The required yards surrounding an existing building shall not be separated in ownership from that portion of the lot upon which the building is located, and no part shall be considered as providing a yard for any other existing building on the same or adjacent lot. A yard shall not be reduced to less than the required dimensions for the district in which it is located by enlarging an existing building. A yard of less than the required dimensions shall not be further reduced. Every required yard shall be open and unobstructed from the ground upward except for accessory buildings as provided in Section 1121.16, and permitted projections as provided in Section 1121.17. (Ord. 67-17. Passed 5-15-67.)