Free planning tool
A clearer picture of your ADU budget.
A square-foot price is only the beginning. Put the rest of the project in the same picture, before you compare quotes.
02 / Illustrative example
All-in planning allowance
$318,550
600 sq ft · $531 per sq ft including all categories
- Construction
- $210,000
- Design & engineering
- $25,000
- Permits & fees
- $12,000
- Site & utilities
- $30,000
- Contingency
- $41,550
Contingency is 15% of all four base categories. This worksheet adds your assumptions; it does not predict a contractor’s price or confirm what fits on your lot.
How to use this ADU cost calculator
The starting values are an illustrative scenario created by Local ADU Guide, not a survey of local bids. Change each allowance to match written estimates. An included item belongs in one category only: if your construction proposal includes utility work, remove that amount from the separate site allowance.
For regional context, the San Mateo County ADU Resource Center gives a rough $450–$600 per square foot placeholder that includes hard construction costs and soft design/fee costs. That is not a construction-only input for this worksheet. Actual costs vary by scope and site. Source accessed September 7, 2026.
What to check before relying on the total
- Ask whether demolition, excavation, foundation, utility trenching and electrical service upgrades are included.
- Check allowances for structural engineering, surveys, soils work, agency fees and energy documentation.
- Add any financing costs, taxes, insurance, landscaping or furnishings you need separately; this worksheet does not calculate them.
- Use written contractor and agency estimates before making financial commitments. The entered size is a budget input, not a zoning approval.