The Energy Act 2011 states that all buildings that do not meet the minimum energy performance standard will not be allowed to be let as from 2018 until they have been upgraded.
The secondary regulations, The Energy Efficiency (Private Rented Property) (England and Wales) Regulations 2015, known in the industry as the Minimum Energy Efficiency Standards (MEES), were confirmed and passed into law on the 26th March 2015.
The key points to note are:
Minimum Energy Efficiency Standards make it unlawful for properties with F or G Energy Performance Certificates (EPC) to be let, without implementing cost-effective energy efficiency improvements or fulfilling an exemption criterion.
Minimum Energy Efficiency Standards came into effect on 1st April 2018 for new leases and lease renewals/extensions where there is an EPC already. Minimum Energy Efficiency Standards comes into effect on 1st April 2023 for all existing leases.
More information here