EPC Potential is D. What happens?

A friend has a large Victorian House currently let as an HMO in London. The EPC says the best potential is D if he does some more work. How will it work if these new rules come in saying all property has to be a C?

He will probably have to apply for an exemption. I would advise you to take legal advice.
Current regulations are for an E rated property but that will change as regulations change.