Building Regulations and Planning Permissions

Building Regulations Part L1 | Architectural Glazing to Existing Dwellings

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 to house extensions, ensuring good thermal insulation to all external facades and dictating targets for CO2 emissions, 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 (a house extension).

All works on existing houses (referred to as dwellings), such as renovations, extensions, or conversions must comply with Approved Document L Volume 1 in line with the requirements for works to existing buildings.

Approved Document L was updated in 2022 in line with the government's new energy standards targets. During this update, the old Approved Document L1B (which detailed the thermal performance requirements for house extensions) was overridden. All thermal performance requirements for glazing to extensions are now detailed in Approved Document L Volume 1.

This article details the requirements for glazing as part of works to an existing dwelling as written in Approved Document L Volume 1. For an overview of the thermal performance requirements for glass to a new build house read our article Building Regulations Part L1 | Architectural Glazing to New Build Dwellings.

An Overview of Approved Document L Volume 1 for Existing Dwellings

If you are undertaking building work to an existing dwelling, then you must comply with Approved Document L Volume 1. The energy efficiency performance requirements for works to an existing building differ very slightly from that of a new build house.

When designing the renovation or extension to an existing house you have to keep in mind the following:

  • You need to limit heat gains and heat losses through the building envelope (Section 4)
  • You need to ensure that any new building services are energy efficient (Section 5 and 6)
  • That any new elements to the building comply with the requirements in Approved Document L (Section 10)

That if any works are conducted on existing elements in the house that they also adhere to Approved Document L Volume 1 (Section 11).

Approved Document L Volume 1 applies to any ‘building work’ to an existing dwelling unless it is exempt. There are certain exemptions for glass conservatories, extensions to listed buildings, historical buildings or houses that are in a conservation area. These are detailed later in the article.

highly glazed home extension building regulations

"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.

Thermal Performance of Glazing to a House Extension

The required thermal performance of glazing to a house extension is listed in Table 4.2 of Approved Document L Volume 1. Any glazing in an extension should comply with these figures as a minimum.

Extract from Table 4.2

Windows, roof windows and curtain walling Uw value 1.4 W/m2K
Rooflights* Uw value 2.2 W/m2K
Glazed Doors (over 60% glazed on internal face) Uw value 1.4 W/m2K
External fire door sets Uw value 1.8 W/m2K

*The upstand that the rooflight is installed on should have a max U value of 0.35 W/m2K.

To achieve these thermal performance requirements, you will likely need to specify a double glazed installation with low e coating and argon gas filling as a standard. The framing will also need to be fully thermally broken to achieve these levels.

glass extension thermal performance building regulations L

Thermal Performance of Replacement Glazing

If you are replacing glazing within an existing dwelling the new window or glass door must both comply with the thermal performance requirements listed in Table 4.2 (see above) and have a performance that is no worse that what is already there.

When the replacement glazing is to a traditional building there may be limitations on the changes you can make to the glazing. If you need to maintain the character of the existing building, installing double glazing or new thermally broken frames may be detrimental to the existing design of the house.

If this is the case, you can either:

  • Using glazing with a maximum Ug value (centre pane performance) of 1.2 W/m2K or
  • Use low emissivity secondary glazing.

Any replacement window, door or rooflight must also be draught-proof and use insulated cavity closers on installation where possible.

If in the replacement of a window or door you are also enlarging the window opening, then you need to check the percentage of glazing against the total floor area of the glazing. If the area of windows, roof glazing or glass doors exceeds 25% of the total floor area of a house you need to achieve a higher thermal performance than that detailed in Table 4.2.

approved document L existing dwellings glazing

Thermal Performance of Glass Conservatories or Porches

A glass conservatory has a very specific definition and is different to a glass extension.

A conservatory is classed as an addition to the property, outside the main insulated walls of the house, separated from the existing dwelling by external quality, insulated doors, and not heated by the houses heating system.

A conservatory or a glass porch will be exempt from the energy efficiency requirements under regulation 21 as long as it meets the below criteria:

  • The glass conservatory is on the ground level
  • It doesn’t have a floor area of greater than 30m2
  • The heating system for the home doesn’t extend into the conservatory or porch
  • The space doesn’t have its own heating system.
  • The wall/window/door that separates it from the existing house is being retained
  • if the wall/window/door that separates it from the existing house is being replaced, the new element adheres to the performance requirements of Approved Document L Volume 1.

If the conservatory or porch is not exempt, then it must comply with the thermal performance requirements as if it were a normal extension. There may be further exemptions for glass additions to an existing dwelling if it is listed or historical.

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

work to existing dwellings building regulations glazing

Thermal Performance of Highly Glazed Extensions

Section 10 of Approved Document L Volume 1 gives a detailed list of the requirements of additions to existing dwellings. Although paragraph 10.7 states that you cannot have an extension with over 25% glazing if you read a little further on you will see that is not the case.

If the total area of glazing in the new extension exceeds 25% of the floor area of the extension, then you just need to do a few calculations to show it still complies. You can either use the formula expression given in paragraph 10.9 or use the Standard Assessment procedure (SAP). In essence, what you are trying to show is that the highly glazed extension provides the same level of performance (or better) than a ‘standard’ extension with less glazing. This needs to be calculated as a whole with all aspects of the extension envelope.

It is typical in these instances that you will need to use glazing that has an improved U value over what is listed in Table 4.2. Other thermally insulating elements of the extension may also need to be improved to achieve the right level of efficiency.

thermal performance of replacement glazing

There are certain exemptions from Approved Document L Volume 1 if adherence to these rules would detract or harm the visual character of a building.

This includes:

  • Listed houses (Section 1 of the Planning (Listed Buildings and Conservation Areas) Act 1990)
  • Houses in a conservation area (Section 69 of the Planning (Listed Buildings and Conservation Areas) Act 1990)
  • Houses included in the schedule of monuments (Section 1 of the Ancient Monuments and Archaeological Areas Act 1979.
  • Historical or traditional houses where there is a need to match the external appearance or character of the existing building.

Extensions or renovations to these types of buildings are treated on a case by case basis by the local authority. The building control body should take advice and guidance from the conservation officer as to what is deemed appropriate for that building.

The conservation officer will determine what works can be undertaken to the sensitive building to maintain the character of the building. Any works should adhere to Document L wherever reasonable and possible within this guidance.

thermal performance glazing building regulations

Further Reading

The following technical articles might also be useful if you are designing or specifying glazing for an extension or renovation project.  

What is a Thermal Break and How Does it Work?

Thermal Performance in Modern House Design 

What is a U Value Rating?

Scottish Building Regulations Guide 

Building Regulations Part L1 | Architectural Glazing to New Build Dwellings

home extension building regulations part L1 glazing