Do i need a permit if i change my garage into a temporary room?

I'm afraid you will need planning permission to 'change the use of the room', we were going to do the same for our daughter for when she stays during breaks from uni but have found out that the amount of paperwork and legal crap involved even for temporary use made it a non-starter. There were all sorts of rules about health and safety and required certificates concerning building regulations. Unfortunately unless you are going to do it fully with all the planning permission etc you are standing on very tricky ground, good luck.