(a) The area used in either a site plan or plot plan to determine the number of living units allowed on that area shall not be reused in computing the number of living units for that area or for any subsequent area used with that area.

(b) The building department shall maintain a file of site plans or plot plans and a map which shall evidence which areas have been used in computing numbers of living units. Upon application to the planning and zoning board, the planning and zoning board shall have the discretion to accept a revised site plan, provided the revised site plan shall not exceed the maximum number of living units for any or all of the site.