California Code, Government Code - GOV § 20821

(a) The contribution to the retirement fund of a school employer or a contracting agency electing to be subject to Section 20325 with respect to school members and local members making payments pursuant to Section 20325 for past service that was less than the minimum prescribed by paragraph (2) of subdivision (a) of Section 20305 because of service of less than 20 hours a week, prior to the enactment of Section 20325, shall be increased by an amount or a percentage of the compensation paid to those members determined by the board.

(b) The additional contribution imposed pursuant to subdivision (a) shall include an amount to pay the reasonable administrative expenses incurred by the board in establishing the additional contribution required in subdivision (a).