I'll soon be taking-out an old "back staircase" and closing the ceiling.
AFAIK the staircase was fitted as an afterthought (it's not shown on the
1930s architect's drawings) so I assume they cut the joists and fitted a
trimmer some time in the 40s or 50s. The plan is to simply attach joist
hangers to the trimmer (fixings at about 300mm), fix a wall plate
opposite the trimmer, attach joist hangers to the wall plate and drop-in
the joists. The span is 1500mm and the joists are 8", on 400mm centres.
The ceiling around the opening is lath'n'plaster so I'll probably pull
this down and replace with plasterboard, or I might just over-board it.

The walls are lime plaster over solid concrete blocks so getting good
fixings for the wall plate should be straightforward, probably using
large frame fixers (or ???)

Is there anything else I should be thinking about?