Why ADU plans are the single biggest decision before permits
Your ADU plans determine your timeline, your cost, and what your finished unit looks like. In California today you have two real paths: pick from a pre-approved plan library, or hire an architect for a custom design. Both work; they just suit different projects.
Pre-approved ADU plan libraries
Los Angeles, San Diego, Long Beach, San Jose, and a growing number of California cities publish libraries of ADU plans that have already cleared building plan check. If you pick one and your lot fits the footprint, you skip the longest step in the permit process.
Best for: Standard rectangular lots, homeowners who care more about timeline than design uniqueness, and garage conversions where the footprint is locked in.
Tradeoffs: Limited size and layout options. Most libraries top out around 1,000 to 1,200 square feet. You also cannot mirror, rotate, or modify the plans without re-triggering plan check.
Custom ADU plans
A licensed architect or design-build firm draws plans specifically for your lot. You control the layout, ceiling heights, window placement, and exterior aesthetics. Plans then go through standard city plan check, which adds four to twelve weeks depending on the jurisdiction.
Best for: Hillside lots, irregularly shaped lots, homeowners who want a specific architectural look, and any project larger than 1,200 square feet.
Tradeoffs: Higher design cost and longer permit timeline.
What good ADU plans always include
- Stamped architectural drawings (floor plan, elevations, sections)
- Structural engineering calculations
- Title 24 energy compliance report
- Mechanical, electrical, and plumbing schematics
- Site plan showing setbacks, easements, and existing structures
If a builder hands you "plans" missing any of these, the city will reject the submittal.
How to pick between the two
Walk your lot with your builder. If your backyard is a flat rectangle big enough for a stock plan, the library route saves you weeks. If your lot has slope, irregular setbacks, or you want a two-bedroom design that the library does not offer, custom is worth the extra time.
For a property in the City of Los Angeles, use our pre-approved ADU plan site-fit checklist to compare candidate footprints, entrances, utilities, and foundation conditions before choosing a design.
Related reading
Frequently asked questions
Can I modify a pre-approved ADU plan? Minor cosmetic changes (paint, finishes) are fine. Structural or footprint changes require re-submittal for plan check.
Do all California cities offer pre-approved ADU plans? Not yet. The list is growing — LA, Long Beach, San Jose, San Diego, Sacramento, and several others have libraries.
How long does custom ADU plan design take? Typically four to eight weeks from kickoff to permit-ready, depending on architect workload and revision rounds.