Can I rent out a garage conversion flat?

Only with the right permissions in place. Independent letting is one of the strongest indicators of a separate dwelling in planning terms, so it needs planning permission, full building regulations compliance and sign off from building control. Your mortgage lender will need to agree to it, and your buildings insurance will need updating. Letting a unit that was approved as an ancillary annexe risks a breach of planning control, which is not something to discover after a tenant has moved in.