Allowable Glazing for House Extensions
Allowable glazing for house extensions can be found in Approved Document L1B.
The guidance is for the amount of glazing you are allowed for an extension is as follows:-.
The extension should be designed so that the total area of windows, roof-lights and doors does not exceed more than 25% of the extension floor area.
You can also include any existing openings which as a result of the work no longer exist or are no longer exposed.
For example if the extension has a floor area of 20m², the allowed amount of glazing allowed in the extension would be 5.00m². However if the extension was then to cover an existing patio door (opening) which has an area of 4.00m², the total allowable glazing for the extension would then be 9.00m².
The guidance on allowable glazing for house extensions does not actually allow the designer a great deal of scope as most clients prefer to have a large amount of glazing to an extension.
The good news is that there are ways to incorporate large areas of glazing into the design. This can be achieved by having excessive glazing calculations carried out by a qualified Energy Assessor. The Assessor can carry out one of two calculations.
Area-weighted U-value method
This method allows for justifying glazing that is partially over the guidance limit by increasing the “U” value of the walls, floor, roof and glazing to the extension.
Whole dwelling calculation method
This method, also known as a SAP Rating allows much more glazing to the extension by carrying out improvements to the existing property, such as loft insulation, cavity wall insulation, new boiler and low energy lighting.
So in simple terms by having calculations undertaken to justify allowable glazing for house extensions, you can have substantially more glazing to the extension that the guidance suggests.
I hope you have found this article helpful. Further articles on this topic will be provided.