In the continuing drive to make domestic and non-domestic properties more energy efficient, the UK government launched the Minimum Energy Efficiency Standards (MEES). The scheme started in 2016 with an introduction period of two years - the aim is to improve the energy grades of housing and commercial premises to E or better
1 April 2018 is when MEES comes into force and any new tenancies must have an EPC of E or better. From April 2020 all domestic lets new or existing must achieve at least an E. Non-domestic date is April 2023.
There are penalties for non-compliance but, also grants for insulation, heating and lighting (see ECO 2 details on this site)
There are exceptions and more detail to MEES; which is linked here and can be explained if you call Mark at Assess Energy Solutions UK