California Code, Public Utilities Code - PUC § 29034.5

(a) The district may approve annexation of areas to the district pursuant to Chapter 9 (commencing with Section 29500) or other forms of affiliation;  however, the district shall meet its service commitments within its boundaries as they existed on January 1, 1971, before the district may expend any district funds for the purpose of extending services and facilities outside of those boundaries, except for the extension of services and facilities south of the Daly City Station to the terminus of the BART-San Francisco Airport Extension.

(b) This section does not preclude either of the following:

(c) As used in this section, “service commitments” means the extension of district services and facilities to the Cities of Antioch, Pittsburg, Pleasanton, Livermore, and Fremont and to the northwest section of the City and County of San Francisco.