Hiya, I'm a planning officer so can give you a more definitive answer. If all you are doing externally is taking down the garage door, bricking it up and adding a window flush to the wall then you will only need planning permission if you have what's called an Article 4 Direction over your property restricting permitted development rights, or if there was a condition attached to any planning permission granted at the site that could also restrict permitted development. Even if you don't need planning permission you will need Building Regs.