A couple of things to think about.
Plan your electrics and light placement now before you plaster board over.
How are you planning on heating the room - under floor or a radiator?
You may want to raise the floor to the adjoining room level with screeding.
Also if it’s a garage you will want half a wall and a window.
Won’t need a fire door as your going from garage to room and not the other way.
I don’t believe you need any kind of regs or planning permission because you not extending out, up or down.