Building Regulations and Planning Permissions
IQ Glass Solutions LTD, Sky House, Raans Road, Amersham, HP6 6FT
Building Regulations and Planning Permissions
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 2021 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.
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:
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.
"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.
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.
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:
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.
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:
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."
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.
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.
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.
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
Scottish Building Regulations Guide
Building Regulations Part L1 | Architectural Glazing to New Build Dwellings