Short answer
Addition plans vary with size, structure, city review, and whether the project needs design exploration or just documentation of a clear scope.
| Item | Planning range | Notes |
|---|---|---|
| Concept and feasibility | $1,500-$7,500+ | Smart before committing to full drawings. |
| Permit plans for modest addition | $10,000-$30,000+ | Structural coordination usually required. |
| Design-led addition | $25,000-$75,000+ | Higher when architecture, interiors, and construction support are included. |
How to use these numbers
Treat the ranges as planning context, not a quote. Bay Area homes punish lazy assumptions: existing conditions, utilities, structural scope, planning review, and consultant needs can move the number fast.