The simple answer is yes, you can sleep in a summer house extra bedroom. The answer gets more complex however if you want to use a garden room for only sleeping. There are rules if the primary use of a garden room is sleeping. These are different to if it is only occasionally slept in. To have a garden room as permanent sleeping accommodation you need to check building regulations and get planning permissions. Normally most garden rooms are allowed to be built under Permitted Developments rights which mean you do not need planning permission. For sleeping accommodation however, you will first need to apply for planning permission from your local council.
