Nonconformity created through public taking. Where the dimensional nonconformity of a structure is created by government acquisition of property such structure shall not be regarded as a nonconforming use, and may be continued, structurally altered, reconstructed, repaired or enlarged so long as it remains an otherwise lawful use; provided, that any structural change, repair, addition, alteration or reconstruction, shall not increase, extend, enlarge or affect the dimensions which are deficient.