Building Regulations Part L1B for Glazing

Building Regulations are a set of governmental rules and guidelines to ensure all building works in the UK are safe, accessible and limit waste and environmental damage.

Part L of the Building Regulations deals with the Conservation of Fuel and Power, ensuring good thermal insulation to all external facades and dictating targets for CO2 emission, thermal efficiency, heating and waste management.

These targets and parameters differ depending on the type of property (commercial or residential) and whether the proposed works are a new build or work to an existing dwelling.

All works on existing houses, such as renovations, extensions, or conversions must comply with Part L1B of Building Regulations, these generally hold slightly higher targets for U values and thermal efficiency of glazing than L1A.

Building Regulations Part L1B and Glass Extensions

There are various rules and guidelines for the use of glass in renovations and extensions as stipulated by Part L1B.

If the amount of glazing to the extension exceeds more than 25% of the floor area (Section 4.2) this is then classed as an 'over glazed extension'. If this is the case you need to show that your glazing exceeds the building regulations standards for thermal insulation.

glass side infill extension

"Areas of glazing greater than 25 per cent may be acceptable, especially if this is required to make the extension consistent with the external appearance of character of the host building. In such cases and where practical, either the U-value of the window should be improved relative to the standard set out in paragraph 4.1b, or other compensating measures applied" - Building Regulations 2010 L1B, Section 4.2. 


building regulations part L

Building Regulations Part L1B and Replacement Glazing

Windows, doors, and rooflights are all classed as 'controlled fittings' as part of Building Regulations. This is the entire system which includes the frame. This means that if you are just replacing the glass within an existing frame then this is not notifiable and doesn't need to comply with building regulations (however it is strongly advised that the replacement glazing does comply).

If you are replacing the entire window or door system (glass and frame) then the new glazing will need to comply with building regulations. If the replacement glazing works are straightforward replacements (taking out a window and replacing it with a new one of the same size into the same opening) then you could use a FENSA certified company to prove compliance. This means that the company's systems have already been tested and proven to adhere to building regulations standards. The FENSA approved company will provide the homeowner with a certificate proving compliance.

Please note that this ONLY applies to actual replacements and not glazing to an extension or new glazing into an enlarged opening. IQ Glass is not FENSA certified as the works we do rarely fall into the remit for replacement glazing, however, our aluminium division Hedgehog Aluminium is FENSA certified and is able to carry out complete replacements of windows and doors under the FENSA scheme.


Building Regulations Part L1B and Performance Requirements

Where entire windows, glazed doors, or rooflights are being added or replaced on a building renovation they should be designed to meet Building Regulations performance requirements. These are detailed in Section 4.21 as:

Window/Door TypeRequired Thermal Performance
Window, roof window, roof lightUw Value 1.6 W/m2K or WER Band C
Doors with more than 60% glazingUw value 1.8 W/m2K or Doorset Energy Rating (DSER) Band E
Other DoorsUw value 1.8 W/m2K or Doorset Energy Rating (DSER) Band E

There are some useful notes in this section that help with glazing specification:

  1. The Uw value requirements in the above table are based on glazing in the vertical position. This is important as the thermal insulation of a glass unit changes when it is installed at an angle. All roof glazing used on a project must meet the above performance when vertical. It is then accepted and understood that it will be less when installed.


steel doors to rear extension in London
glazed extension to modern home in London

2. The U values in the performance table are the Uw values (weighted U valued or overall thermal performance of the entire window installation). It is important that you ensure any thermal performance data you receive from glaziers is the Uw and not the Ug value (centre pane of the glass). (Read ‘What is a U Value? ’)

3. You should also ensure that the windows or doors are 'draught proofed in both the installation and the system performance (Section 4.19). This refers to the air permeability testing of the door or window.

The Uw values listed above will only be able to be achieved if you specify a highly insulating window/door system with double glazing as a minimum.  Also ensuring that a well engineered, thermally broken framework is used where necessary will guarantee a high-performing Uw value for the whole installation.

Building Regulations Part L1B and Conservatories

It is important to note that a glass extension is different than a conservatory. A conservatory is classed as an addition to the property, outside the main insulated walls of the house, separated from the residence by external quality, insulated doors, and not heated by the houses heating system. If you are adding a true conservatory to a building then you may not need to comply with building regulations thermal performance requirements.



"Regulation 21 exempts some conservatory or porch extensions from the energy efficiency requirements." - Building Regulations Part L1B Section 3.15. 


If your glass conservatory:

  •  - is on ground level
  •  - has a floor area less than 30 m2
  •  - uses glazing that complies with Part K of Building Regulations (uses safety glass)
  •  - retains the existing insulating walls, windows and doors of the original house
  •  - does not use the heating system from the main house/building

Then it does not need to comply with the thermal performance requirements of building regulations.

If the conservatory does not adhere to all of the exemption criteria above then it will need to be designed to ensure that:

  •  - there is an effective thermal separation between the existing building and the new glass extension
  •  - there are independent heating controls for the glass extension from the house
  •  - the glazing adheres to the thermal performance requirements of Building Regulations as above
glazed minimal extension
glazed extension to modern home in London

Part L1B of Building Regulations applies to building works on existing dwellings. This means house extensions or renovations. If your project is a new build house then it must adhere to the regulations in Building Regulations part L1A - 'Conservation of fuel and power in new dwellings'.

You can view the latest versions of Building Regulations Part L1B (and other parts) on the governments Planning Portal: Approved Document L1B: Conservation of fuel and power in existing dwellings

If you found this guide useful then share it with your colleagues or friends. If you have any questions about the glazing on your project and its compliance with Building Regulations Part L please just get in touch with the team at IQ.

Rebecca Clayton
Technical Sales
Rebecca is Head of Marketing at the IQ Group and has worked in glazing specification for many years. She has a broad range of technical knowledge about all our glazing products and offers technical advice and guidance to architects for specification. Her easy to digest technical advice is often quoted in magazines and publications. You might also recognise her as one of the IQ Glass CPD presenters.
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