Foundation

The three common foundations

Most Portland DADUs use one of three:

1. Slab on grade. Cheapest. Common on flat lots. ~$22,000-$32,000 for an 800 sq ft footprint.
2. Stem wall + crawl space. Good in Portland's wet climate; gives access to plumbing under the floor. ~$30,000-$45,000.
3. Pier + grade beam. For sloped lots or where geotech requires deeper bearing. ~$35,000-$60,000+.

The geotech report at step 1.2 (if you needed one) drives the choice.

Inspections

• Pre-pour. PP&D's structural inspector verifies rebar size, spacing, embedment, and depth match the engineered drawings. Schedule 24-48 hours ahead.
• Post-pour foundation survey. Some lots need a foundation-location survey to verify setbacks before framing — your survey contract from step 1.1 should cover it.

What goes wrong here

• Rain. Portland gets 36+ inches a year, mostly October-May. Pouring through the rainy season is doable, but it adds tarp and temp-cover costs.
• Underground surprises. Old foundation footings, abandoned cisterns, buried oil tanks. That last one is a real Portland-specific risk — many older lots have decommissioned heating-oil tanks that turn up during excavation. Decommissioning runs $2,500-$8,000.