Ruby28, I’d echo what Callistemon said. I wanted a study but didn’t want to lose a small bedroom. We saw Studybeds (www.studybeds.co.uk) at an Ideal Home exhibition, and bought one. Lovely big desk, and if you pull the bed down the desk self-levels and slips underneath it, so you don’t have to take computer, printer and assorted stuff off it every time. We got a small double, they also do singles (which I gather they sell to university halls of residence) and full doubles. Delighted with the ease of use and quality ( we got a set of drawers which slide underneath the desk too). Can recommend. As Callistemon said, other models are available, but I only have experience of that one. It gets used maybe 2 weeks in a year, the rest of the time I have my study and craft room to myself.
Ctussaud, a word of caution. We have a 1 single bedroom annex with a sitting area, shower room and kitchenette, over our garage here in west Wales, originally built by the previous owner in the 1960s. It is the width of a path, about 1 metre, from the house. The Valuation Office have designated it a second home as it has a toilet and kitchenette, and our local council have slapped 150% council tax on it, which we expect to go up to 200% next year, which they are allowed to do according to Welsh Senedd law. I gather areas of England such as Cornwall and Devon are also taking punitive second home measures. You just need to be aware of what local legislation there is, or might be in the near future, if thinking of building anything at the bottom of the garden to be lived in