I converted an outhouse, no bigger than a prison cell (so I'm told) into a habitable room. I wanted it as a quiet sitting room and guest bedroom.
I bought from Argos a cheap, �89 I think, junior bed. Not sure if they still do this. It had sliding door storage in the divan base. I had some large thick foam which I formed into a sofa back, and another piece to form an arm. Both just sat on top of the mattress. I covered the bed/sofa in a thick quilt cover I already had, match covered the foam pieces and had a very smart result.
Although it was only a junior size, 2' 6" instead of 3', and a bit shorter, my well built friend was very comfy in it.