House design · Choose the structure and foundations · design-14

Choose and design the foundations

Foundations carry all loads to the ground and protect walls from moisture and frost. The type depends on the geotechnical report from the site module, the wall system and the climate.

How

Safe room first: if you plan one, finish secure-1 to secure-4 before this step. FEMA notes ICC 500 often needs more wind resistance than a typical slab gives, so the room may need its own foundation.
Get soil bearing capacity, reactive clay, groundwater and frost depth from the site module's geotechnical work.
Match foundation width to wall type: straw bale and thick earth walls need wider footings and slabs (Your Home).
In cold climates consider a frost-protected shallow foundation, which uses perimeter insulation to raise frost depth. HUD's NAHB study showed depths as shallow as 16 inches in severe US climates.
Insulate slab edges and the underside of the slab in cool and cold climates (Your Home).
Keep moisture-sensitive walls well above ground: the US straw bale appendix requires bales at least 8 inches above earth.
Plan service penetrations through the slab (water, waste, power conduits, earthing) with the other modules before pouring.
Hazards: set the floor level against the local design flood elevation (design-27), follow the seismic detailing in design-26 and resist uplift in high wind areas (design-25).

Do these first

Sources

Frost-Protected Shallow Foundations, Phase II Final Report. NAHB Research Center for US Department of Housing and Urban Development, accessed 2026-11
Construction systems. Your Home, Australian Government, accessed 2026-11
Thermal mass. Your Home, Australian Government, accessed 2026-11
2015 IRC Appendix S: Strawbale Construction. International Code Council (copy hosted by Ecological Building Network), accessed 2026-11
Foundation and Anchoring Criteria for Safe Rooms (fact sheet, 2021). US Federal Emergency Management Agency (FEMA), accessed 2026-11

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