Coal-door

It was hard to work out at first what the function of this little door was - or what, when it was opened, you might see.

A passer-by explained that it was an old coal-hole.
Most houses' coal-holes would be at ground-level, as the coal-store was often in the house cellar, and the coal merchant could just pour his delivery straight in and down. Presumably this house had no cellar, or coal-shed, so a back-room must have been used - and this unusually high coal-hole was where the load was poured in.