Short answer
Many ADUs do not need a full architect-led process, but custom detached ADUs, hillside sites, tricky garages, and design-forward backyard homes may justify one.
| Item | Planning range | Notes |
|---|---|---|
| Pre-approved or adapted ADU plan help | $2,500-$8,000 | Best when the design is mostly settled. |
| Custom ADU design and permit set | $8,000-$28,000+ | Custom design and city comments raise cost. |
| ADU design-build preconstruction | $3,000-$15,000+ | Often credited or folded into construction if you proceed. |
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.