There are lots of simple ways to do this.
1) Fix a joist to the rear wall using frame fixings. Fit two 'plants' to the
side walls using the same method (no need to disturb rooms on either side).
Put the landing on top and use the stairs to hold the front of the landing.
2) As above but add a front joist using joist hangers on the plants.
3) Like 2) but using angle plate to screw into the faces of plant and joist.
4) Get posh and build the frame using proper wood joints (cross-lap would do
it nicely.) Then, disassemble the frame, fit the plants to the wall and
enjoy the satisfaction of a job well done as the joists slip neatly into
I'd do 3). I know 1) should work, but I wouldn't have the confidence to do
it. My chippie mate would do 4).