Cal. Pub. Res. Code § 21159.23

(a) This division does not apply to any development project that consists of the construction, conversion, or use of residential housing consisting of 100 or fewer that is affordable to low-income households if both of the following criteria are met:
1. (1) The developer of the development project provides sufficient legal commitments to the appropriate local agency to ensure the continued availability and use of the housing units for lower income households, as defined in Section 50079.5 of the Health and Safety Code, for a period of at least 30 years, at monthly housing costs, as determined pursuant to Section 50053 of the Health and Safety Code.
2. (2) The development project meets all of the following requirements:
  1. (A) The project satisfies the criteria described in Section 21159.21.
  2. (B) The project site meets one of the following conditions:
    1. (i) Has been previously developed for qualified urban uses.
    2. (ii) The parcels immediately adjacent to the site are developed with qualified urban uses, or at least 75 percent of the perimeter of the site adjoins parcels that are developed with qualified urban uses and the remaining 25 percent of the perimeter of the site adjoins parcels that have previously been developed for qualified urban uses, and the site has not been developed for urban uses and no parcel within the site has been created within 10 years prior to the proposed development of the site.
  3. (C) The project site is not more than five acres in area.
  4. (D) The project site is located within an urbanized areaor within a census-defined place with a population density of at least 5,000 persons per square mile or, if the project consists of 50 or fewer units, within an incorporated city with a population density of at least 2,500 persons per square mile and a total population of at least 25,000 persons.
(b) Notwithstanding subdivision (a), if a project satisfies all of the criteria described in subdivision (a) except subparagraph (D) of paragraph (2) of that subdivision, this division does not apply to the project if the project is located within either an incorporated city or a census defined place with a population density of at least 1,000 persons per square mile.
(c) Notwithstanding subdivision (b), this division applies to a project that meets the criteria of subdivision (b), if there is a reasonable possibility that the project would have a significant effect on the environment or the residents of the project due to unusual circumstances or due to the related or cumulative impacts of reasonably foreseeable projects in the vicinity of the project.
(d) For the purposes of this section, “residential” means a use consisting of either of the following:
1. (1) Residential units only.
2. (2) Residential units and primarily neighborhood-serving goods, services, or retail uses that do not exceed 15 percent of the total floor area of the project.