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Building Regulations Approved Document L1

Windows & Doors

The above Building Regulation was introduced in April 2002 to regulate the thermal efficiency of all new and replacement windows and doors. The regulations are lengthy but in simple terms mean that replacement timber windows and doors will need to achieve a thermal insulation rating (U value) of 2.0W/m2 with new installations needing to achieve 1.8W/m2.

Another requirement of Approved Document L1 is that both new installations and replacement windows & doors will need either :-

  1. Obtain formal Building Regulation approval from the Local Authority before any work is started.


  2. The installer must be a member of FENSA (Fenestration Self Assesment Scheme), which allows them to certify that the product installed meets the Building Regulations. The installer will register the installation with FENSA who will in turn issue a certificate of compliance.

D.G.M JOINERY have been FENSA registered members since its inception Company Registration No. 11160.


It is essential when considering whether a structure is a conservatory and therefore not part of the main structure of the property to understand the definition as quoted in the Building Regulations.

The structure needs to have:

  • Not less than ¾ of the roof area made from translucent material, and
  • Not less than ½ of the wall area made from translucent material
  • Either unheated, or heated by a system with its own and separate heating controls, and
  • Be separated from the main residence by external quality doors

If the structure meets this definition, in full, then it is exempt from the requirement of Approved Document L1.

Furthermore if the structure is less than 30m2 then it is generally not necessary to obtain Building Control approval for its construction work.

It should be noted that whether or not a structure is deemed to be a conservatory or an extension to a dwelling, the requirements of Approved Document N still apply.

If the structure is deemed to be a conservatory then all doors and windows between the conservatory and the main residence must, when replaced, comply with Approved Document L1. The conservatory is not to be registered with FENSA but replacing the dividing doors and windows needs to be.

If the structure does not meet the full definition of a conservatory it does not have an exempt status and must comply with Approved Document L1. In this situation any doors and windows between the residence and the extension will be classed as internal and exempt from Approved Document L1.