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SAP Calculations For Overglazed Extensions

When your proposed extension has over 25% glazing, then you may need sap calculations. We can figure out the most effective way for you to achieve compliance with the building regs.

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Heat Loss Calculations

It’s now quite stylish for contemporary extensions to generally be bright and light and this is usually accomplished by having such things like bi-fold doors and large amounts of glazing. Having said that, this isn’t compliant with the building regulations as mentioned in paragraph 4.2 of the official document AD L1B, that states that the maximum amount of glazing has to be:

  1. a) No more than 25% of the floor area : plus
  2. b) the total area of any doors or windows that are removed as result of the extension.

So for example if we have an extension with a total floor area of 10 m2, and we intend to take out the existing patio door of 4 m2 to produce an opening from the house to the extension. Then this is the maximum amount of permitted glazing.

Extension floor area 10.0m2
25% glazing allowance 2.5m2
Plus existing opening made by patio door 4.0m2
Maximum permitted amount of glazing 6.5m2


If you imagine a typical external door being around 2 m2, you will realise that 6.5m2 is not a lot of allowed glazing.

If you still desire that light and bright extension in London, you will have to demonstrate to building control that your proposed extension still is compliant with AD L1B. To do this you will need Sap calculations.

The Whole Dwelling Method

This method allows greater design flexibility by using Sap calculations to show that the CO2 emissions from the entire dwelling including the overglazed extension are no worse than a ‘notional’ property of the identical shape and size with an extension having no more than the maximum amount of glazing.

This way upgrades in the original house for example walls or loft insulation, or home heating upgrades can be done to overcompensate for the overglazing. If those improvements have been made already and are over and above that required by AD L1B then the property can be credited with this.

To be able to carry out the Sap calculations no site visit is necessary, but we will require the following:

• Floor Plans to include sections and elevations
• Orientation of the property
• Construction specification of walls, roof and floors
• Specification for all doors and windows
• Primary and secondary heating
• Provision for providing hot water
• Information of renewable technology (if any)
We’ll further assistance in supplying the required information.

So if you require a Sap Assessor or Sap calcs in or around London, then get in touch now.


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