Yes ignore my straightjacketed and blinkered friend.
With all building work, extensions, alterations, modifications whatever, the first thing to ask in this context is "Do I require building regulation approval?". To determine this, the next question is "What building regulations actually apply?" - ie what is and is not 'controlled works'.
Any garage conversion to a habitable room requires building regulation approval. End of.
The issue is to what extent - ie what conversion work is 'controlled works', and therefore what part of the conversion work needs permission and is going to be inspected. This will differ on a case by case basis depending on the work being done - bricking up the garage door, foundations or a beam, the window, the floor, the roof the doorway from the house etc. It's not like an extension say, where almost everything comes under building regulations, with a conversion only certain things do.
So for once again for clarity, a garage conversion requires building regulation approval.