My studio, above the garage is 28x32 so I have a small kitchen, a shower room and a propane stove. A 50inch wide staircase is useful as I can get things up and down without struggling. And, with an eye to the future,it would be easy to fit a stairlift.
I had electrical outlets sunk into the floor at 4 points so that I don't have wires trailing everywhere, plug points in the walls are at counter height. I put in open shelving designed for fabric storage but, on reflection, would have waited to see how I used the room before having permanent fixtures. There are windows all along one wall, window seats under windows on two other walls but no one ever sits there so they are just storage for little used items. There is room for two couches - one is a futon in case I want to stay there when guests are in the main house - and a big old rocking chair. If there is one thing I would change, apart from the spacing of the fixed shelving, it's the lighting. I didn't think I would need more than the recessed ceiling spot lights and wall lights but a couple of industrial lighting strips would make the space more useable in dreary weather. Despite having 12 windows there is not enough directional lighting. I was so concerned about it looking nice that I painted it neutral, had a finished floor with underfloor heating and quite forgot that I might want to splash paint around, do a bit of sanding, dye fabric in the industrial sink or generally make a bit of a mess or screw things to the walls. So it's a room that could become an apartment if anyone could get me out of it but, the point made about not using it as much as if it were attached to the house is worth considering. I tend not to go there if the rain is really bouncing, the snow is deep or it's the dead of night ...but once I get there I lose track of time. A covered walkway would fix wet weather access and a screened in porch would make it perfect. But almost perfect is good too.