You want to know how big an Accessory Dwelling Unit (ADU) can be in California, and this article is going to answer that question, no matter what part of California you’re in.

Each jurisdiction can have its own rules about the minimum and maximum ADU size limits, but here’s the basic breakdown!

Most of the laws referenced in this article are summarized in the official HCD (California Department of Housing and Community Development) ADU handbook and you can watch a video about the California ADU Legislation that went into effect in 2020.

The information is up to date as of November 23, 2020. Since your city or county can amend the rules and issue new guidance, it’s best to get in touch with them about the rules for your property.

The most general answer about Maximum ADU Size Limits

If you can build a detached ADU, then here are the easiest, broadest answers I can share before diving into the detail…

Everywhere should allow up to 800 sq ft detached ADU

No matter what California jurisdiction you are in, detached ADUs can be built up to 800 square feet, as long as they are less than 16 feet high and respect 4 foot rear and side yard setbacks.

Most places allow up to 1200 sq ft detached ADU

There are something like 391 cities and counties using the state code, where an ADU can be up to 1200 square feet.

Some very specific restrictions in about 150 cities and counties

Scroll down to the section titled “What to do if your jurisdiction has its own ADU size rules?“ to figure out the rules in these specific jurisdictions…

These cities and counties might limit you to 1000 square feet for 2+ bedrooms, 850 square feet for 1 or fewer rooms, and 800 square feet because of bulk and massing limits (aka minimum lot size, open space, lot coverage, and FAR requirements).

Different rules for attached ADUs and converted structures

If you’re converting an existing structure, the HCD says there are no size restrictions on your ADU. So there is no maximum size of an ADU made from a converted space - or rather the maximum size is determined by the largest existing accessory structure in the jurisdiction!

If you’re converting an existing structure into a JUNIOR ADU (these are different than ADUs) then you generally have to be under 500 square feet.

If you’re building onto an existing structure (an attached ADU), there are other limits, generally the attachment should not exceed 50% of the existing square footage of the home.

How to figure out the rules on your property…

So far, this article has had the broadest statements I can make for folks who are in a hurry and don’t want to read a long post full of more precise information.

If you’re seriously considering an ADU, you should read the rest of this article - at least the bold words - to get a sense of how to proceed with your research.

The exact size limit depends on what jurisdiction you’re in

First step: what jurisdiction is your property in?

There are 539 different jurisdictions (58 counties, and 482 cities. Those don’t add up perfectly because San Francisco is weirdly both.)

Figure out your jurisdiction and then check if it’s on this list of jurisdictions that have approved local ADU rules and sent them to the HCD as of November 2, 2020.

What to do if your jurisdiction has its own ADU size rules?

If your jurisdiction is on this list, they have a local ordinance adopted in 2020 - you should look that up and use it to determine your maximum ADU size.

These ordinances can have a number of different effects:

1. They can loosen restrictions - some jurisdictions actually allow larger than 1200 square foot detached ADUs

2. They can confirm the state code - they just repeat the 1200 sq ft restriction from the state code

3. They can tighten restrictions up to a point

   1. Pretty much everybody can build an 800 square foot detached ADU

   2. Barring any lot coverage, FAR, minimum lot size, and open space requirements, people should be able to build an 850 square foot detached ADU

   3. If you’ve got 2+ bedrooms, again barring any lot coverage, FAR, minimum lot size, and open space requirements, people should be able to build a 1000 square foot detached ADU

The state code says that local jurisdictions can adopt ADU rules that limit ADU size up to a point.

There are two ways they can restrict your ADU size.

Maximum Size Limits on ADUs

The first way is just putting a maximum size on detached ADUs. That maximum size should be no smaller than 1000 square feet for a 2+ bedroom ADU, and no smaller than 850 square feet otherwise. If your local law is more restrictive than that, you may have grounds to argue that it is unenforceable.

Bulk & Massing Regulations and Development Limitations

The second type of restriction is that the city can implement controls like maximum lot coverage, minimum lot size, Floor-Area-Ratios (FAR), or open space requirements.

An example would be a local ordinance that says you can’t build a structure that would leave less than 50% of your lot open.

Even if the maximum size allowed for an ADU is 1,200 square feet, you may find that a lot coverage requirement limits you to less surface area than that.

The state law acknowledges that local ordinances are allowed to have these kinds of requirements, but it explicitly says that there should be exemptions to allow the construction of an ADU up to 800 square feet, less than 16 feet high, respecting 4 foot rear and side yard setbacks.

So even if those requirements would limit you to a very small ADU, or to not build an ADU at all, you may be able to build up to 800 square feet, 16 feet high, with 4 foot rear and side yard setbacks.

What if your jurisdiction is not on that list, but they have size limits?

There are a number of jurisdictions that have adopted ADU size limits without submitting their ordinance to the HCD.

The most common one my subscribers keep telling me about is Irvine.

One option - work with the city or county size limits

One is to work with the city or county despite the fact that they have not sent their ADU ordinance to the HCD. You can often achieve your goals fastest by working WITH the planning department.

Another option - speak to the HCD or an attorney

Another option is to write to the HCD or an attorney to ask about all your available options.

I’m not here to give you legal advice since I don’t know the specifics of your situation, but if you do want to go down this route, I can share contact information for ADU specialists at the HCD.

Write to hello@how-to-adu.com with the details of your situation including the street address and any correspondence you think highlights the challenge you’re facing with your jurisdiction.

Please note that this article is being written in November 2020 and that cities and counties can meet every month to amend the rules or issue guidance. And they have 60 days to send those ADU rules to the HCD for approval. So don’t be surprised if my list from Nov 2 quickly gets outdated!

What is the minimum size an ADU can be?

The minimum size for an ADU in the state code is 150 square feet, equivalent to the minimum for an efficiency unit defined in Section 17958.1 of the Health and Safety Code.

That said, every rule has its exception if you’re willing to apply for a discretionary permit like the Sonoma couple who got a 108.5 sq ft above the garage ADU in this video.

What is the best-selling size ADU in California?

Okay, now that we’ve gone through what the literal maximum California ADU sizes are, I’m going to tell you what I wish everybody knew…

Most homeowners should be building a 749.99 square foot ADU.

I’ll write a longer article about all the various financial benefits of this, but the short version is:

1. ADUs under 750 square feet are exempt from impact fees

2. ADUs under 800 square feet are exempt from bulk & massing development limits like FAR, open space, lot coverage and min lot size requirements

3. The value of that extra 50 square feet is often not worth the additional fees, restrictions and hassle

It won’t surprise you that most of the best-selling ADU floor plans and prefabs are under 750 square feet.

This Design Trick Can Increase the Maximum Size of an ADU House

When you apply for a permit for a detached ADU over 750 square feet, you may incur impact fees.

Additionally, when you apply for a permit for an ADU over 800 square feet, you may trigger bulk and massing limits like FAR, open space, minimum lot size and maximum lot coverage requirements.

On the other hand, if you submit a permit for a detached structure that has an ADU combined with a JADU, you may not trigger either the impact fees or the size limits.

Email ryan@how-to-adu.com for a copy of the “Detached ADUs Over 750 sq ft” ebook for more information.