SAP calculations from £95

The Standard Assessment Procedure, SAP, is the government's approved methodology for demonstrating that a new dwelling complies with Part L of the Building Regulations, which sets the energy efficiency and carbon emissions standards new homes must meet. Unlike RdSAP, which surveys an existing building, a SAP calculation is built from the design specification of a dwelling that does not yet exist, or has only just been completed, and it produces both the compliance figures building control needs and the on-construction EPC required before the property can be sold or occupied.

Anyone constructing a new dwelling, or carrying out a conversion that counts as a material change of use, needs a SAP calculation as part of the building control process.

A design-stage SAP calculation should be commissioned once drawings and a specification are settled enough to model, ideally before a building control submission is made, so any shortfall against Part L can be corrected on paper rather than on site. Leaving the calculation until drawings are finalised and construction has started removes the ability to adjust the design cheaply, since changes at that point usually mean altering what is already being built rather than what is still on paper.

A design-stage calculation needs scaled drawings, a specification covering wall, roof and floor construction and U-values, glazing type and area, the intended heating and hot water system, and details of any renewable technology. The more complete the specification at this stage, the more reliable the calculation, and the less likely a late change will force a re-run once construction is already underway.

Pricing depends on the number of dwelling types on a project, the complexity of the design, and whether we are producing a single design-stage calculation or a full design-stage-plus-as-built package. A single self-build dwelling is priced individually; a development with several house types benefits from a reduced per-plot rate once a type has been modelled once and then confirmed against each plot's actual specification.

A design-stage calculation typically takes a few working days from receipt of a complete specification, and can be turned around faster where a submission deadline is tight and the specification is already settled. The as-built calculation follows construction, once the air test result and final specification details are available, and is normally completed within a similar timeframe once that evidence is in hand.

We are used to working directly with architects, energy consultants and building control officers throughout a project, flagging early where a specification is unlikely to pass so changes can be made before they are locked in, and reconciling any changes made on site — a substituted boiler, a different glazing specification — against the original calculation rather than assuming they have no effect.

Where a design-stage calculation shows a shortfall, we identify which specific inputs are driving it — fabric performance, airtightness target, heating efficiency — and work through the most cost-effective route to compliance with the design team, rather than defaulting to the most expensive fix such as adding renewable technology when a cheaper fabric or controls change would close the gap just as effectively.

We produce both design-stage and as-built calculations under SAP 10.2, model realistic fabric-first specifications rather than idealised ones, and coordinate delivery of the on-construction EPC alongside the as-built figures so building control has everything it needs in one package.

A SAP calculation is produced twice on a typical new dwelling project, and the two versions serve different purposes. The design-stage calculation, based on drawings and specification, is what building control uses to check the scheme is capable of meeting Part L before work starts, and it is also what most building control bodies expect submitted alongside the initial notice or full plans application. The as-built calculation is produced once construction is complete and reflects what was actually installed — the airtightness result from testing, the heating system commissioned, and any specification changes made on site — and it is this version that generates the on-construction EPC lodged for the property. A scheme that passed comfortably at design stage can fail at as-built stage if airtightness underperforms or a specified measure was substituted on site, which is why we treat the two stages as genuinely separate calculations rather than a formality repeat of the first.

The current version of the methodology, SAP 10.2, is sensitive to a specific set of inputs more than others: the fabric U-values for walls, roof, floor and glazing, the thermal bridging or psi-value approach used at junctions, the airtightness target and achieved result, the heating system's efficiency and controls, and — increasingly significant since SAP 10.2 revised the carbon factors — whether the primary heating is gas, a heat pump, or another low-carbon source. Getting these inputs right early avoids two common problems: a scheme designed to a generic specification that turns out not to meet target once real junction details and airtightness are modelled properly, and a scheme over-specified with expensive measures where a cheaper combination would have passed just as comfortably. We model realistic, buildable inputs from the outset rather than an idealised specification that the site team cannot actually deliver.

A fabric-first approach — prioritising a well-insulated, well-sealed building envelope before relying on renewable technology to close the remaining gap — tends to produce a more robust compliance margin than a specification that relies heavily on a single low-carbon heating system to do most of the work. This matters practically because fabric performance, once built, is far harder and more expensive to correct than a heating system, and a scheme with generous fabric performance has more headroom to absorb minor as-built shortfalls such as an airtightness result slightly worse than assumed at design stage. We routinely model the trade-off between spending on fabric improvements against spending on renewable technology, so a developer can see where the marginal pound produces the largest compliance benefit for that specific dwelling type.

Part L 2021 compliance for a new dwelling is demonstrated by showing the Dwelling Emission Rate, DER, is no worse than the Target Emission Rate, TER, calculated from a notional specification for that dwelling, and separately that the Dwelling Fabric Energy Efficiency, DFEE, is no worse than the Target Fabric Energy Efficiency, TFEE. These are two distinct tests that must both be satisfied: a scheme can pass on carbon emissions by leaning on a very efficient heating system while still failing the fabric energy efficiency test if the building envelope itself is weak, and vice versa. We report both figures clearly rather than a single pass or fail headline, because a design close to failing on one test but comfortable on the other tells a very different story to a design with a healthy margin on both.

Since the introduction of Part O of the Building Regulations, new dwellings must demonstrate an assessment of overheating risk, and for many dwelling types this is done using the CIBSE TM59 methodology, which models internal temperatures against defined comfort criteria across a representative summer weather file rather than relying on a simple rule of thumb. Overheating risk is driven by orientation, glazing area and shading, ventilation strategy, and thermal mass, and a highly airtight, well-insulated dwelling designed purely for winter performance can be at greater risk of summer overheating if ventilation and solar gain have not been considered together with the fabric specification. We run the overheating assessment alongside the SAP calculation rather than as an afterthought, since changes made to satisfy Part O — additional openable window area, external shading, a mechanical ventilation strategy — can also affect the SAP result and need reconciling with it.

The as-built SAP calculation should reflect the actual measured air permeability result from a certificated air test, not the design target, and a result worse than assumed at design stage can move a scheme from a comfortable pass to a fail if there is no margin elsewhere in the specification. We recommend air testing is arranged early enough in the build programme that, if the result is disappointing, there is still time to identify and seal obvious leakage paths before final sign-off rather than after the as-built calculation has already been submitted. Other as-built evidence that should be gathered as construction finishes includes commissioning certificates for the heating system, manufacturer data for any installed renewable technology, and confirmation that glazing and insulation actually installed match the specification the design-stage calculation assumed.