7) Step 5: Budget framework: where ADU costs really come from
Instead of searching for a single “ADU cost in San Diego” number, build your budget from the drivers that actually change pricing:
Major cost drivers
- Site work: grading, drainage, retaining needs
- Foundation type and soil considerations
- Utility connections: sewer tie-ins, electrical upgrades, trenching distance
- Structural complexity: rooflines, multi-level builds, framing complexity
- Finish level: basic durable finishes vs custom/luxury details
- Kitchen and bathroom scope: cabinetry, tile work, fixture quality
- Windows and doors: quality, size, and placement
- Permits, engineering, and required reports (project-specific)
Practical budgeting approach
- Decide your target finish level early (rental durable vs premium)
- Prioritize performance (insulation, windows, waterproofing)
- Set a contingency buffer for surprises, especially on older properties
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