Your local council has to address that. (Or certifier maybe..But its still probably going to need to go through Council)
A thread a few months back had council breathing down a owner as his tenants were living in main house, poolshed and also garage. Council insisted that neither be occupied.
Of course its taxable income and the deductions may be very limited. Plumbing, electricals, kitchen etc all contain in a dbl garage. ....All on a concrete slab ? May be expensive for the outcome. Resale ?? Loss of garage may be a buyer concern. 90% of buyers dont want a houso's shed in their garage - Restricts buyers. Refit could also then be costly. Sure you may have no plans to sell...Nobody ever does.