Pub. Util. Code, § 131102

(a) (1) A retail transactions and use tax ordinance for a tax of either one-half of 1 percent or 1 percent applicable in the incorporated and unincorporated territory of a county, except as provided in paragraph (2), may be imposed by a county transportation authority or the commission in the manner prescribed in Section 131103 and Part 1.6 (commencing with Section 7251) of Division 2 of the Revenue and Taxation Code, if two-thirds of the electors voting on the measure vote to approve its imposition at an election which shall be called for this purpose by the board of supervisors within one year after the adoption of a county transportation expenditure plan.

(2) The authority to impose a tax under paragraph (1) shall apply to the incorporated and unincorporated territory of the County of San Mateo and the City and County of San Francisco only until December 31, 2025. The elimination of the authority pursuant to this paragraph to impose a tax shall not affect the validity or operation of a tax approved before December 31, 2025, in the incorporated and unincorporated territory of the County of San Mateo or the City and County of San Francisco pursuant to paragraph (1).

(b) Commencing January 1, 2026, a retail transactions and use tax ordinance for a tax of 1/8 of 1 percent or multiple thereof, up to 1 percent, applicable in the territory of the City and County of San Francisco may be imposed by the county transportation authority or the commission in the manner prescribed in Section 131103 and Part 1.6 (commencing with Section 7251) of Division 2 of the Revenue and Taxation Code, if two-thirds of the electors voting on the measure vote to approve its imposition at an election which shall be called for this purpose by the board of supervisors within one year after the adoption of a county transportation expenditure plan. The combined rate of taxes in the territory of the City and County of San Francisco imposed pursuant to this subdivision and paragraph (1) of subdivision (a) shall not exceed 1 percent.

(c) Commencing January 1, 2026, a retail transactions and use tax ordinance for a tax of 1/8 of 1 percent or multiple thereof, up to 1 percent, applicable in the incorporated and unincorporated territory of the County of San Mateo may be imposed by the county transportation authority or the commission in the manner prescribed in Section 131103 and Part 1.6 (commencing with Section 7251) of Division 2 of the Revenue and Taxation Code, if two-thirds of the electors voting on the measure vote to approve its imposition at an election which shall be called for this purpose by the board of supervisors within one year after the adoption of a county transportation expenditure plan. The combined rate of taxes in the incorporated and unincorporated territory of the County of San Mateo imposed pursuant to this subdivision and paragraph (1) of subdivision (a) shall not exceed 1 percent.

(d) The ordinance shall take effect at the close of the polls on the day of the election at which the proposition, as set forth in Section 131108, is adopted. The ordinance shall specify the period, as determined by the adopted county transportation expenditure plan during which the tax will be imposed. The tax may be terminated earlier if the projects in the adopted plan are completed and any bonds outstanding issued pursuant to this division are redeemed.