Are you referring to a block of flats where mail is delivered to the main outer door of the building ? Or are you enquiring about a dwellinghouse (two storey building) where the letter box is positioned at the front door ? There may be complications if it is a flat used as combined dwelling by other occupiers and just the single communal letter box. Royal Mail does have a legal responsibility to deliver post to the appointed address, but you need to be sure as to the location and use of the letter box, which must be clearly visible and accessible.