Cal. Civ. Code § 1954.205

(a) As part of the regular bill for water service, a landlord shall only bill a tenant for the following water service:
1. (1) A charge for volumetric usage, which may be calculated in any the following ways:
  1. (A) The amount shall be calculated by first determining the proportion of the tenant’s usage, as shown by the submeter, to the total usage as shown by the water purveyor’s billing. The dollar amount billed to the tenant for usage shall be in that same proportion to the dollar amount for usage shown by the water purveyor’s billing.
  2. (B) If the water purveyor charges for volumetric usage based on a tiered rate schedule, the landlord may calculate the charge for a tenant’s volumetric usage as described in subparagraph (A) or the landlord may instead divide each tier’s volume evenly among the number of dwelling units, and the rate applicable to each block shall be applied to the consumption recorded for each dwelling unit.
  3. (C) If the water purveyor charges the property rates on a per-dwelling unit basis, the tenants may be charged at those exact per unit rates.
2. (2) Any recurring fixed charge for water service billed to the property by the water purveyors that, at the landlord’s discretion, shall be calculated by either of the following:
  1. (A) The tenant’s proportion of the total fixed charges charged to the property. The tenant’s proportion shall be based on the percentage of the tenant’s volumetric water use in relation to the total volumetric water use of the entire property, as shown on the property’s water bill during that period.
  2. (B) Dividing the total fixed charges charged to the property equally among the total number of residential units and nonresidential units at the property.
3. (3) A billing, administrative, or other fee for the landlord’s and billing agent’s costs, which shall be the lesser of an amount not to exceed four dollars and seventy-five cents ($4.75), as adjusted pursuant to this paragraph or 25 percent of the amount billed pursuant to paragraph (1). Beginning January 1, 2018, the maximum fee authorized by this paragraph may be adjusted each calendar year by the landlord, no higher than a commensurate increase in the Consumer Price Index based on a California fiscal year average for the previous fiscal year, for all urban consumers, as determined by the Department of Finance.
4. (4) A late charge as assessed pursuant to Section 1954.213.
(b) If a submeter reading for the beginning or end of a billing period is, in good faith, not available, the landlord shall bill the tenant according to Section 1954.212.
(c) This section does not prohibit a landlord or the landlord’s billing agent from including any other lawful charges, including, but not limited to, rent, on the same bill.