L1B – SAP calculations for heavily glazed extensions

L1B – SAP calculations for heavily glazed extensions

If you build an extension where the glazing is greater than 25% of the floor area, then we can undertake a SAP assessment to demonstrate compliance with Approved Document L1B.

Using SAP, we can provide you with an economic and feasible solution to ensure your extension achieves compliance, no matter how much glazing you wish to include in the design.


Three steps to compliance:

  1. Use SAP to calculate the CO2 emissions for the existing house along with the proposed extension.

  2. Calculate the CO2 emissions for the existing house with a notional extension. A notional extension is an extension that meets the minimum standards of Approved Document L1B.

  3. Compliance is achieved provided the “proposed” CO2 emissions are no greater than the “notional” CO2 emissions.

How Energytest can help

Fast and accurate SAP calculations at a competitive price.

Comprehensive advice and support.

One fully-inclusive fee covering:

  • SAP calculations.
  • U-value calculations.
  • All the consultancy required to ensure every dwelling achieves a pass.

We shall assess the extension based on the design-specification provided. The vast majority of projects we assess achieve a pass first time. If a pass is not achieved we shall put forward cost effective options to achieve a pass. For example, increase in insulation, better performing glazing, or improvements to the existing dwelling.

Once a satisfactory specification has been agreed we shall issue a copy of the SAP reports for submission to Building Control.

  • Floor plans, elevation and section drawings for both the existing dwelling and the proposed extension;

  • A detailed construction specification for the proposed extension;

  • An approximate age and construction type for all floors, roofs and external walls within the existing dwelling (for example, 1970s cavity wall);

  • A description of the windows within the existing dwelling (for example, 10 year old UPVC double glazed);

  • Details of the primary heating system (such as the make and model of the boiler);

  • Details of any mechanical ventilation (for example, the number of mechanical extract fans);

  • The approximate proportion of low energy light bulbs;

  • Details of any hot water storage;

  • Details of any fixed secondary heating systems (such as open fire place); and

  • Details of any low or zero carbon technology, such as solar hot water or photovoltaics.

What our clients say:

“An excellent service, easy to use with a fast turnaround!”

Sanders Design Services