Mr F built an agricultural shed with an upper floor (with full planners approval). However, it's used for storage and he also intends to have his train set up there
. He hasn't built the stairs yet so it's only accessible by ladder, and as I hate ladders I don't go up there. We do though have a 'tea corner' in the main part of the building. There are chairs, a kettle, water and biccies etc, plus a heater - one of those Dyson oval ones which can blast out the heat. Also a dehumidifier, as the steel walls, although they're lined with wood, nonetheless collect moisture. There are double glazed windows to keep the place from freezing in winter, and grass outside to the south so we can sit outside for our tea when it's warm.
We did consider a wood burning stove, but we decided that it was too much of a fire hazard (with agric vehicles and fleece stored there), plus by the time we'd got it heated up we would have finished whatever we were doing there.
I think you will fall foul as you are calling it living accommodation, rather than a rest area. For us, our house is only about 100yds from the building, so we only use it for tea when we're too lazy or too busy to trot down to the kitchen.