Where a window or windows is/are completely replaced (as opposed to repaired) in existing dwellings, they must comply with Approved Documents Parts L1B and K4 (England) or N (Wales) (safety in relation to impact). In addition, the building should not end up with a worse level of compliance with respect to other applicable parts of the Building Regulations, which includes Parts A (Structure), B (means of escape in case of fire), C (Moisture Penetration, F (ventilation), J (combustion appliances and fuel storage systems), M (access for the disabled) and Regulation 7 (Workmanship and Materials).
The Building Regulations for replacement windows state that “The situation must be made no worse than the outgoing windows”. For example if the outgoing windows contained trickle vents the new placement windows must include them.
Compliance with the Gas Safety (Installation and Use) Regulations 1998 is also essential.