An Accessory Dwelling Unit (ADU) is a fully independent second home on the same lot as a primary residence — with its own kitchen, bathroom, and entrance. ADUs are the fastest-growing way Californians add rental income, house family, or right-size their property without selling and moving.
You may have heard them called granny flats, casitas, in-law suites, backyard cottages, secondary units, or guest houses. The legal name in California is “accessory dwelling unit,” and the definition is very specific:
A guest room with a mini-fridge is not an ADU. A garage with a couch is not an ADU. The legal status — and the rental income, property-value lift, and financing options that come with it — only kicks in once it's permitted and inspected as an independent dwelling unit.
California recognizes four ADU configurations. Most lots can host more than one — for example, a JADU plus a detached ADU, or a conversion ADU plus a new detached unit.
A free-standing structure built somewhere else on the lot — usually in the backyard. Maximum privacy, fastest path to higher rent, and no shared walls with the main home.
An addition to the existing home — for example, a bump-out off the back or a unit above an existing garage. Shares one or more walls with the primary residence.
Existing space reborn — a garage, basement, or accessory building turned into a legal independent unit. Conversions usually have the lowest cost per square foot.
A unit up to 500 sq ft carved out of an existing single-family home. Must have an efficiency kitchen and may share a bathroom with the main home (per AB 1154 / SB 543).
The ADU boom in California is driven by a handful of very different goals. Most of our customers fall into one or two of these buckets.
Offset the mortgage or generate passive income — a typical California ADU rents for $1,800–$3,500/month.
House aging parents or adult kids on the same lot with private space, separate kitchen, and an independent entrance.
Get hard separation between work and home life without commuting or paying for outside office space.
A dedicated, comfortable space for visiting family, friends, an au pair, or a live-in caregiver.
Downsize into the ADU, rent the main home, and stay on the property you love as you age.
A code-compliant ADU adds 20–35% to most property values in California — and the unit can sell separately under AB 1033.
California sets a strong baseline that every city and county must respect. Local jurisdictions may be more permissive — they cannot be more restrictive.
Site, utility, and budget realities mean very few lots will host the full stack — but knowing the ceiling helps you plan.
Local objective standards still apply — site layout, utility capacity, and Coastal Zone overlays drive what is actually buildable.
The full state code is California Government Code §§ 66310–66342. Here's the homeowner's short version of what every city must allow.
Pre-approved plan programs, Coastal Development Permits, fee waivers, hillside overlays, and more. We track 49 California cities individually.
Every ADU project hits friction somewhere. The four below catch most homeowners off guard.
Every California city has its own ordinance on top of state law. State law caps the decision to 60 business days on a complete application — but completeness review and city back-and-forth can stretch the calendar.
Lot size, slope, setbacks, and fire-zone overlays all narrow what is buildable. A few feet in the wrong direction can add tens of thousands in foundation or fire-rated construction costs.
A 1BR ADU in California typically runs $150K–$200K all-in; a 2BR/1BA around $250K–$335K. Cash, HELOC, renovation loan, and CalHFA grants are all on the table — but the right mix depends on your equity and goals.
Cheapest bid is rarely the lowest cost when delays, change orders, and warranty issues hit. A clean scope, fixed-price line items, and a written schedule matter more than the headline number.
Everything you need to go from “I'm thinking about it” to a permit-ready set — without paying for a single hour of design work upfront.
Sketch a floorplan in our drag-and-drop builder, then walk through it in 3D. No sign-up, no software to install.
Verified ADU rules, fees, parking, and pre-approved plan programs for 49 California cities — with links to every official city page.
Search active rental listings in your zip code to see what 1BR and 2BR ADUs actually rent for near you — before you commit to a size.
Send us your sketch and our planners check it against your city’s setbacks, height, parking, and fee thresholds before you spend money on a permit set.
Sketch a custom layout in minutes, walk through it in 3D, then send it to our planners for a free city-specific code check.