California submetered water billing rules for mobile-home parks (2026)

California regulates park utility billing more tightly than almost any state, through the Mobilehome Residency Law (MRL, Civil Code §798 and after). The 2024 update (AB 604) rewrote the water-billing math specifically. Here's what a park owner has to get right in 2026.

Part of our state-by-state series — submetered water billing rules by state. Other states: Texas · Florida · Oregon · Washington

What you may charge at all

Under Civil Code §798.31, a homeowner can only be charged rent, utilities, and reasonable charges for services actually rendered. A new charge that isn't in the rental agreement — including newly itemized utilities — needs 60 days' written notice before it starts (§798.32).

The water-billing math after AB 604

Since January 1, 2024, Civil Code §798.40 spells out how submetered water is billed. Two pieces are allowed:

• Usage, billed in proportion to the homeowner's own submeter reading against the park's total — the same dollar-proportion the water purveyor charged the park. No markup, and common-area water may not be folded in.
• A service charge, allocated proportionally or split equally — but combined administrative and service fees are capped at the lesser of $4.75 a month (adjusted annually for inflation) or 25% of the billed usage.

Every billing period must state the water charge separately, with the opening and closing meter reads, and the park must conspicuously post the serving utility's current residential rate schedule (or its website). If a third-party billing company prepares the bills, its name, address, and phone must appear on the statement.

Unbundling water from rent? There's a required rent cut

A park may switch from utilities-in-rent to separate billing (§798.41) — but when it does, it must simultaneously reduce base rent by the average monthly amount that utility cost the park over the previous 12 months. Unbundling is a transparency move, not a raise. Skipping the rent reduction is the fastest way to turn a billing change into a legal fight.

The meter itself is regulated too

Submeters used for billing are trade devices under California's weights-and-measures rules: they must be approved device types, installed by certified service agents, and registered with the county sealer, who requires periodic re-testing (commonly on a ten-year cycle). A park that's been billing off the same meters since the 90s without a test has a compliance gap it doesn't know about.

Common California mistakes

• Charging service/admin fees above the AB 604 cap (pre-2024 fee setups are often over it)
• Rolling common-area or irrigation water into tenant bills
• Skipping the opening/closing reads on the statement, or not posting the utility's rate schedule
• Unbundling water from rent without the matching rent reduction
• Never re-certifying old submeters with the county

The per-tenant math itself is simple — the water submeter calculator does reads-to-bill in one step. For how California compares to other states, start at the state-by-state overview.

Sources

• Civil Code §798.31, §798.32 (charges and notice): https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CIV&sectionNum=798.31.
• Civil Code §798.40 (billing; as amended by AB 604): https://leginfo.legislature.ca.gov/faces/billNavClient.xhtml?bill_id=202320240AB604
• Civil Code §798.41 (separately billed utilities; rent reduction): https://law.justia.com/codes/california/code-civ/division-2/part-2/title-2/chapter-2-5/article-4/section-798-41/
• County weights & measures, utility submeter programs (example): https://www.nevadacountyca.gov/1455/Utility-and-Mobile-Home-Park-Meters

General information, not legal advice. The MRL changes almost every year — confirm the current text (or ask a California mobilehome-park attorney) before you change how you bill.