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