Understanding Maximum ADU Size Limits in California

When planning to build a detached ADU in California, here are the general guidelines:

Basic Guidelines:

- Up to 800 sq ft: Allowed in all jurisdictions, provided the ADU is under 16 feet tall and maintains 4-foot rear and side yard setbacks.

- Up to 1200 sq ft: Permitted in most areas, with around 391 cities and counties adhering to this standard.

Specific Restrictions:

- Some cities may limit ADU sizes to 1,000 sq ft for 2+ bedrooms or 850 sq ft for 1 bedroom due to bulk and massing limits like lot size and open space requirements.

Converted Structures:

- No size restrictions for converting existing structures, except Junior ADUs, which are capped at 500 sq ft.

Attached ADUs:

- Should not exceed 50% of the existing home’s size.

How to Determine ADU Size Limits for Your Property

1. Identify Your Jurisdiction:

- California has 539 different jurisdictions, including 58 counties and 482 cities. Check your jurisdiction's specific ADU rules, which may be updated frequently.

2. Jurisdiction-Specific Rules:

- Some local ordinances might allow larger than 1200 sq ft, confirm the state code, or have tighter restrictions.

Recommended ADU Size:

A 747-749 sq ft ADU is optimal:

- Impact Fees: ADUs over 750 sq ft may incur a $10,000 annual fee.

- Solar Requirement: ADUs over 800 sq ft require solar panels, costing around $30,000.

- Efficiency: A 747-749 sq ft ADU maximizes land use and efficiency without additional costs.

Planning Your ADU

- Detached ADUs Over 750 sq ft: May trigger additional fees and restrictions.

- Design Tricks: Combining an ADU with a Junior ADU (JADU) may avoid some limitations.