The back of our garage was converted into a utility by previous owners, it too was integral but the new door was off the back of the kitchen.
Be aware that they don't allow you to "step down" into the garage so make sure the flooring levels are sorted. (this was why my mate couldn't convert hers)
Prev owners just needed building regs no planning as it is a part conversion so they only nicked about 5ft off the back of the garage.