Quick Answer: An Accessory Dwelling Unit (ADU) is a secondary housing unit on a single-family or multi-family residential lot in California. As of 2025, California state law allows ADUs on virtually all single-family lots with few restrictions. ADU types include detached (new construction), attached (addition to main home), garage conversions, and Junior ADUs (JADU, within the primary home). ADUs in the Bay Area cost $100,000–$400,000 depending on type and can generate $2,000–$4,000/month in rental income.
ADU Types in California — What You Can Build on Your Lot
Helpful Resources:
| ADU Type | Description | Typical Cost (Bay Area) | Max Size |
|---|---|---|---|
| Detached ADU | Separate new structure in backyard | $180,000–$400,000 | 1,200 sq ft |
| Attached ADU | Addition to existing home with separate entrance | $150,000–$350,000 | 50% of primary unit |
| Garage conversion ADU | Convert existing attached/detached garage | $80,000–$180,000 | Existing garage footprint |
| Junior ADU (JADU) | Convert room within primary home | $30,000–$100,000 | 500 sq ft |
| Manufactured/prefab ADU | Factory-built unit installed on-site | $120,000–$280,000 | 1,200 sq ft |
California ADU Laws — Key Rules for 2025
- Allowed on all single-family lots: California state law (AB 2221, SB 897) overrides local zoning to allow ADUs on virtually all residential lots
- Permit approval in 60 days: Cities must approve or deny ADU permits within 60 days of a complete application
- No owner-occupancy requirement (through 2025): You don’t need to live on the property to build an ADU — though some cities retain this for JADUs
- No parking replacement: Converting a garage to an ADU does not require building replacement parking spaces
- Solar panels required: Most new ADUs in California must include solar panels (Title 24 requirement effective 2020)
- One ADU + one JADU per lot: Most single-family lots can have one ADU plus one JADU (500 sq ft max within the home)
ADU Rental Income Potential in the Bay Area
Bay Area ADU rental rates in 2025: Studio ADU (300–450 sq ft): $1,800–$2,400/month. One-bedroom ADU (450–700 sq ft): $2,200–$3,200/month. Two-bedroom ADU (700–1,000 sq ft): $2,800–$4,000/month. San Jose, Sunnyvale, and Cupertino command the highest ADU rents in the Bay Area due to proximity to tech employers.
Frequently Asked Questions
Can I build an ADU in San Jose if I have an existing garage? Yes — converting a garage to an ADU is one of the most popular and cost-effective ADU options in San Jose. You don’t need to provide replacement parking when converting a garage within half a mile of a transit stop (which covers most of San Jose). The conversion requires building, plumbing, electrical, and mechanical permits.
What is a Junior ADU (JADU) in California? A Junior ADU (JADU) is an independent living unit of up to 500 square feet created within the existing walls of a single-family home. It must have a separate entrance but can share plumbing with the main home. JADUs are the most affordable ADU option — often $30,000–$80,000 — and require minimal permits in most California cities.
Do I need solar panels on a new ADU in California? Yes — under California’s Title 24 2019 Building Energy Efficiency Standards (effective January 2020), new ADUs require solar photovoltaic (PV) systems. Minimum system size is determined by an energy calculation. The CalHFA ADU Grant Program offers grants up to $40,000 that can cover solar installation costs.
Quartz Construction & Remodeling (CSLB #1003436) is a full-service ADU builder serving San Jose and the Bay Area — from design and permits through final inspection. Call (650) 749-7436 for a free ADU feasibility review and cost estimate.
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"Quartz Construction did an amazing job on our kitchen remodel. The team was professional, on time, and the results exceeded our expectations. Highly recommend to any Bay Area homeowner!"
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"Just finished our ADU garage conversion. The project came in on budget and on schedule — rare in the Bay Area! The rental income already started. Could not be happier with Quartz Construction."
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