Cal. Civ. Code § 163

  § 163. Construction and arrangement. Garden-type maisonette dwelling
projects may be erected on condition that they comply with the following
provisions:
  1. The dwelling units in such projects, together, or in their
aggregate, do not exceed in superficial area thirty-five per centum of
the area of the site or plot upon which such projects are erected.
  2. The units in such projects do not exceed two stories in height.
  3. Each section contains not more than two apartments in any unit.
  4. Every one or two-family unit is separated from all other such units
by an unpierced partition constructed of materials having a
fire-resistive rating of at least one hour and such partition extends
from the first floor above the basement or cellar of the dwelling to the
top of the roof boards in such manner as to prevent the passage of fire,
smoke or gases between units.
  5. a. A project section shall not be constructed of units which,
together, are of a greater area than three thousand square feet unless
divided by unpierced fire walls of masonry or other materials having a
fire-resistive rating of at least three hours. Such wall shall be
continuous from the floor of the lowest level of the dwelling to the top
of the roof boards and be fire-stopped at floors and roof in such manner
as to prevent the passage of fire, smoke or gas between areas.
  b. Fire walls shall extend at least two feet above the level of the
finished roof or, in lieu thereof, where there is a peaked or sloped
roof, the ceiling of the attic of all units comprising such section
shall be covered with gypsum plaster-boards three-eighths of an inch in
thickness or with other materials having the same fire-resistive rating.
  6. The facing of the exterior walls shall be of brick veneer not less
than four inches in thickness or may be of one or a combination of other
materials; and the interior facing of such walls shall be covered with
one-half-inch thick plaster boards and three-eighths-inch thick sanded
gypsum plaster or a combination of other materials which, when combined
with the exterior materials shall have a fire-resistive rating of at
least one hour.
  7. Cellar ceilings throughout shall be covered with gypsum plaster
boards three-eighths of an inch in thickness or with other materials
having the same fire resistive rating. A heating plant located within a
section shall be completely enclosed with incombustible materials having
a fire resistive rating of at least three hours and all openings
therefrom to other parts of the dwelling shall be equipped with
fire-proof doors and assemblies with the doors self closing.
  8. Concealed vertical and horizontal draft openings shall be
fire-stopped with incombustible materials.