An accessory dwelling unit (ADU) is a second, self-contained home on a residential lot, such as a detached backyard cottage, a garage conversion or an attached suite. Under California law, a city must approve or deny a complete ADU application within 60 days, or it is deemed approved. Since January 1, 2026, cities must also confirm whether an application is complete within 15 business days.
Types of ADUs We Build
- Detached ADUs and guest houses. A freestanding home in the backyard offers the most privacy and usually the strongest rental value.
- Garage conversions. Turning an existing garage into living space reuses the existing structure and is often the most economical path.
- Above-garage ADUs. Building a new garage with a living unit above, like our detached garage and ADU in Westlake Village, adds a home without giving up parking.
- Attached ADUs. A unit attached to the main house with its own entrance, kitchen and bathroom.
- Junior ADUs (JADUs). A smaller unit created within the existing home, often from a bedroom with an efficiency kitchen.
California ADU Rules in 2026: What Homeowners Should Know
State law now strongly favors homeowners who want to build ADUs, and it overrides many local restrictions. Several updates took effect recently, and they make the process more predictable:
- A 60-day decision clock. Once your application is complete, the city must approve or deny it within 60 days. If it misses the deadline, the application is deemed approved.
- A completeness check within 15 business days. Under SB 543, effective January 1, 2026, cities must tell you within 15 business days whether your application is complete, which closes a common source of delay.
- Coastal ADUs on a clock too. AB 462 requires local agencies with a certified Local Coastal Program to decide on a Coastal Development Permit for an ADU within 60 days, which matters for properties in Malibu and other coastal areas.
- No short-term rentals. ADUs built under state law can only be rented for terms longer than 30 days.
- Junior ADU occupancy. Owner-occupancy is now required only when a JADU shares a bathroom with the main house.
ADU law changes frequently, and local standards still apply where they do not conflict with state law. We confirm current requirements with your city before design begins.
Site and Weather Considerations
Drainage
Backyard ADUs are often placed at the lowest point of the lot. During winter storms, water that once soaked into a lawn now has to be directed around two buildings. We design grading, gutters and drainage so water moves away from both the ADU and the main house.
Fire zones
In fire hazard severity zones, a detached ADU must meet the same Chapter 7A wildfire-resistant standards as a new home, including ember-resistant vents, Class A roofing and ignition-resistant exterior materials. We account for this in design and budgeting.
Heat and orientation
In the San Fernando and Conejo valleys, a small ADU with a large west-facing wall can overheat quickly in late summer. Careful window placement, roof overhangs and an efficient heat-pump system keep the unit comfortable and inexpensive to run.
Start with utilities. The size of your sewer lateral, electrical panel capacity and water meter often drive ADU cost more than square footage. We review all three during feasibility.
Run the numbers as an investment. Compare expected long-term rent in your neighborhood against the total project cost and financing to understand your payback period before you commit.
Pick the right type for your goal. A garage conversion is usually the lowest-cost route, while a new detached ADU typically offers the best privacy and rental value.
How Long an ADU Takes
| Phase | Typical duration | What happens |
|---|---|---|
| Feasibility and design | 6 to 10 weeks | Site review, utilities, design, engineering and energy calculations |
| Permit review | Up to 60 days after a complete application | State law sets the decision deadline |
| Construction | 4 to 8 months | Site work, foundation, framing, systems, finishes and inspections |
What Affects the Cost of an ADU
ADU cost depends on the type of unit, its size, site access, utility upgrades, foundation and drainage needs, and finish level. Garage conversions usually cost the least because the structure already exists, while new detached units with full kitchens and bathrooms cost more but deliver more value. Fire-zone requirements and difficult access can also add cost. We provide a realistic budget range during feasibility and a fixed-price proposal once design is complete.
Frequently Asked Questions
How long does it take to build an ADU in Los Angeles?
Plan on roughly two to three months for feasibility and design, up to 60 days for permit review once the application is complete, and four to eight months of construction.
Do I need to live on the property to build an ADU?
No. State law no longer allows cities to require owner occupancy for ADUs. Owner occupancy only applies to junior ADUs that share a bathroom with the main house.
Can I rent my ADU on Airbnb?
No. ADUs built under California ADU law can only be rented for terms longer than 30 days.
Is a garage conversion cheaper than a new ADU?
Usually, yes, because the foundation, walls and roof already exist. A new detached ADU costs more but typically offers more space, privacy and rental value.
Can you build an ADU in the coastal zone?
Yes. Coastal properties also need a Coastal Development Permit, and a recent state law requires agencies with a certified Local Coastal Program to decide on ADU coastal permits within 60 days.
Last reviewed: November 2026 by the Elliott Construction Group team. Building codes and permit rules change often; we confirm current requirements for your property before design begins.