Any increase in building or floor area shall not exceed an aggregate of more than 25% of the building or floor area, whichever is less, existent at the date the use became nonconforming under this or previous ordinances, during the life of the nonconformity, and in any event shall be permitted only by special exception under the provisions of this chapter. However, a single-family detached dwelling (Use B1), which is nonconforming as of use, may be permitted an expansion without percentage limit of building or floor area, by right, without necessity of a special exception. Structures or land uses that have reached their maximum expansion allowance under previous ordinances are not eligible for any increase in building or floor area under this chapter. A structure, which is nonconforming in terms of height, shall not be extended to increase the height.