Per-square-foot comparisons often omit design, engineering, utility work, permit-related costs or difficult site conditions. Build the budget from the property outward: feasibility first, then the building, then finishes.
A practical way to look at it
Before debating countertops, locate the sewer connection, check electrical capacity and think through how concrete, framing and equipment will reach the site. Those questions can change an ADU budget long before finishes do.
ADU type changes the starting point
Garage conversions, attached ADUs, detached units and additions above garages have different structural and utility requirements. Existing construction is only valuable when it is documented, suitable and adaptable.
Site and utility work matter
Sewer route and capacity, water, electrical service, gas strategy, drainage, grading, trenching and equipment access can materially affect cost before interior selections begin.
Small units still contain expensive rooms
Every legal dwelling needs essential systems, and kitchens and bathrooms are cost-dense. A smaller ADU may have a higher cost per square foot even when the total is lower.
Separate soft costs
Track survey, design, engineering, energy documentation, utility coordination, permits and other professional services separately from construction until the proposal clearly includes them.
Verify current local rules
California and local ADU rules change. Confirm zoning, setbacks, height, parking, fire access, utility and fee requirements with the authority having jurisdiction for the actual property.
Move from idea to feasibility
ADELIE can help organize the site questions, desired unit, access and finish expectations needed for a useful early budget.
Sources and methodology
This guide separates official requirements from planning estimates. Government sources support permit and contractor statements; cost figures are broad market observations, not promises or project quotes. A dependable number still requires a site review and written scope.
- California HCD accessory dwelling unit guidance
- City of San Diego ADU and JADU information
- Swell Contracting — 2026 San Diego Remodel Cost Guide
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