An Accessory Dwelling Unit is a secondary home on the same lot as a primary residence, with its own kitchen, bathroom, and entrance. Smaller than the main house, fully independent, and legal to rent under California state law.
Also called: granny flat · in-law suite · casita · backyard cottage · secondary unit · guest house
Start the Guide ↓Every ADU falls into one of four shapes, defined by how it relates to your primary home. Most homeowners pick based on lot constraints, budget, and how independent the unit needs to feel.
A stand-alone structure behind or beside the main home. Maximum privacy and the most rental-friendly format, both units feel like complete homes.
An addition that shares a wall with the primary residence, often a bump-out or wing. Construction is simpler because utilities and structure already exist.
An existing structure, a garage, basement, or accessory building, turned into a permitted unit. Lowest cost of the four because the shell already exists.
A unit carved out from inside the primary home with its own entrance and efficiency kitchen. Compact, fast to permit, perfect for aging-in-place or a home office.
Every industry has its shorthand. Here are the terms you'll see on permit drawings, zoning codes, and lender paperwork, in plain English.
Accessory Dwelling Unit, the umbrella term for any legal secondary residence on a single lot.
Junior ADU, up to 500 sf, carved inside the main home, with an efficiency kitchen.
The minimum distance between a structure and the property line. CA mandates 4 ft sides & rear for ADUs.
Floor-Area Ratio, total built square footage divided by lot size. Some cities cap your project here.
California's energy code, insulation, glazing, HVAC, and solar requirements every new ADU must meet.
The city's review of your construction documents before issuing a permit. Usually 1–3 rounds of comments.
A by-right approval, no public hearing, no neighborhood vote. State law makes most ADUs ministerial.
A regulated band along the coast where Coastal Commission permits apply, adding 2–4 months to timelines.
School & infrastructure fees charged by the city. Waived for ADUs under 750 sf.
The portion of the lot built upon. State law caps coverage rules at the zone's base, never lower.
Standard ADUs no longer require the owner to live on-site. JADUs still do, in either unit.
The 2024 law that lets cities opt in to allowing an ADU to be sold separately, as a condo. Adoption is per-city.
An ADU isn't just an investment, it's a tool. Different homeowners use the same unit for very different reasons.
A well-designed 600 sf unit rents for $2,200–$3,000/mo in San Diego. Most ADUs cover their own mortgage within 12 years.
Aging parents, adult kids, or in-laws get full independence under the same roof. The most common ADU use case in California.
A real workspace 30 ft from your back door, with its own bathroom and a door that closes. Hybrid workers love it.
Downsize without leaving the property. Move into the ADU, rent the main house, stay in the neighborhood you built your life in.
An ADU typically adds 25–35% to your property's appraised value, more in supply-constrained San Diego.
Even without renting it, an ADU is a hedge: optionality, flexibility, and a second usable structure on a single lot.
All-in totals, design fees, city fees, materials, and labor included. Numbers below are typical San Diego County ranges for a well-built turn-key ADU.
Two ways to think about return: long-term property appreciation, and monthly rental income. Both stack, you don't have to pick one.
Assumes 4% baseline appreciation; an ADU adds ~25% to appraised value at completion, compounding from there. San Diego's supply constraints typically push these numbers higher.
San Diego median market rents, Q2 2026. A typical 1 BR ADU pays back its construction cost in ~12 years; in high-demand zip codes (coastal, urban core), closer to 9.
Click any question. Answers are practical, San Diego-specific, and grounded in current state & local law.
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