If you are renting it will need to meet regulations for safety purposes.
It may also need consent if it is used as a separate dwelling as the side extension may have stated it is incidental to the main dwelling and not to create a separate dwelling.
The council will advise what you need to do, if anything, to continue renting it out.
You may have to split for council tax.
Insurance, ring your insurer. You may have to change cover. You may need additional cover.
Mortgage, you probably need consent of the mortgage company to rent out. Having an occupier can effect the lender's security.
Do you have a tenancy agreement? You should to have control and protect your interest.
What about deposit, is that registered with the protection scheme. You can be sued if not.
Type of tenancy will depend on letting. If you regard them as a lodger then room rental agreement may be fine. If you have created a separate dwelling then you may be better off with a n assured shorthold tenancy, then you will need to protect the deposit.
The council should be helpful. If anything needs to be done they will advise and give you time to comply.