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.
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).
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.