From 1st October 2010 it will only be legal for window companies to take orders for windows with a Window Energy Rating (WER) of band C or above, or a combined U Value of 1.6 W/m2K for installation into existing dwellings. Likewise, all doors ordered on or after 1st October 2010 will have to have a maximum U value of 1.8 W/m2K (U values are a universal method of measuring thermal insulation)
Any products installed on or after 1st October 2010 will have to comply with the new stricter Building Regulations and have been approved and certified by the BFRC as attaining the new higher energy ratings.

Building Regulations.

Replacing windows and doors now needs to meet certain standards in the Building Regulations to reduce energy loss. This means that you need to comply with the Building Regulations when installing replacement windows, doors,
When selling property, surveyors will ask for evidence that replacement glazing installed after April 2002 complies with the new Building Regulations. There are two ways to prove compliance:-

1. a certificate from the local authority saying that the installation has approval under the Building Regulations.

or

2. a certificate showing that the work has been done by an installer who is registered under a registered Self-Assessment Scheme such as  Fensa.

DIY projects or installations by non-registered firms need Council approval under the Building Regulations and ultimately this may be your responsibility. Therefore be sure to ask whether an installer is able to self-certify. If not, either they, or you, will need to make an application to the Council for approval under the Building Regulations.