Housing development as used in this section, means a development project for five or more residential units, including mixed-use developments. For the purposes of this section, “housing development” also includes a subdivision or common interest development, as defined in Section 4100 of the Civil Code, approved by a city, county, or city and county and consists of residential units or unimproved residential lots and either a project to substantially rehabilitate and convert an existing commercial building to residential use or the substantial rehabilitation of an existing multifamily dwelling, as defined in subdivision (d) of Section 65863.4, where the result of the rehabilitation would be a net increase in available residential units. For the purpose of calculating a density bonus, the residential units shall be on contiguous sites that are the subject of one development application, but do not have to be based upon individual subdivision maps or parcels. The density bonus shall be permitted in geographic areas of the housing development other than the areas where the units for the lower income households are located.