spanned multiple floors.

Thanks for any advice in advance!


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## RLGA (Sep 30, 2013)

1. Correct.

2. Correct.

3. Correct.

4. Correct.  When the floor level of a basement is located more than 30 feet below the lowest level of exist discharge, then the building must conform to the requirements for underground buildings.  Whether or not basement levels are part of an underground building, the areas of the basements (excluding the first level basement) must be included when determining compliance with allowable building area.

5. Yes, if stories are not separated.


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