Within the aforesaid fire limits, no structure shall be built, altered, moved or enlarged unless such structure will be enclosed with walls constructed wholly of stone, well-burned brick, terra cotta, concrete or other such non-combustible materials; provided that, properly owners in the fire limits may construct out-buildings on the rear of their lots of frame material, the sides to be covered with sheet iron or metallic siding and the roof with fire-proof roofing; also the owners or the lessees of the right-of-way of the Union Pacific Railroad Company’s grounds and the owners and lessees of the Burlington Northern Railroad Company’s grounds may construct buildings of frame material, the sides to be enclosed with sheet iron, metallic siding or hard board and the roof to be covered with fire-proof roofing; but before constructing such a building, a plan of the building proposed to be built is to be submitted to the city’s Building Official and a permit obtained to construct such building or buildings, as provided in this article.