I understand where you're coming from hc, but let me explain. It wouldn't be a change of use because the existing class C3 will exist on the land where the garage sits. What is required is a planning application for development of a domestic garage into a separate dwelling. It's a similar process as those who create a granny annexe on the side of their dwelling. However whilst a granny annexe will invariably contain a condition that prevents the permanent separation of house and annexe into two dwellings, in this case the house has already been legally split from the garage, so it HAS to be independent.
If the area of land is big enough and a new dwelling could be accommodated, albeit for a smaller house than perhaps those around, the access is OK and the garage is within the existing settlement area (in town/ city), then most of the policies applying would be ticked, and one might get consent, but my guess is that it isn't.
It is easier to split a house / garden / garage into two dwellings whilst one owns the whole, not just one bit of it.