Unless otherwise expressly provided, the terms "rear yard," "front yard," "side yard" and "court," when used in this chapter, shall be deemed to refer only to a rear yard, front yard, side yard or court required by this chapter. No lot area shall be so reduced or diminished that the yards, courts or open spaces shall be smaller than prescribed by this chapter. No existing building shall be altered, enlarged or rebuilt except in conformity with the regulations herein prescribed.