News · October 10, 2025 · From January 1, 2026, SB 9 makes a city's ADU ordinance void if the city misses the state's 60-day filing or 30-day reply deadline. [1][2]
The ADU rules your city posts may not be the rules that apply to your permit. That's worth knowing before you pay for drawings.
What changed in 2026 (SB 9 of 2025)
A city must send its ADU ordinance to HCD, the state housing department, within 60 days of adopting it. HCD may then send written findings on whether it complies. If HCD finds it doesn't, the city gets no more than 30 days to respond. It must either amend the ordinance or readopt it with findings that explain why it complies [2].
Before SB 9, if a city neither amended its ordinance nor readopted it with findings, this section's remedy was a notice: HCD had to notify the city, and could notify the Attorney General, that the city was violating state law [1]. That rule still stands [2].
SB 9 added a second consequence. If the city misses either deadline, "that ordinance shall be null and void." The city must then apply the state's ADU standards until it adopts an ordinance that complies, including sending it in on time [2]. Since the same date, junior ADU ordinances work the same way under a 2025 law, SB 543 [3]. A junior ADU (JADU) is a unit of up to 500 square feet inside a single-family home [4].
An older rule also voids an existing ADU ordinance that fails to meet state law, and the state's standards apply then too [5].
Glendale shows how that older rule plays out. HCD told the city in May 2024 that its ADU ordinance was null and void, and the city replied that state law doesn't let HCD declare it void "without a formal legal adjudication by a court," and kept applying it [6]. ADU applicants complained to HCD about rules such as the city's ban on ADUs above detached garages. Only on October 3, 2025, more than 16 months after that May notice, did the city agree to stop enforcing that ban, while maintaining it was not in violation of state law [6][7]. What's at stake can be the size of the cottage: where a city has no compliant ordinance, HCD's handbook puts the cap for a new detached ADU at 1,200 square feet of interior livable space [8][9].
How did San Mateo answer the state within 30 days?
HCD sent San Mateo findings on October 18, 2024 and asked for a written response by November 17; the city replied on November 15 [10][11]. That exchange came before SB 9; under the rule in force now, missing a reply date like that voids the ordinance [2]. San Mateo's updated ordinance took effect March 4, 2026 [12]. For the city's current rules, see our San Mateo guide.
How can you tell if your city's ADU ordinance is void?
SB 9 doesn't say how a homeowner would learn that an ordinance has gone void [2]. HCD posts letters to cities on its website. Its December 5, 2025 letters to Los Altos and Saratoga list the new rule among recent changes to state ADU law [13][14]. Our note on those letters covers what they ask.
So ask the planning department, in writing:
when the city adopted its current ADU ordinance, and whether it sent it to HCD within 60 days;
whether HCD sent findings, and whether the city answered within 30 days;
which rules it will apply to your application: its ordinance, or state law.
Get the answer before the design is set.
Amiari Construction builds ADUs and additions on the Peninsula.
How we checked this: In October 2026, Amiari Construction read SB 9's chaptered text and history and Government Code §§66313, 66314, 66316, 66326 and 66333.5 in the legislature's own database, HCD's 2024 findings to San Mateo and the city's reply, San Mateo's Ordinance 2026-02, HCD's December 2025 letters to Los Altos and Saratoga, HCD's 2025 notice to and agreement with Glendale, and HCD's ADU Handbook.
