A nonconforming structure may not be enlarged, extended, increased in height, width or depth, moved or relocated, modified in such a way so as to increase habitable or usable space, number of dwelling units or number of bedrooms, unless such structure is changed to a structure conforming to the requirements of this chapter, except that an existing one- or two-family structure (principal or accessory) may be enlarged, extended or added to, provided:
(1) The structure is located on a lot which conforms to minimum lot area requirements; and
(2) The enlargement, extension or addition conforms to all yard requirements; or
(3) The portion of the enlargement, extension or addition which does not conform to yard requirements consists entirely of the enclosure of existing side or rear porches; or
(4) The portion of the enlargement, extension or addition which does not conform to yard requirements is located entirely to the rear of the existing nonconforming structure, has a side yard setback not less than the existing nonconforming structure and conforms to all other bulk requirements.