Nothing contained in any law or enactment for the time being in force to regulate the erection, re-erection, construction, alteration or maintenance of buildings within the limits of any municipality shall apply to any building used or required for the public service or for any public purpose, which is the property, or in the occupation, of the Government, or which is to be erected on land which is the property, or in the occupation, of the Government :
     Provided that, where the erection, re-erection, construction or material structural alteration of any such building as aforesaid (not being a building connected with ¹*** defence, or a building the plan or construction of which ought, in the opinion of ²[the Government concerned], to be treated as confidential or secret) is contemplated, reasonable notice of the proposed work shall be given to the municipal authority before it is commenced.