On 1 April 2018 it became unlawful to grant new leases of residential or commercial property with an EPC rating of less than E.
From 1 April 2020, this applies to all residential leases (both new and existing) lettings, unless a valid exemption is in place.
From 1 April 2023 this will be extended to include all existing commercial leases.
You will never be required to spend more than £3,500 (including VAT) on energy efficiency improvements.
If you cannot improve your property to an E rating for £3,500 or less, you should make all the improvements which can be made up to that amount, then register an ‘all improvements made’ exemption.
If your property is currently empty, and you are not planning to let it, you don’t need to take any action to improve its rating until you decide to let it again.
More information can be found at https://www.gov.uk/guidance/domestic-private-rented-property-minimum-energy-efficiency-standard-landlord-guidance