Building Regulations for Glass Box Extensions
A recent trend has seen demand for extensions that join internal living spaces to gardens using large areas of glazing and bi-fold doors.
Here at Natralight, we regularly work with customers who are looking to extend their property, increasing the value and improving the look of their home with structural glass and rooflights.
This sort of transparent look can be at odds with Building Regulations Part L, which sets targets for the building envelope's energy efficiency.
For our latest blog, we are going to explore Building Regulations in greater detail, and give you the best guidance for compliance.
Understanding Structural Glass Extensions
Part L of the Building Regulations limits the total area of openings or glazed elements, such as windows, roof lights and doors, to a maximum of 25% of the extension's floor area. A smaller extension with a set of patio doors and a lantern light, for example, can easily take up this 25% allowance.
Extensions that exceed the allowance are generally referred to as an 'over-glazed extension', but can be known as glass box extensions, sun rooms or orangeries.
If you and your architect believe you will exceed the 25% rule mentioned above, there are alternative methods to ensure you comply with the Building Regulations.
Firstly, deduct the total area of any windows or doors that, as a result of the extension, no longer exist or are no longer exposed. The heat lost through these elements is no longer being lost, which helps with compliance.
If this still isn't an option then you will need to demonstrate that the proposed extension is no less compliant than if an extension of the same size and shape was built, according to the 25% limit.
Using the Standard Assessment Procedure
If your extension is heavily glazed (over 50%) you need to use the Standard Assessment Procedure (SAP) which demonstrates that the calculated carbon dioxide emission rate from the dwelling with its proposed extension would not be greater than the dwelling and a notional fully compliant extension of equal size and shape.
This allows you to keep almost 100% glazing by upgrading existing elements and services within the original part of the house instead. For example, the installation of more efficient heating and a hot water system which increases the insulation within the existing roof.