B. Those dwellings in the locality with assessed values in the most recently ended tax year that are not in excess of the average assessed value for such year of a dwelling situated on property that is zoned as single family residential shall qualify for a total exemption from real property taxes under this article. If the value of a dwelling is in excess of the average assessed value as described in this subsection, then only that portion of the assessed value in excess of the average assessed value shall be subject to real property taxes, and the portion of the assessed value that is not in excess of the average assessed value shall be exempt from real property taxes. Single family homes, condominiums, town homes, manufactured homes as defined in § 46.2-100 whether or not the wheels and other equipment previously used for mobility have been removed, and other types of dwellings of surviving spouses, whether or not the land on which the single family home, condominium, town home, manufactured home, or other type of dwelling of a surviving spouse is located is owned by someone other than the surviving spouse, that (i) meet this requirement and (ii) are occupied by such persons as their principal place of residence shall qualify for the real property tax exemption. If the land on which the single family home, condominium, town home, manufactured home, or other type of dwelling is located is not owned by the surviving spouse, then the land is not exempt. For purposes of determining whether a dwelling, or a portion of its value, is exempt from county and town real property taxes, the average assessed value shall be such average for all dwellings located within the county that are situated on property zoned as single family residential.