Any dwelling unit constructed by or subsidized by an agency of the government or government the United States or of the Commonwealth of Pennsylvania and which has restrictions on occupancy by reason of age, income or other unique condition, shall meet the minimum habitable floor area standards established by that agency, or, in the absence of such standard, shall meet the standards set forth in Subsection E. Any dwelling unit which, pursuant to this section, is constructed with less habitable floor area than that required by Subsection E, shall not thereafter be occupied by persons not having the unique condition upon which the special permit was based unless it is conclusively demonstrated to the Board that the intended change in occupancy will not result in overcrowding or cause an increase in density or traffic congestion.