SAP Calculations For Extensions

Residential Extensions (Part L1b)

If you are building an extension Part L1b of the building regulations requires existing dwellings to have a SAP calculation if they are classed as excessively glazed. This occurs when an extension includes glazing of more than 25% of the extension’s floor area, common examples of when these calculations are required include the construction orangeries, conservatories and extensions with large bi-fold doors, glazed elevations and glazed roofs.

The requirements for residential extensions aren't a strict as new build standards and allow for existing elements of the building to be incorporated into the design.

Upon completion of the building works the SAP calculation is used to prepare the Energy Performance Certificate (EPC).

Methods Of Compliance

Method 1 - Area Weighted U-Value Calculation Method

The area weighted U-Value calculation assesses the proposed U-Values of the actual extension against the notional U-Values set within the requirements of Part L1b table 2 to demonstrate that the heat losses (W -watts) are no worse than the allowable notional equivalent.

Cheaper than Method 2 due to simplicity
Process ignores solar gains
No requirement for full building design
Often requires more insulation and/or better glazing compared to Method 2
Requires only extension drawings for calculation
Offers less design flexibility than Method 2

Method 2 - Whole Dwelling Calculation Method (Actual vs Notional SAP Calculation)

The whole dwelling calculation method assesses the dwelling as a whole, taking into account the existing dwelling and any improvements being made as a result of the extension and the proposed extension. The method is able to take into account solar gains, improvements to the existing dwelling including boiler/heating upgrades, window upgrades and insulation upgrades. This method of calculation does however require a full set of drawings for the existing dwelling and proposed extension.

Includes benefits from upgrading existing elements
Requires more information gathering
Usually requires less insulation
Can take longer than Method 1
Incorporates the benefits of solar gains
Requires a full set of drawings of the whole dwelling
Offers greater design flexibility than Method 1
Slightly more expensive than Method 1 due to larger calculations

Why Use BEAT?

  • Experienced and qualified assessors providing bespoke low cost solutions saving you money and time

  • We have preformed hundreds of assessments just like yours

  • Fast turnaround times

  • Reports in pdf format 

  • A dedicated assessor from start to finish

When should I get my extension SAP?

Quite simply, the earlier we are involved, the more influence we can have on easily incorporating small changes with minimal cost which can make big impacts and huge savings in build cost and carbon preformance.

How long does it take to get the completed calculations?

Generally a SAP calculation takes about 2-3 days to produce, we suggest however you allow 1 week from appointment to allow for busy periods.

What do you need to produce a SAP for an extension?

Depending on whether compliance is being sought via method 1 or 2, we require following:- Method 1 - Drawings including plans, sections, elevations and specification of just the extension. Method 2 - Drawings including plans, sections, elevations of both the existing whole dwelling and of the extension. Also we require a site plan together with a proposed specification for the materials and insulation together with the heating and hot water appliances. Additionally, we'll need information about the age of the existing property and the existing building services.

What happens if my design fails?

Even if your proposed design and specification fails we will provide you solutions and recommendations to successfully achieve a PASS free of charge!

How to get a quote?

Just send us your project drawings via email to or use our quote request form and we'll send you an electronic quote by return.

Frequently Asked Questions