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SAP Calculations for Extensions and Conservatories

Whole house SAP Calculations for extensions with more that 25% glazing for building regulation compliance

SAP Calculation Extension

Have you been asked by Building Control to provide a Notional vs Compliant extension SAP Calculations because your extension has more that 25% glazing and not sure what you need? We can help guide you through the process and provide all the documentation required for sign off.

Why have I been asked for a SAP Calculation for my extension?

SAP stands for the Standard Assessment Procedure, which is the Governments Standard for assessing the Energy Ratings of Dwellings. When you register your extension building works with you local Building Control, they will be charged with ensuring that the works complies with current Building Regulations. Part L of the building regulations (England and Wales) relates to the Conservation of Fuel and Power. In specific Part L1B of the Building Regulations sets out specific criteria that you are required to comply with when constructing an extension to an existing dwelling. These include include meeting u-value requirements on all new build elements and designing no more than 25% off the extended floor area in glazing. If your extension has lest than the 25% glazing then you will not be required to undertake a full sap, you will, however, need to ensure that the extension in constructed as to meet the fabric u-value requirements. Building Control will offer guidance on this, but if you would like some independence advice we will be more than happy to help. You can view the Part L1B Document on the Governments website by visiting Part 1LB SAP Requirements.

You will need a SAP if:

  • Your extension is open to the main house
  • Your extension has over 25% glazing to extension floor area

What if I have over 25% glazing?

If you extension has over 25% glazing to floor area, which is often the case with Orangery style builds you will need a Whole House SAP Assessment, sometimes referred to as a notional vs compliant SAP Assessment. Under section 4.4 of L1B an optional approaches that offer more design flexibility.

The first approach is the area weight calculations. Here we assess the area weighted u-value of your extension using the default values for the u-value fabric requirements. We then compare this to the extension with the u-value calculated based on the proposed building fabric for the extension.

The second approach is to undertake the whole house SAP Assessment to show that the calculated carbon dioxide (CO₂) emission rate from the dwelling with its proposed extension is no greater than that for the dwelling plus a notional extension which is built to the current standard Building Regulations for extensions. This offers the greatest design flexibility.

As side note all new build and renovated elements will still be required to meet the elemental U-value targets, however is is best to treat these value as worst case and improving on these significantly will provide greater flexibility when it comes to the amount of glazing desired.

To meet the Building Regulation requirements, it may be necessary to make additional improvements to the existing dwelling, such as installing extra layers of mineral wool insulation within the loft space. Again, these additional improvements to the building fabric will need to meet the minimum Building Regulation standards.

Conservatories and porches are exempt where the work is at ground level, the floor area is less than 30m2, the heating system does not extend into the conservatory and the external wall and door separating the dwelling and the conservatory is retained.

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