On any building devoted in whole or in part to any nonconforming or noncomplying use, work may be done in any period of 12 consecutive months on ordinary repairs, or on repair or replacement of nonbearing walls, fixtures, wiring or plumbing, to an extent not exceeding 50% of the true value of the building, provided that the cubic content of the building as it existed at the time of passage or amendment of this chapter shall not be increased.