The garage conversion boom: what the permit record shows

What did we find?

California’s ADU boom is real, large and directly countable. It is not a modeled estimate. An ADU is a second, independent home on a lot that already has a house. The state’s Department of Housing and Community Development (HCD) publishes permit data project by project, as a public CSV file.

We filtered its Table A2 to ADUs only. That gives 281,321 ADU permit records statewide. They grew from 12,806 in 2018 to 55,167 in 2025, a 4.3-times increase in seven years.

Of those, 3,357 records name a garage in their own project description. We searched the free-text notes field for the word "garage" and found 3,357 ADU permits describing a garage conversion. That is a minimum, not a full count, because the notes field is optional.

Most are rentals, not rooms for the owner’s family. Of the 3,357 records, 2,981 are recorded as renter tenure and 376 as owner. That is roughly 89 percent meant as rental housing.

Los Angeles County alone accounts for 127,850 of the 281,321 statewide ADU permits. That is more than 45 percent in one county.

Did the new laws drive the growth?

Growth sped up after each wave of laws, but the data alone cannot prove which law caused what. The yearly counts line up closely with the legal history.

The 2018 starting point (12,806 permits) already reflects the first wave of statewide easing (SB 1069/AB 2299, effective 2017).

The sharpest jumps came after the second wave (AB 68, AB 587, AB 670, AB 881, SB 13), which took effect January 1, 2020. Permits rose roughly 18% from 2019 to 2020 and kept growing through 2022-2023. Those laws removed minimum lot sizes statewide and cut side and rear setbacks to 4 feet.

HyreGarage analysis: this is a correlation, and we state it carefully. Several bills often took effect in the same session. Other things moved at the same time: interest rates, building costs and how fast each city put the rules in place.

Why are most garage conversions rentals?

Probably because a garage is the cheapest place to add a rentable unit. The 89 percent renter share is close to the stated purpose of California’s ADU laws, which eased permits in a series of bills from 2016 onward. The goal was more rental housing inside existing neighborhoods, without new subdivisions or large projects.

A garage conversion is one of the cheapest ways to add a rentable unit to a single-family lot. The structure, foundation and often the electrical service already exist. A new backyard ADU starts from scratch.

That cost advantage plausibly explains why garage conversions lean even more toward rentals than ADUs in general.

HyreGarage analysis: the dataset cannot tell us why any one homeowner converted a garage. It shows the pattern across more than 3,300 recorded projects. That pattern fits garage conversion as mainly a low-cost rental strategy for existing owners, rather than extra space for the household.

Why do we say "at least 3,357"?

Because the true number is higher, and we cannot say by how much. The word "at least" reflects how the data is collected. It is not just caution.

Each of California’s roughly 540 cities and counties sends its own APR data to HCD. Each uses its own permit software and its own habits for filling in the free-text NOTES field.

Some write detailed descriptions. Stanislaus County’s "CONVERT EXISTING DETACHED GARAGE / APARTMENT TO ACCESSORY DWELLING UNIT AND ADDITION OF BATHROOM" is typical of a detailed one. Others record only a generic category or a permit number, or leave the field blank, even for an identical garage conversion.

So the 3,357 figure leans toward places that write detailed notes, not necessarily places with more garage conversions. A large city with short notes could have hundreds of uncounted garage conversions inside its "ADU" total. In this dataset they look the same as a new detached unit with no garage at all.

HyreGarage analysis: we could have tried to correct for this, for example by assuming short-note cities convert garages at the same rate as detailed ones and scaling up. We have no independent way to test that assumption. A wrong correction with false precision would be worse than an honestly labeled minimum.

What could we not find?

The true total of garage-to-ADU conversions in California. The 3,357 figure comes from a keyword search of optional notes. We did not scale it up, because we do not know what share of real garage conversions leave "garage" out of their notes.

Cost or size for these projects. HCD’s dataset has no construction cost or unit size. So this page cannot report an average garage conversion cost. Our garage conversion cost calculator works from your own line items instead.

Why any one permit was filed, beyond the tenure field. We report the overall renter/owner split. We did not interview any homeowner or read any project file beyond the structured fields.

A similar dataset for any other state. No other state we could find publishes permit data this detailed, project by project, as a public download.

How well each city reports. We did not check which cities or counties write detailed notes and which write short ones. That would show how unevenly the 3,357 minimum undercounts across the state.

Whether the trend continues past 2025. The data covers submissions through the latest APR cycle at retrieval. Future years could show growth, a plateau or a decline, depending on the economy and new laws.

What do "1,005,686 records" represent?

Each record is one reported housing project, not a summary line. It is unusual for a state agency to publish data this detailed. Table A2 has one row per project, across every city and county in California, over eight reporting years.

That detail is what makes our garage count possible. A summary table of "ADU permits by county by year" would give the totals above, but nothing about how the units were created: new build, garage conversion or another kind of conversion.

The free-text NOTES field, however unevenly filled in, is the only part of the dataset that shows that difference. That is why this page’s main number depends on it. Before you plan a conversion, our garage conversion code requirements study covers the building code, and the conversion feasibility checker lists questions for your city.