Ours has brick walls either side and a dwarf wall at the back, so it’s about 20ft by 8ft.
Great for extending the time of year you can spend ‘in the garden’, because you can have a BBQ etc then bring food/drink in rather than have everyone freeze their nuts off after 8.
In winter it’s just a store room, utility and mini-gym. Hot/cold is a none issue, it cools down in minutes if you fling the windows open, and it’s always going to be more insulating than just having the door/window/wall.
If doing it myself rather than it coming with the house it’d have a solid roof + velux windows, and I’d have taken out the doors/windows from the kitchen and dining room, but that’d require building regs etc.