Additions and alterations. No additions or structural alterations shall be made to any building nonconforming as to use, unless such additions or alterations are required by direction of a federal, state or municipal authority or unless such nonconforming building and the use thereof is made to conform to all regulations of the district in which it is located, except that one addition to a building nonconforming as to use may be made which does not exceed 25% of the full assessed value of the building alone, valued at the time of the proposed addition.