(a) "Separate interest" has the following meanings:
(1) In a community apartment project, "separate interest" means the exclusive right to occupy an apartment, as specified in Section 4105.
(2) In a condominium project, "separate interest" means a separately owned unit, as specified in Section 4125.
(3) In a planned development, "separate interest" means a separately owned lot, parcel, area, or space.
(4) In a stock cooperative, "separate interest" means the exclusive right to occupy a portion of the real property, as specified in Section 4190.
(b) Unless the declaration or condominium plan, if any exists, otherwise provides, if walls, floors, or ceilings are designated as boundaries of a separate interest, the interior surfaces of the perimeter walls, floors, ceilings, windows, doors, and outlets located within the separate interest are part of the separate interest and any other portions of the walls, floors, or ceilings are part of the common area.
(c) The estate in a separate interest may be a fee, a life estate, an estate for years, or any combination of the foregoing.