SO, the only thing I can think of is to do this:-
http://www.pciq.co.uk/pics/joists2.gif

Rather than trying to extend the existing joists beside the stairs, I
would be tempted to strengthen A (and B if required) to allow you to do
the more "natural" additon of new joists over the stairwell.

You may find that you could insert a temporary joist parallel to A but
say 6" away from it, and then fix B to it. Then cut B a couple of inches
short of A and insert a new joist beside A and bolt it to the existing
A. Now rehang B back onto the strengthened A. If B is not up to carrying
the extra load over the stairs then you could also double this at that
stage. Finally hand the new joists over the stairwell from B to the wall.


Mmm... but, how can I insert a temporary joist parallel to A but 6" away
from it without cutting B first (and our neighbour's ceiling falling down)?
Or have I totally missed the point? :-)

Oh hold on, do you mean a temporary joist above the floor and strap it onto
B to hold it up temporarily while I cut the end of it?

Wouldn't you still be relying on a strengthened Joist A to support 1/2 of
the entire floor weight?

Andy