elfycat that does only apply to separate dwellings where you could potentially rent it out or sell it, I used to work in Council Tax.
We had our garage converted but as there was a door from the garage into the back garden at the rear of the garage we retained a "store room" at the back, so we ended up with somewhere to stash stuff, plus we put a shed in the garden for all the kids outdoor toys.
We also didn't just brick up the bottom of where the garage door used to be and put a window in. We matched size wise to the window on the other side of the front door which is 1.7m wide, so they made an opening for the window of that size. It looks much less like a garage conversion that way. Even the building regs guy commented on that.
Ours is a playroom with a sofa in it, I banked a whole wall with Ikea Pax wardrobes so we can store coats, tv, wii, games, toys and the hoover in there.
As it seems like it is all part of the house it should be double skinned wall wise, ie you have breezeblock/concrete block in the garage so just make sure it is insulated well.
Think about power sockets then install a couple more, you won't regret it!