You will need planning permission and building regs approval to convert a detached garage into a bedroom. I don't know your local planning policies or where the garage is on your plot but round my way this would be unlikely to be approved. You can normally convert to a home office, gym, garden room etc without approval.
If you had PD rights you could convert it. It won't matter to planners what you call it, it's what you do with it that matters, and a bedroom for ancillary use, not renting would be permitted.
It needs to be for a purpose incidental to the enjoyment of the house. It is debatable what this includes but most of the councils I work with specifically state that sleeping accommodation in outbuildings is not PD. I haven't tried an appeal but Im sure there is one out there somewhere.
I don't think you should build this without speaking to your planning office or getting a lawful development certificate.
If the OP wanted to he could use his front room as a bedroom and it's not a planning issue (note: there is no actual conversion involved). In the same way, he can convert his garage into his new "front room", and then use it as a bedroom.
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