ADU and Granny Flat Guide: California Regulations and Costs

California has streamlined ADU regulations to address the housing shortage. Here's everything you need to know about adding an accessory dwelling unit to your property.

What is an ADU?

An Accessory Dwelling Unit is a secondary housing unit on a single-family residential lot. Types include:

• Detached ADU: Standalone structure in backyard
• Attached ADU: Connected to primary residence
• Garage conversion: Existing garage converted to living space
• Internal ADU: Converted space within existing home
• Junior ADU (JADU): Under 500 sq ft, within existing structure

California ADU Regulations

Size Limits

• Detached ADU: Up to 1,200 sq ft
• Attached ADU: Up to 50% of primary home (max 1,200 sq ft)
• JADU: 150-500 sq ft

Setback Requirements

• 4-foot side and rear setbacks (often reduced from standard)
• No front setback requirement in many cases

What Cities Cannot Require

State law limits local restrictions:

• No minimum lot size for ADUs
• No owner-occupancy requirement during construction
• No parking requirement if near transit
• Must approve ministerially (no discretionary review)

ADU Costs in California

ADU Type	Cost Range
Garage Conversion	$100,000-$180,000
Attached ADU	$150,000-$280,000
Detached ADU	$180,000-$350,000
Prefab/Modular ADU	$120,000-$250,000

The ADU Process

1. Feasibility study: Site evaluation, regulations review
2. Design: Architectural plans (2-4 weeks)
3. Permit application: 60-day max review period
4. Construction: 4-8 months typically
5. Final inspection: Certificate of occupancy

Financing Options

• Home equity loans/HELOCs
• Cash-out refinance
• Construction loans
• ADU-specific financing programs

ROI Considerations

ADUs can provide:

• $1,500-$3,500/month rental income (varies by location)
• Property value increase of $100,000-$300,000
• Multi-generational living flexibility

Ready to build an ADU? Connect with ADU specialists on Bidroom.