Cal. Civ. Code § 9

  § 9. Buildings converted or altered. 1. On or after December
fifteenth, nineteen hundred sixty-one, no multiple dwelling shall be
enlarged or its lot diminished so that the yard or other unoccupied
areas shall be less in size or area than the minimum dimensions
prescribed in section twenty-six.
  2. A building not a dwelling, if converted or altered after April
eighteenth, nineteen hundred twenty-nine, to a multiple dwelling, shall
thereupon become subject to all the provisions of this chapter
applicable to dwellings of like class and kind erected after such date.
  3. A dwelling of one class or kind, altered or converted after April
eighteenth, nineteen hundred twenty-nine, to another class or kind,
except as hereinafter in this section and in articles six and seven
otherwise provided, shall thereupon become subject to all the provisions
of this chapter applicable to a building of that class or kind, erected
after such date, to which it is altered or converted.
  4. No dwellings shall be altered so as to be in violation of any
provision of this chapter relating to dwellings of like class and kind
erected after April eighteenth, nineteen hundred twenty-nine, except
that it shall be sufficient for the purposes of this section that
tenements shall comply with article seven, converted dwellings comply
with article six, and lodging houses comply with section sixty-six.
Nothing in this section shall, however, be deemed to prohibit the
conversion or alteration of any multiple dwelling, other than a
converted dwelling and a lodging house, from a class A to a class B
multiple dwelling, or vice-versa, provided that the entire dwelling is
of fireproof construction and is made to conform to the applicable
provisions of section sixty-seven, and to all other provisions of this
chapter applicable to multiple dwellings of like class and kind erected
before April eighteenth, nineteen hundred and twenty-nine.
  5. a. Any multiple dwelling may be altered to conform with any
provision of this chapter applicable to dwellings of like class and kind
and not expressly limited in application to dwellings erected after
April eighteenth, nineteen hundred twenty-nine; or to conform to the
provisions of this chapter relating to egress, or to exits from
apartments, in dwellings of like class and kind erected after such date
if such dwelling also conforms or is made to conform to all the
requirements relating to stairs and public halls in dwellings erected
after such date, except that existing dimensions of stair landings,
treads and risers need not be changed.
  b. An apartment in any part of a fireproof multiple dwelling erected
before April eighteenth, nineteen hundred twenty-nine, may be altered or
subdivided and need not conform to the requirements of paragraph a of
this section relating to stairs and public halls provided each newly
created apartment has access to a public hall which provides horizontal
egress to at least two stairs. If both of such stairs are not arranged
and designed as required by sections two hundred thirty-six and two
hundred thirty-seven, at least one of such stairs shall be so arranged
and designed, and the other shall be arranged and designed as required
by section two hundred thirty-nine. Both stairs shall be protected by
automatic sprinkler heads throughout.
  6. Any tenement previously converted to other uses may be altered or
reconverted to a tenement by complying with the provisions of article
seven, provided, however, that its height and bulk were not enlarged
prior to such alteration or reconversion except as permitted by and in
accordance with the provisions of sections two hundred eleven and two
hundred twelve of this chapter.
  7. In any class B multiple dwelling, except a rooming house or lodging
house, any apartment may be occupied as an apartment in a class A
multiple dwelling, provided such apartment complies with all the
requirements for apartments in class A multiple dwellings erected after
April eighteenth, nineteen hundred twenty-nine. In any rooming house or
lodging house, one apartment may be occupied as an apartment in a class
A multiple dwelling, provided such apartment is occupied solely by the
owner, janitor, superintendent or caretaker.
  8. Any apartment in any class A multiple dwelling may be occupied for
single room occupancy only if such dwelling complies with the provisions
of section two hundred forty-eight and all other provisions of this
chapter applicable to such dwelling.
  9. Excepting a frame dwelling, any dwelling three stories or less in
height erected after April eighteenth, nineteen hundred twenty-nine as a
one or two-family dwelling may be converted to a multiple dwelling to be
occupied by not more than three families in all, with a maximum
occupancy of two families on each floor in a two story building and one
family on each floor in a three story building, provided however that it
shall be unlawful for any such dwelling converted at any time since
October fifteenth, nineteen hundred fifty-two, to have any boarders or
roomers. In each such instance, compliance shall be required with all
the provisions of article six, including section one hundred seventy-a
of said article.
  10. If any class A dwelling erected before April eighteenth, nineteen
hundred twenty-nine, is altered so as to increase the number of living
rooms by more than twenty per centum, such dwelling, except as otherwise
provided in sections two hundred eighteen, two hundred thirty-five and
two hundred thirty-six, shall be made to conform to the requirements of
this chapter with respect to class A dwellings of like class and kind
erected after such date.