Any building, other structure or use of land which is made nonconforming by any lot size, open space or height regulation of this chapter or by any subsequent amendments thereto, may be continued, except as hereinafter provided.

Any nonconforming building, other structure or use of land may be enlarged or extended, provided that any such enlargement or extension shall comply with the regulations of this chapter.

Any nonconforming building or other structure may be repaired or structurally altered, provided that alterations creating enlargements or extensions shall conform to the regulations of this chapter.

Any nonconforming building or other structure which has been damaged or destroyed by any means to the extent of 50% or more of its equalized assessed value, as determined by the Town Assessor of the Town of Boston, or which has been condemned by the Code Enforcement Officer and ordered to be demolished, shall not be rebuilt or repaired except in conformance with the regulations of this chapter. In the case of a permitted restoration of any such nonconforming use, the degree of nonconformance of the original building or other structure shall not be increased.