A SAP calculation is a spreadsheet based calculation to work out heat losses and gains of a building. It stands for ‘Standard Assessment Procedure’. It is used at design stages to ensure Building Regulations compliance and produce an energy rating.

Generally speaking for existing buildings, (newly created dwellings will need a more in depth SAP and energy certificate) basic SAP calculations are necessary for all extensions that have more than 25% of the floor area as glazing. This is because on existing building projects, each building element, eg wall, roof, floor, windows has minimum thermal performances to be achieved to be compliant. These have been set out based on ‘typical’ % areas. So if there is a higher percentage of glass than expected, heat loss will be higher (and potentially overheating), therefore compliance has to be proved with a SAP calc.

The calculation takes into account the window sizes, areas, orientation and room sizes etc. It then has a series of choices for heating systems and such. From this you get a few numbers that indicate a pass or a fail compared to the “notional” house that doesn’t have too much glazing (eg. one that would pass building regs for your extension’s size without additional proof).

Under the SAP calculation process however, other upgrades and alterations can be undertaken to mitigate this heat loss or gain from too much glazing. For example, upgrading the boiler, increasing insulation elsewhere or renovating thermal performances in other parts of the building.

SAP Calcs can be done remotely with just the drawings, but only individuals or firms licensed with the software can do it as it can be complex to navigate. Most architects will use an external consultant, and depending upon the design complexity (new build or intricate plans are more complex for example) will depend upon the consultant’s involvement. On the more basic or marginally non-compliant projects a simple fast service will be sufficient, rather than more of a consultancy approach. For a simple project I would expect to cost in the region of £150-200 for a SAP calc.

In some cases building control are happy to proceed without a SAP calculation, even if an extension exceeds the 25% rule of thumb, provided substantial other improvements are taking place, but this is dependent upon each project and Building Control discretion. TSA can arrange for a SAP calc to be carried out should building control request, or the glazing is significant.