FENSA & Document L | UK Building Regulations

FENSA, Document L & The UK Building Regulations?

‘FENSA is a government-authorised scheme that monitors building regulation compliance for replacement windows and doors. Every FENSA Approved Installer, from large national brands to small local companies, is assessed regularly by us to ensure its standards are continually maintained.’


2002 saw the introduction of Document L in the UK, which was a new set of regulations for replacement windows.   Introduced to achieve national energy saving targets, these changes can still cause some confusion.

When selling property, surveyors will ask for evidence that replacement windows installed after April 2002 comply with the new Building Regulations.  There are two ways to prove compliance:-

  1. A certificate showing that the work has been done by an installer who is registered under the Fenestration self-assessment scheme by FENSA Limited, CERTASS Limited or the British Standards Institution.
  2. A certificate from the Building Control Partnership (local authority) saying that the installation has approval under the Building Regulations.

Replacement sashes do not require compliance.

Document L regulations only apply to ‘new or replacement windows’ and therefore replacement sashes are not subject to the requirements.  Replacement sashes are simply a repair or modification to an existing window and may be either single or double glazed.  Nobody can issue a FENSA certificate for replacement sashes.

Conservation areas may have other restrictions that apply. 

Although compliance is not mandatory our double glazed, low-e, sashes do comply with the spirit of the regulation.

Full window replacement does require compliance.


All new windows must meet minimum thermal standards, expressed as a ‘U’ Value.  Some exemptions apply for conservation areas and listed properties.  New glazing needs to have a maximum U-value of 2.0 w/m2k for window frames or 2.2 w/m2k for doors. 

To achieve the required U value in a new sash window, it must be double glazed using the thickest possible low ‘E’ Argon filled glass unit.   

Inevitably these additional requirements increased the cost of replacing whole windows, making repair work the preferred option where possible.

We are not a FENSA registered company because the majority of our work is restoration rather than replacement.   Whilst FENSA membership allows for instant certification, alternative compliance certification can easily be obtained from the local authority. The specification of our replacement windows will be accepted by local authorities, but the homeowner will need to make a submission to the Council for consideration under the Building Regulations.



Building Control Partnership