To report a suspected unauthorised alteration to a listed building you can report a suspected unauthorised development online.
If you are looking for more general advice for homeowners you may find the information below useful:
Planning (LB&CA) Act 1990 https://www.legislation.gov.uk/ukpga/1990/9/contents
Historic England – Advice to homeowners page https://historicengland.org.uk/advice/your-home/
SPAB – homeowners advice https://www.spab.org.uk/advice