I converted a double garage 2 years ago.
We moved into a 4 bed newish build and we knew that we would convert a garage into a playroom. We have off road parking for 4 cars, we are in the old show home and I have 2 front gardens plus an inverted T shaped drive. So the parking didn't come at the cost of a garden. As it was the show home the garage already had a steel lintel so we had a huge space.
Both mine and DH's cars are parked in front of where the double garage was, and as it is the playroom for the children I don't look out onto the cars.
I personally find it incredible that someone would provide a room for a car 
I didn't want our garage to look like a converted garage and the way I did this was to not just brick up the bottom of the garage door opening and fill the top half with a window.
Instead the dining room is to the front of the house so we matched the two new windows with that one size wise and style wise. The only thing that gives it away is the vertical bricks that extend past the window but you have to look hard. Even the building control guy commented how good it looked.
We retained a store at the back of the garage as the door to the garden was at the back, so the playroom is about 5.5m long by 3.6m wide and the store is 5.5m long by 1.6m wide. So I basically had a wall built with an internal door leading from the playroom into the store.
We had to have a door knocked through from the hall too. I'm in Yorkshire and it cost £12k. I was ballpark quoted £8k for a single garage as that is what I thought we would buy, and lucked out with a double garage. Mine included relocating a soil pipe, radiators, flooring (gulp) building a wall, creating a new loft hatch, electrics, alarm relocating of PIRs and alarm pad.
Hope this helps.