Oregon's ADU rules opened up a lot of property for a second dwelling unit, but the foundation requirements are the same as for any other code-compliant residential structure. Here is what that means in practice.
Permit and inspection
Every ADU foundation in Oregon requires a building permit and at least one foundation inspection before the pour. The permit is pulled by the owner-builder or the GC, not by the concrete contractor. The inspection happens after the footing is dug and the rebar is set, before any concrete goes in the forms.
Engineered plans
Most jurisdictions require stamped engineered plans for the foundation. Your designer or architect typically includes the foundation in the structural drawing set. We pour to whatever the engineer specs; we do not redesign foundations.
Foundation types that work for ADUs
Three foundation types cover almost every ADU we pour in the Umpqua Valley.
- Monolithic slab-on-grade with thickened edge: fastest, cheapest, works on flat sites
- Continuous footing with stem wall: better on sloped lots, allows crawl space access
- Pier and grade beam: for expansive or poor soils, or hillside builds
Timing realities
From signed contract to poured ADU foundation is typically six to twelve weeks: permit approval (two to six weeks depending on jurisdiction), site prep and excavation (one week), forms and rebar (two to four days), inspection (one to three days), pour and cure (one week before framing). Plan accordingly.