In 2018 regulations regarding the Energy Performance Certificate changed to state that a property must have an EPC of level E and above in order to be rented in the private sector.
The new regulation states that if the tenancy has already started before 2018 and the EPC was below an E rating, the landlord has a responsibility to improve the rating to bring it up to an E. This must be done before April 2020.
EPC’s can be improved by things like a new boiler or new windows. However, there are less expensive options too such as new radiator valves, wall insulation or having LED bulbs.