California Code, Government Code - GOV § 66324

(a) Fees charged for the construction of accessory dwelling units shall be determined in accordance with Chapter 5 (commencing with Section 66000) and Chapter 7 (commencing with Section 66012).

(b) An accessory dwelling unit shall not be considered by a local agency, special district, or water corporation to be a new residential use for purposes of calculating connection fees or capacity charges for utilities, including water and sewer service, unless the accessory dwelling unit was constructed with a new single-family dwelling.

(d) For an accessory dwelling unit described in paragraph (1) of subdivision (a) of Section 66323, a local agency, special district, or water corporation shall not require the applicant to install a new or separate utility connection directly between the accessory dwelling unit and the utility or impose a related connection fee or capacity charge, unless the accessory dwelling unit was constructed with a new single-family dwelling, or upon separate conveyance of the accessory dwelling unit pursuant to Section 66342.

(e) For an accessory dwelling unit that is not described in paragraph (1) of subdivision (a) of Section 66323, a local agency, special district, or water corporation may require a new or separate utility connection directly between the accessory dwelling unit and the utility. Consistent with Section 66013, the connection may be subject to a connection fee or capacity charge that shall be proportionate to the burden of the proposed accessory dwelling unit, based upon either its square feet or the number of its drainage fixture unit (DFU) values, as defined in the Uniform Plumbing Code adopted and published by the International Association of Plumbing and Mechanical Officials, upon the water or sewer system. This fee or charge shall not exceed the reasonable cost of providing this service.