What Is an ADU?
ADU stands for accessory dwelling unit. That is a small second home on the same lot as your main house.
It has its own kitchen, bathroom, and door. This guide tells you what you can build and how to start.
What counts as an ADU?
An ADU is a real home. People can sleep, cook, eat, and use the bathroom there. It sits on the same lot as your main house.
You may hear other names for it. People say granny flat, in-law unit, casita, backyard home, or guest house.
One thing to watch: a guest room with no kitchen is not an ADU. A real ADU needs a place to cook.
Four kinds of ADUs
The big difference is where it goes on your lot. Your land and your city will help you pick.
Detached ADU
A new small home in your yard. It stands on its own. This gives you and the people inside the most privacy.
Attached ADU
New space built onto your house. It shares a wall with your home but has its own door.
Conversion ADU
We turn space you already have into a home. A garage is the most common pick.
Junior ADU
A small unit inside your home, up to 500 square feet. It can share a bathroom with the main house. If it does, the owner must live on the lot.
Why people build one
- Rent money. Rent it out to help pay your mortgage.
- Family close by. Give your parents or grown kids a place of their own.
- Room for guests. Friends and family get their own space when they visit.
- A place to work. Keep your office away from the noise of the house.
- Stay on your land. Move into the ADU and rent out the main house.
- Home value. More living space can add to what your home is worth.
How many ADUs can you build?
It depends on what is on your lot now. State law says your city must allow at least this much:
Lot with one house
- 1 ADU made from space you already have
- 1 Junior ADU, up to 500 square feet
- 1 new detached ADU, up to 800 square feet
Lot with an apartment building
- At least 1 ADU made from space no one lives in, like a storage room or garage. You can go up to 25% of the number of units.
- Up to 8 new detached ADUs. You can't build more than the number of units you have now.
Keep in mind that the law is one thing and your lot is another. The size of your yard, a slope, or your budget may limit what makes sense. We check all of this for you.
California ADU rules in plain words
California made ADUs much easier to build. These state rules matter the most. Your city can have its own rules too, so we always check them.
- Size
- Your city must allow an ADU of at least 800 square feet if it fits the other rules. Many cities allow bigger ones.
- Space from the fence
- A setback is how far a building must sit from your property line. Your city can't ask for more than 4 feet on the side and back.
- Height
- The base limit for a detached ADU is 16 feet. Some lots can go up to 18 or 20 feet.
- Lot size
- A city's lot size rule can't stop you from building an 800 square foot ADU.
- Permit time
- A permit is the city's OK to build. Once your forms are complete, the city has 60 days to say yes or no. If it asks for changes, the job can take longer.
- Living on the lot
- You don't have to live on the lot to build or rent an ADU. The one exception is a Junior ADU that shares a bathroom with the main house.
- Parking
- A city can ask for one parking spot at most. Many ADUs need none. That includes ADUs within a half mile walk of public transit.
- HOA
- An HOA is a group that sets rules for a neighborhood. On lots zoned for single-family homes, it can't ban ADUs. It can still have fair rules.
- Renting
- Many ADUs must be rented for more than 30 days at a time. Think long-term renters, not weekend guests.
- Selling
- In most cases you sell the ADU along with your home. Some cities allow you to sell it on its own as a condo.
Source: California ADU Handbook, March 2026. This guide is not legal advice. Rules can change.
How to start, step by step
You don't need to know it all to get started. Most detached ADUs take about 8 months, give or take, from our first meeting to move-in day. It can take longer, based on how fast your city approves the plans.
Free call
Tell us what you want and share your address. We go over your goals.
Site check and budget
We look at your lot, your utilities, and your city's rules. Then you get a budget.
Design and plans
You pick the layout and the look. Our team draws the plans.
Permits
We turn in your plans and answer the city for you.
Build
We build your ADU for a fixed price with no hidden change orders.
Final check and move in
The city does its last check. Then you get the keys.
Common problems and how we handle them
Slow permits
Each city has its own forms, rules, and wait times. One missed detail can cost you weeks.
What we do: We learn your city's rules first and handle the back and forth with the city.
A lot that won't fit the plan
A slope, a tree, or a far-off sewer line can change what you can build.
What we do: We walk your lot with a builder before any design starts.
Surprise costs
Many people plan for the building but forget the work around it, like digging and power lines.
What we do: We show you the whole budget early. See what goes into ADU costs.
Too many companies
When one firm draws and another builds, each can blame the other.
What we do: One team does the design, plans, permits, and build.
The downsides, so you know
An ADU is not right for every home. Think about these first:
- It costs a lot. Most people need a loan or savings to pay for it.
- It takes time. Plan on months of design, permits, and building.
- You will have noise and dust in your yard while we build.
- You give up some of your yard.
- If you rent it out, you become a landlord.
- Your property taxes may go up. Ask your county for details.
Questions people ask
What does ADU stand for?
ADU stands for accessory dwelling unit. It means a second home on the same lot as your main house.
Is a guest house an ADU?
It is if it has a kitchen, a bathroom, and a place to sleep, and it has permits as an ADU. If it has no kitchen, it is not an ADU.
Is a tiny home an ADU?
It can be. It must meet the state's rules for an ADU. Manufactured homes can count too. Ask us about your lot.
Can I put my ADU on Airbnb?
Often, no. Many ADUs must be rented for more than 30 days at a time. Check your city's rules.
Can my HOA stop me?
On lots zoned for single-family homes, an HOA can't ban ADUs. It can still have fair rules, such as some rules on how it looks.
Can I convert my garage?
Often, yes. A garage can be a good way to get an ADU without building from the ground up. Your city can't make you replace the parking you lose.
How much does an ADU cost?
It depends on the size, your lot, and your city. Read our ADU cost guide.
Find out what fits your lot
Send us your address. We will tell you what you can build and go over your budget.