ADU basics for Orange County homeowners
Yes, most Orange County homeowners with a single-family lot can add an accessory dwelling unit. California law now requires cities to allow ADUs in most residential zones. What changes from city to city is the detail: design rules, fees and how plan check works.
What counts as an ADU
An accessory dwelling unit, also called a granny flat or in-law unit, is a second, smaller home on the same lot as your house. It has its own living, sleeping, kitchen and bathroom space. There are four common types:
- Detached ADU. A separate small house in the backyard.
- Attached ADU. An addition built onto your home with its own entrance.
- Conversion ADU. An existing garage or other space turned into a unit.
- Junior ADU (JADU). A unit of up to 500 square feet carved out of your existing home.
Why people build them
Room for a parent or a grown child. A rental that helps with the mortgage. A way to stay on your property as you get older, living in the ADU and renting the main house. More usable space and more equity in the home you already own.
California ADU rules at a glance
| Rule | What state law sets |
|---|---|
| Junior ADU size | Up to 500 sq ft, inside the existing home |
| Side and rear setbacks for a new ADU | A city can’t require more than 4 ft |
| City review of a complete application | Within 60 days |
| Impact fees | None for ADUs under 750 sq ft |
| HOA rules | Can’t ban ADUs outright; reasonable design rules allowed |
These are statewide rules as of 2026. Your city’s planning department has the final word on your lot, and coastal zones can add steps.
The steps, start to finish
- Feasibility. We look at your lot, utilities, access and any easements, and tell you which ADU type fits.
- Design. Floor plan, exterior style and finishes, drawn to your city’s standards.
- Permits. Plans go to the city for plan check. If you have an HOA, their architectural review usually runs alongside.
- Construction. Foundation, framing, utilities, inspections, then finishes.
- Finishing touches and cleanup. Final inspection, then you move in or rent it out.
OrangeBlue handles each of those steps, from design and permits to construction, finishing touches and cleanup. Because we’re a one-stop shop, you deal with one team instead of a designer, a permit runner and a builder.
What drives the cost
Size, type and site conditions. A garage conversion uses walls and a slab you already have. A detached unit needs a new foundation and longer utility runs. Sewer and electrical capacity can add work if your existing service is small. The honest answer on price is a site visit, and we’re happy to give you a free estimate. Read more about our accessory dwelling units and room additions.
Quick answers
Can I build an ADU on my lot in Orange County?
In most cases, yes. California law requires cities to allow ADUs on lots with a single-family home in residential zones. Your city sets design details, so confirm with its planning department.
How big can a junior ADU be?
Up to 500 square feet, built within the walls of your existing home.
Can my HOA stop me from building an ADU?
In California, an HOA generally can't ban ADUs outright, but it can apply reasonable rules on things like design and placement. Plan for their review alongside the city's.
How can I pay for an ADU?
Common options include home equity loans, construction loans, personal loans and savings. Some homeowners plan the unit as a rental so the income helps cover the cost.
Ready to start your project?
Call us or send a message and we’ll come out with a free, accurate estimate.