The maximum height of such altered, enlarged, relocated or reconstructed building with non-conforming uses, shall not exceed the maximum height permitted by the applicable district regulations, except that for non-conforming residences in C8 Districts or Manufacturing Districts, the maximum height of such altered, enlarged, relocated or reconstructed building, shall comply with one of the following options:

(a)        for single or two-family residences, a horizontal plane equivalent to a height of 35 feet, and for buildings other than single- or two-family residences, the applicable sky exposure plane for the district; or

(b)        for all residences a horizontal plane equivalent to the pre-existing height of such building, as measured from the top of the lowest usable floor, to the highest point of such pre-existing building, as measured from the reference plane.