The Underground World of McMansion: The Basement

We often get asked why we don’t typically have basements in Texas, but places like Missouri have them.  As a general rule, a concrete foundation should be approximately 12” below the frostline to prevent the ground from freezing below your foundation and lifting the foundation upwards due to expansion.  In Austin, the frostline is about 4-6” deep but in Missouri, the frost line can be as deep as 3 feet.  In cold locations then, a basement is an extension of space that was required to dig the foundation.

This doesn’t mean you can’t have a basement in Austin. If you live in one of the urban neighborhoods, one reason for a basement is that basement square footage is not counted as Buildable Area under the “McMansion Ordinance”.  

In the new version of the City of Austin Land Development Code, several unconditioned spaces, such as covered patios and garages, count against the Buildable Area maximum, so people are choosing to go underground to increase their Buildable Area without increasing the scale of the building above grade. Because of this, basements have become popular in Austin.

For a basement to qualify, it has to meet the following two criteria from the City of Austin Land Development Code (3.3.3):


1. The habitable portion does not extend beyond the first-story footprint and


    a. Below natural or finished grade, whichever is lower; and


    b. Surrounded by natural grade for at least 50% of its perimeter wall area, if the
        habitable portion is required to be below natural grade under paragraph 1.a.


2. The finished floor of the first story is not more than three feet above the average elevation at the intersections of the minimum front yard setback line and the side property lines.

The code relevant to new residential construction in Austin is the 2009 International Residential Code. In this code there are quite a few considerations affecting basements. Basements need a second exit such as an operable window or second door into a window well or another method of exiting.  The second means of egress needs to be a minimum 5.7 square feet with a sill no more than 44” above the floor.

In Austin, which is in climate zone 8, the Residential Code requires either insulating the walls or the floor between the basement and first level.  We suggest using rigid insulation board outside the walls; a thickness of 2” for an R-10 value, is required by code, as well as insulating the ceiling, mainly for acoustical separation. The building code also requires a minimum head height of 7’-0” for a basement.