Your EPC report will include a list of recommendations detailing measures which should improve the energy efficiency of your property. It will include both a short list of top actions you can take, and a more detailed list further down setting out all recommended measures. The recommendations will help you choose which measure or combination of measures to install.
The MEES Regulations refer to the concept of ‘relevant energy efficiency improvements'. This is a measure, or package of measures, recommended in your EPC report, which can be purchased and installed for £3,500 or less (including VAT) - the cost cap.
If you have installed all ‘relevant energy efficiency improvements' for your property, but your property's EPC rating is still below E, you can register an exemption on the grounds that ‘all relevant improvements have been made and the property remains below an E'.
You are free to install any energy efficiency measure(s), but if your chosen improvements do not appear in the list of ‘recommended energy efficiency improvements' and they fail to improve your property to EPC E, you will not be able to let the property or register an ‘all relevant improvements made' exemption.
You will then need to make further attempts to improve the rating to a minimum of E, in order to let the property.