I've now added the stonework to the bottom section of the walls, which cover up the extensions to the height. These are cut from some embossed plastic sheets that I had in the 'bits box', which were originally from a model railway water tower I think, but I can't recall the manufacturer. I tried to create an uneven finish to the line of foundation stones, which should contrast with the rendered upper sections of the walls. I made a mould for the door and took a casting from ProCreate putty; it sits a little bit too proud but I hope it'll blend in when painted. I filled in some of the missing stones at the corners by both cutting out individual stones and gluing on and by filling any remaining gaps with modelling putty. I also found an excuse to use Games Workshop's Liquid Green Stuff for the first time to smooth over cracks and it seems to work pretty well.
Next step will be to construct the roof.