A structure or building which is nonconforming as to dimensions or lot density may be repaired, improved, altered or enlarged, provided that the repair, improvement, alteration or enlargement conforms to the requirements of this and other codes and does not increase any existing nonconformity of dimension or density, and further provided, that whenever nonconformity exists because the number of principal buildings exceeds the maximum number of principal buildings permitted on a lot in that district, no repair, alteration or improvement shall be permitted which would increase the size (i.e., height, width, length) or intensify the use (e.g., create a two apartment buildings out of a single-unit guesthouse) of any secondary principal building. For purposes hereof, "secondary principal building" shall mean any principal building on a lot containing two or more principal buildings which, because of its size location, amenities, nature and frequency of use or other factors cannot be considered the primary principal building. By way of example, but not in limitation, secondary principal buildings would include garage apartments, guest houses and cottages occupied or used less frequently than other principal buildings on the lot.