Having just bought a place with a converted garage / annex….
Are they registered as 2 separate properties with 2 separate deeds?
If so big pain in the ass to change.
If not when I looked into the council tax liability you can ask for a discount based on the fact you are using it for your own use, should be able to find out on the councils website. I was also told if you decommission the food preparation area it would no longer count as a separate dwelling for tax purposes. So basically disconnect and remove the oven (which isn’t used) in our case.
As for utilities, ours were combined on a single bill but had a seperate slave meter to be able to work out the elec usage in the annex and presumably charge it privately. Check that’s not the case.
It makes a fantastic turbo cave with ensuite shower 🙂