California Code, Government Code - GOV § 37101.7

(a) In accordance with the provisions of subdivision (b), the legislative body may license for revenue, and fix the license tax upon, persons who transact in the city the business of a contractor licensed pursuant to Chapter 9 (commencing with Section 7000) of Division 3 of the Business and Professions Code.

(b) The ordinance which adopts the license and license tax shall not impose a greater license tax upon those persons subject to it who, as contractors, have no fixed place of business within the city, than upon those contractors who have a fixed place of business within the city;  provided, however, that such ordinance may impose a license tax graduated according to gross receipts attributable to contracting work done within a city, regardless of whether or not the contractor has a fixed place of business within the city.