ADU Size Limitations in California: What You Must Know (2025 Guide)

1. Statewide Size Standards 

• Minimum ADU size: 150 sq ft 

• Detached ADUs: up to 1,200 sq ft max

• Attached ADUs: limited to 50% of the main home’s living area or 1,200 sq ft, whichever is smaller 

• Junior ADUs (JADUs): up to 500 sq ft, interior conversion units 

• Efficiency ADUs: max 500 sq ft, with minimum habitable dimensions 

2. Bedroom-Based Minimums

• One-bedroom ADU: must allow at least 850 sq ft 

• Two-bedroom (or more): at least 1,000 sq ft 

• Interior room minimums: bedrooms/living rooms ≥ 70 sq ft, kitchens ≥ 50 sq ft, bathrooms ≥ 30 sq ft 

3. Height & Setback Regulations

• Height limits:

  • Detached or attached ADUs: 16 ft minimum; up to 18 ft near transit/multistory homes; attached can reach 25 ft (same as main house) 

• Setbacks:

  • State allows 4 ft side/rear setbacks; front yard must accommodate an 800 sq ft ADU

4. Conversion & Existing Structure Rules

• Garage, basement, attic conversions: no max size cap if structure exists; JADUs still limited to 500 sq ft 

• Covered conversions: existing footprint remains valid — no expansion limits unless you extend structure

5. Multi‑Family and Lot Density

• Single‑family lots: one detached ADU + one JADU allowed 

• Multi‑family lots: up to 8 detached ADUs (one per existing unit), plus interior conversions 

6. Local Ordinance Variations

• Many cities/counties adopt stricter caps (e.g., San Francisco: 850 sq ft for 1 BR; Marin: up to 1,200 sq ft with conditions) 

• Always check with your ADU local planning department first!

7. Why 750 sq ft Matters

ADUs under 750 sq ft qualify for streamlined approvals and waived or lower fees 

8. Recent Legislative Updates (2022–2025)

• AB 68 (2019): added legal JADUs

• SB 13, AB 670, AB 881 (2019): capped local size, eased permit review, reduced fees 

• SB 1211 (2024): allows up to 8 detached ADUs 

• 2025: Agencies must offer pre‑approved ADU plans and enforce height/size minimums 

9. Practical Planning Tips

• Use pre‑approved plans for cost/time savings

• If aiming under 750 sq ft, benefit from reduced fees & simplified process

• Determine your lot type early (single vs. multi‑family) to understand eligibility

• Consult local codes upfront — variations are real and impactful

Final Thoughts