Cal. Civ. Code § 56

  § 56. Frame buildings and extensions. 1. Except as provided in section
one hundred ninety-three and subdivision seven of this section, no frame
multiple dwelling shall be erected and no frame dwelling not used as a
multiple dwelling on April eighteenth, nineteen hundred twenty-nine,
shall be altered or converted to such use or occupancy.
  2. No existing frame multiple dwelling shall be increased in height
nor shall it be altered to permit a greater occupancy on any story than
provided for on April eighteenth, nineteen hundred twenty-nine; except
that, if the walls of such a frame dwelling are faced with brick veneer
or with another material or combination of materials having a fire
resistive rating of at least one hour, and the entrance story thereof is
occupied by not more than one family, such entrance story may be altered
so that it may be occupied by not more than two families.
  3. No frame building of any kind whatsoever shall be placed or built
upon the same lot with any multiple dwelling.
  4. No multiple dwelling shall be placed or built upon the same lot
with any frame building.
  5. No frame multiple dwelling, no wooden structure of any kind or
class on the same lot with any frame dwelling or with any multiple
dwelling, and no other building on the same lot with any frame dwelling,
shall be altered or converted so as to be enlarged, extended or
increased in height or bulk or in the number of rooms, apartments or
dwelling units therein; except that:
  a. An extension seventy square feet or less in ground area the side
walls of which are of frame and brick filled or of masonry construction
may be added to any existing frame multiple dwelling if used solely for
bathrooms or waterclosets; and
  b. An extension constructed with fireproof walls may be made to a
frame building if the first story of such extension is used solely for
business not prohibited by any local law or ordinance, or if such
extension contains not more than one living room on any story. No yard
or court shall be diminished by such extension so that its area or least
dimension is less than required by this chapter for a yard or court of a
multiple dwelling erected after April eighteenth, nineteen hundred
twenty-nine.
  6. None of the provisions of this section shall apply to dwellings
erected under the provisions of article five-a of this chapter.
  7. In any city the department may approve for any such dwelling three
stories or less in height, the alteration and conversion of that portion
of said dwelling used as a store or other non-residential use to no more
than one additional dwelling unit; provided, however, that: (a) such
space has been vacant for at least one year, and (b) such space has a
minimum of three hundred square feet of floor area, and (c) the
conversion must be for a class "A" use, and (d) said unit shall contain
a cooking space and a complete bathroom, and (e) all walls and ceilings
of the new dwelling unit shall be fire-retarded with one hour rated
fire-retarding materials, and (f) the height and bulk of the dwelling
shall not be increased, and (g) the dwelling will be in full compliance
with this chapter and other related and local ordinances, except that
the owner of said dwelling shall be entitled to consideration for
variances permitted in subdivision one and subparagraph five of
paragraph a of subdivision two of section three hundred ten of this
chapter for multiple dwellings and buildings existing prior to November
first, nineteen hundred forty-nine.