A. On April 1st 2002 new regulations came into force for installing replacement windows and doors. Standards in thermal insulation now have to be met and a certificate issued as proof. This is supplied by the council’s building control officers and, like obtaining planning permission for any building work carried out at the property, will be needed to sell your property. With 1.2 million installations of double glazing a year the task of checking each one would be over whelming for building control. To help this situation the replacement window industry has created FENSA (Fenestration Self-Assessment Scheme). This scheme regulates double glazing companies and certifies their installations, It is preferable and I would highly recommend that you appoint a company regulated by FENSA. Consequently from 1st April 2002, all replacement windows and doors need to be double glazed to meet the new requirements, have a FENSA certificate, or have approval through the building regulations.