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, basic SAP calculations are necessary for all extensions that have more than 25% of the floor area as glazing. (Newly created dwellings will always need a more in-depth SAP and energy certificate.) This basic SAP calc is necessary to judge if an extension will loose too much heat or gain too much heat in winter/summer respectively. (Even triple-glazed, coated-glass is far worse thermally than a normal wall).

The thermal performance part of the building regulations (Part L) on existing building projects, can be considered using the ‘elemental method’ whereby 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 of a ‘nominal’ extension design. So if there is a higher percentage of glass than expected (there usually is in modern design), heat loss will be higher (and potentially overheat in summer), 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 so the overall design need not be dramatically changed. 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 with the licensed 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 £150-300 for a SAP calc by a consultant, more if things get complicated or several attempts are necessary to fine tune the design to a pass.

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.