If it is self-contained accommodation, it pays its own council tax.
You would require planning consent for change of use to rent it out as a separate darling and of course need to install a kitchen of some description.
You can’t rent it as residential accommodation unless it has a kitchen facility of some description
There is no CGT payable until second dwelling is sold and only then if it sold at a profit.
If you are going to rent it out, and you don’t have your lenders consent, you breach your mortgage conditions and the lender may not consent or they put you on higher rate.
This can be problematical for you if there is a falling out with the children (it does) because they have a financial share in the house then.
Can I clarify anything for you?
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You don’t need planning permission if the use is ancillary to the main house. Hence, if you were renting it out even without a kitchen, you would need planning permission for change of use.
CGT would be payable on the increase in value of the rental element.
You would still need consent from the mortgage lender