Nearly half — 48% — of all new housing units permitted in San Mateo County in 2024 were accessory dwelling units. This is more the double the statewide rate. As California has struggled to build enough housing to meet demand, ADUs have become one of the few housing types that have shown strong annual growth over many years, and nowhere more so than here on the Peninsula.
This growth is due in large part to the California Legislature, which began passing legislation in 2016 to lower barriers to ADU zoning and permitting. Since then, new ADU laws have gone into effect every year.
Last year alone brought two significant changes. Assembly Bill 1033 gave cities the option to allow ADUs to be sold separately as condominiums — a potential new path to creating new, more affordable homeownership opportunities that six California cities have adopted (though none in San Mateo County yet). Meanwhile, AB 2533 made it substantially easier to legalize unpermitted ADUs, creating an opportunity to bring unpermitted housing units into compliance with local regulations.
This year’s slate of ADU legislation generally falls into a category that policy makers call “clean up” bills — bills that make some clarifications around the edges (clarifying what fees can and cannot be charged for ADU permits, for example), but do not substantively change the rules.
However, one sentence added to the state Department of Housing and Community Development’s ADU Handbook in March has transformed what is possible on any single-family lot in the state.
Since 2024, HCD has stated that every single-family lot can have up to three ADUs. HCD’s latest ADU Handbook, published in March, now makes it explicitly clear that single family lots allow up to four ADUs in addition to the primary dwelling, if the ADUs meet certain criteria. Allowable unit combinations are: 1). A detached ADU under 800 square feet, 2). A junior ADU (defined as a conversion of interior living space that must be under 500 square feet), 3). A conversion of an existing structure such as a detached garage, and 4). Any regular ADU that meets state and local standards (generally, ADUs can be up to 850 square feet for a studio or one-bedroom and 1,000 square feet for a two-bedroom detached unit).
Recommended for you
Taken together, these rules mean that a single-family lot can now legally accommodate up to five housing units: a primary residence plus four ADUs of specified combinations. These units can be built in any order, and all must be approved by local permitting agencies without discretionary review.
This update also means that, for the first time, California allows applicants to build two new ground-up detached ADUs on their single-family property, as long as one of those new cottages is under 800 square feet and the other meets local design and development standards.
For most homeowners considering ADUs, the choice of a single backyard cottage, an addition to their home (called an attached ADU), or internal conversion of their existing space is still the most viable option. Among homeowners who reach out to my organization for free consultations, more than 65% are interested in an ADU because they are looking to house a family member. However, the range of what is now possible has expanded substantially. For some lots, ADU rules now enable small housing clusters.
Even as ADUs have become a more systematic source of new housing units across the state and the range of options has become more variegated and complex, ADUs remain the only housing type delivered in a fully distributed way, with individual homeowners taking on the complex role of developer.
San Mateo County has built fewer homes than it needs for decades. ADUs alone won’t close that gap, but with nearly half of the county’s new units now coming from this single category, they have become a more meaningful contributor than anyone might have expected a decade ago. What began as a set of incremental policy changes has, over time, reshaped the meaning of a single-family lot. The shift has been gradual, but its implications for how housing may be added across the Peninsula are anything but small.
Anna Alekseyeva is the director of the ADU Resource Center of San Mateo County, a nonprofit that helps homeowners navigate ADU planning and works with local governments on ADU policy.
Thanks, Ms. Alekseyeva, for your guest perspective on updated ADU rules and progress. You say 48% of new housing permits were for ADU’s. But 48% of what total? 10, 100, 1000? Context matters. And how many of these permits have moved ahead to construction/completion? Regardless, if folks move forward on constructing ADU’s then that potentially a good sign.
That being said, I caution folks on building ADUs for rental purposes. Remember that when the rubber hits the road and a renter decides to stop paying rent, it is likely to cost the owner plenty of money to evict that renter. That’s assuming so-called officials allow owners to evict the renter. Remember the COVID years where landlords were forced to carry the burden of the renter without being compensated? Of course, an owner may have to deal with rent control. And on the fiscal side, a home reassessment where an ADU has a higher property tax portion than the main home.
Bottom line, think twice or three times as to whether you want the potential headaches associated with an ADU rental. If anything, I get the feeling owners will sell their homes to investors and these investors will add multiple ADUs in the effort to make a profit. Investors make money and there is more housing. Of course investors won’t care about parking or the potential lowering of neighborhood home values.
ADUs were sold to the public as granny flats — a modest way to house a family member or earn a little rental income. That's not what this is anymore.
The state has quietly rewritten the rules so that a single-family lot can now hold five housing units. Five. On a lot zoned for one home.
That's not a granny flat.
That's a 5-unit apartment complex in a residential neighborhood — built without discretionary review, without neighborhood input, and without any serious study of the cumulative impact on traffic, parking, infrastructure, or neighborhood character.
Our streets weren't designed for this density. Our water and sewer lines weren't built for it. And our neighbors weren't asked about it.
The housing crisis has now become a neighborhood wrecking ball.
Yes, I am sure the next owner of our lot, 50 x 300 feet, will be building to the max. In the process, destroying the peace and quiet that we moved here for.
Wow, this is sobering. So those against R1 zoning are now masquerading multiple ADU’s on a property as a housing strategy? Folks, it’s time to fight back - please consider joining Our Neighborhood Voices (our neighborhood voices.com) and fight back.
Perhaps this is part of the reason condo and townhome prices have been declining. The other side to the reason for price declined is state over regulation requiring expensive balcony repairs.
Keep the discussion civilized. Absolutely NO
personal attacks or insults directed toward writers, nor others who
make comments. Keep it clean. Please avoid obscene, vulgar, lewd,
racist or sexually-oriented language. Don't threaten. Threats of harming another
person will not be tolerated. Be truthful. Don't knowingly lie about anyone
or anything. Be proactive. Use the 'Report' link on
each comment to let us know of abusive posts. PLEASE TURN OFF YOUR CAPS LOCK. Anyone violating these rules will be issued a
warning. After the warning, comment privileges can be
revoked.
Please purchase a Premium Subscription to continue reading.
To continue, please log in, or sign up for a new account.
We offer one free story view per month. If you register for an account, you will get two additional story views. After those three total views, we ask that you support us with a subscription.
A subscription to our digital content is so much more than just access to our valuable content. It means you’re helping to support a local community institution that has, from its very start, supported the betterment of our society. Thank you very much!
(5) comments
Thanks, Ms. Alekseyeva, for your guest perspective on updated ADU rules and progress. You say 48% of new housing permits were for ADU’s. But 48% of what total? 10, 100, 1000? Context matters. And how many of these permits have moved ahead to construction/completion? Regardless, if folks move forward on constructing ADU’s then that potentially a good sign.
That being said, I caution folks on building ADUs for rental purposes. Remember that when the rubber hits the road and a renter decides to stop paying rent, it is likely to cost the owner plenty of money to evict that renter. That’s assuming so-called officials allow owners to evict the renter. Remember the COVID years where landlords were forced to carry the burden of the renter without being compensated? Of course, an owner may have to deal with rent control. And on the fiscal side, a home reassessment where an ADU has a higher property tax portion than the main home.
Bottom line, think twice or three times as to whether you want the potential headaches associated with an ADU rental. If anything, I get the feeling owners will sell their homes to investors and these investors will add multiple ADUs in the effort to make a profit. Investors make money and there is more housing. Of course investors won’t care about parking or the potential lowering of neighborhood home values.
ADUs were sold to the public as granny flats — a modest way to house a family member or earn a little rental income. That's not what this is anymore.
The state has quietly rewritten the rules so that a single-family lot can now hold five housing units. Five. On a lot zoned for one home.
That's not a granny flat.
That's a 5-unit apartment complex in a residential neighborhood — built without discretionary review, without neighborhood input, and without any serious study of the cumulative impact on traffic, parking, infrastructure, or neighborhood character.
Our streets weren't designed for this density. Our water and sewer lines weren't built for it. And our neighbors weren't asked about it.
The housing crisis has now become a neighborhood wrecking ball.
Yes, I am sure the next owner of our lot, 50 x 300 feet, will be building to the max. In the process, destroying the peace and quiet that we moved here for.
Wow, this is sobering. So those against R1 zoning are now masquerading multiple ADU’s on a property as a housing strategy? Folks, it’s time to fight back - please consider joining Our Neighborhood Voices (our neighborhood voices.com) and fight back.
Perhaps this is part of the reason condo and townhome prices have been declining. The other side to the reason for price declined is state over regulation requiring expensive balcony repairs.
Welcome to the discussion.
Log In
Keep the discussion civilized. Absolutely NO personal attacks or insults directed toward writers, nor others who make comments.
Keep it clean. Please avoid obscene, vulgar, lewd, racist or sexually-oriented language.
Don't threaten. Threats of harming another person will not be tolerated.
Be truthful. Don't knowingly lie about anyone or anything.
Be proactive. Use the 'Report' link on each comment to let us know of abusive posts.
PLEASE TURN OFF YOUR CAPS LOCK.
Anyone violating these rules will be issued a warning. After the warning, comment privileges can be revoked.