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The Standard Assessment Procedure (SAP) is the government methodology behind every new build and conversion Energy Performance Certificate (EPC). If a rating has come back lower than you hoped, this page explains why the number lands where it does, and lets you test it yourself on an interactive rating explorer further down.
First, a clarification. SAP assessors do not have the ability to adjust the final rating. The software calculates it from the data entered, and that data comes from your design: the drawings, the specification and, on a new build, the test certificates. Our job is to collect that data, enter it correctly, interpret the result and recommend improvements. There is no judgement to exercise and no dial to turn. We cannot change the result without changing the design.
So when a rating disappoints, the explanation lies in the design, and in what the calculation is forced to assume where the design leaves a gap. Both are covered below.
The rating exists to let one dwelling be compared with another. It is not a forecast of what a particular household will spend, and reading it as one is the most common misunderstanding of the whole system.
It is a cost-based index: broadly, the estimated annual cost of heating, hot water, lighting, ventilation and pumps, worked out under standardised assumptions and then adjusted for floor area, so that a large house and a small one can be compared fairly.
Those standardised assumptions are not a shortcut. They are what makes the comparison possible. For every dwelling it assesses, SAP fixes how many people live there, when the heating runs, how warm the rooms get, and even the weather: it uses UK average weather whatever the dwelling's location, so an identical house in Cornwall and in Newcastle carries the same rating. Without that, two identical houses would score differently because one household works from home and the other is out all day, and someone comparing two certificates would learn nothing about the buildings.
The cost of holding all that constant is that the assumptions will not describe how any particular dwelling is really used. A household heating the whole house to 23 degrees will spend more than the certificate implies; one heating two rooms to 18 will spend less. Neither changes the rating, because neither changes the dwelling. The rating measures the building, not the people in it.
SAP assigns each fuel a cost per unit of energy. Mains gas acts as the benchmark: the price table behind the current rating puts it at 3.64 pence per kilowatt hour, against 16.49 pence for standard-tariff electricity, about four and a half times as much. Because the rating is cost-based, a higher fuel cost means a lower rating.
Insulation and window efficiency play their part, since a more efficient fabric needs less fuel. But the price of the fuel sets the limit on how far they can carry the rating, and electricity is the extreme case. Liquefied petroleum gas (LPG) delivered to a tank runs at about 1.9 times the gas price, and heating oil at about 1.4 times, so both pull the rating down without approaching the penalty that direct electric heating carries. That is how the calculation is designed to operate, not a quirk of any one assessment.
The same logic explains the pecking order within electric heating. Storage heaters use the same fuel as panel heaters, but charge overnight on a cheaper tariff. Heat pumps deliver around three units of heat per unit of electricity, so they beat storage heaters, yet can still trail a gas boiler because the electricity they use costs so much more per unit than gas.
The rating is a cost index, so electricity drags it down. Building Regulations compliance is not judged on cost at all: it is judged on carbon, on primary energy (all the energy the dwelling draws in, including what is lost generating and delivering it) and on the performance of the fabric. On carbon, electricity does well, because the grid has decarbonised faster than its price has fallen. Under SAP 10.2 a unit of grid electricity carries 0.136 kg of CO2 against mains gas at 0.210, about a third less.
So one decision can move the two measures in opposite directions. In the explorer's sample dwelling, the gas combi boiler earns the best rating of any heating system, 76 in band C, while missing its carbon target by more than double: 26.9 against 10.6. The air source heat pump rates lower, 62 in band D, and is the only option that meets that target, 8.5 against 10.4. The best-rated system is not the cleanest one, and the three compliance targets are set out further down.
The certificate half-acknowledges this with its second A to G chart, the environmental impact rating, where the electric options score far better than they do on the headline number. The fuller answer arrives in 2027, when the Home Energy Model replaces the single cost-based rating with separate metrics.
SAP was designed first and foremost with new buildings in mind. A new build assessment includes a measured air pressure test result and, usually, a calculated thermal bridging allowance, and new builds pass or fail Building Regulations on the strength of them.
Neither figure is required for work on an existing dwelling under Approved Document L Volume 1, the section of the Building Regulations that covers conversions. (Assessors still routinely call this Part L1B, its name in the 2013 edition.) So the software falls back to default values, and SAP defaults are deliberately poor:
This is why a thoughtfully converted building can still score a middling rating: the calculation is not allowed to give credit for airtightness or junction detailing that has not been evidenced. Commissioning an air pressure test is often the single cheapest way to lift a conversion's rating.
Every rating in the explorer is a real SAP 10.2 calculation, not an approximation. The settings below combine 9,720 different ways, and all 9,720 were calculated in advance. Even so, they are a small slice of a real SAP assessment, which takes far more inputs than these few.
The sample is an existing detached house, about 110 square metres over two storeys, insulated to the improved standard for existing elements. Above it sits a Part L check, answering a different question: not what the dwelling's EPC rating would be, but whether the same specification would pass Building Regulations. It is judged both ways, as work on an existing home and as a new build, because the two face different standards. A good rating and a compliant design are not the same thing, and the section below the tool explains where they diverge.
These five rows are the explorer's own presets, with the equipment behind each heating system named. Same dwelling throughout, insulated to the improved standard for existing elements: only the heating system and, in the final row, the airtightness change.
| Specification1 | SAP EPC rating |
|---|---|
| Electric panel heaters & electric immersion hot water | 30 F |
| High heat retention storage heaters2 & electric immersion hot water | 52 E |
| Air source heat pump3 & hot water cylinder | 62 D |
| Gas combi boiler4 | 76 C |
| Gas combi boiler & airtightness tested at 5.0 | 77 C |
Every figure above is calculated by our SAP 10.2 engine for the explorer's
sample dwelling, so the table and the tool agree row for row. They remain illustrative
and are not a guarantee for a specific dwelling or report.
1 U-values based on the maximum allowable U-values for existing elements,
column (b) improved, in Approved Document L Volume 1.
2 High heat retention storage heaters, six of them, taken from the SAP
table (code 409) on a seven-hour off-peak tariff.
3 Air source heat pump used is a Vaillant aroTHERM plus 5kW & AI VIH RW,
Product Characteristics Database table 362, its efficiency resolved from the plant size
ratio for each specification.
4 Gas boiler used is a Worcester Greenstar 2000 GR2300iW 30 C NG, Product
Characteristics Database reference 18821, 88.4% winter and 88.0% summer
efficiency.
Only the heating system changes across the first four rows, and the rating moves 46 points, from band F to band C, on an otherwise identical dwelling.
A dwelling can earn a good rating and still fail Building Regulations, or scrape a modest rating and pass. The rating is the A to G number on the certificate. Compliance is a separate calculation, and a new dwelling in England faces three targets. It has to meet all three:
Each target comes from a fictitious "notional" twin of the design: the same size and shape, but built to a fixed specification written into the regulations. Whatever the twin achieves becomes the figure the real design has to beat. The explorer builds that twin for the sample dwelling and calculates all three targets, which is what the "New build targets" row reports.
The third target decides most designs. Of the 9,720 specifications in the explorer, 3,351 meet the carbon target, 470 meet primary energy, and only 90 meet fabric energy efficiency. Sixteen meet all three. Because fabric energy efficiency ignores the heating system, a heat pump cannot rescue a design that fails it: insulation, airtightness and junction detailing are the only levers that move it. That is why they matter more than their effect on the headline rating suggests.
The tool makes the point directly. Its opening specification, a gas combi boiler with the insulation at the improved standard, rates 76 in band C and meets none of the three targets. Switch to the heat pump and drop the insulation sliders to minimum: the carbon target now passes, 10.2 against 10.4, while fabric energy efficiency comes in at 112 against a target of 39. Neither result predicts the other.
One caveat. The sample dwelling is an existing house, so these targets answer "what if this were built new today" rather than describing an obligation it actually carries. Compliance in the end rests with building control, not with a calculation.
Our accredited assessors produce SAP calculations and Energy Performance Certificates for new dwellings, conversions and extensions across the UK.
Get a quoteThe current rating system is weighted towards gas heating, which sits awkwardly with the government's aim of phasing out gas boilers on the way to net zero. Two things are changing, and they are often confused. The Home Energy Model replaces SAP as the calculation engine behind certificates. Separately, the single A to G rating is being replaced by four metrics: fabric performance, heating system, smart readiness and energy cost.
Reformed certificates are targeted for the second half of 2027, and the existing A to G rating continues alongside the new metrics until at least October 2029. The same dwelling will most likely produce a rather different looking certificate.
Not directly. The software calculates the rating from the data entered, so the result only moves when the inputs move. What an assessor can do is tell you which inputs are worth changing. On a conversion the usual candidates are an air pressure test in place of the default assumption, a thermal bridging calculation in place of the default y-value, and low energy lighting to every fixed fitting.
SAP is a cost-based index, and standard-tariff electricity costs about four and a half times as much per unit as mains gas, the benchmark fuel: 16.49 pence per kilowatt hour against 3.64. Even a well-insulated dwelling scores markedly lower on panel heaters than the identical dwelling on a gas boiler. Cheaper off-peak tariffs (storage heaters) and heat pump efficiency claw much of that back, and all-electric dwellings score far better on the certificate's second chart, the environmental impact rating.
The Building Regulations do not require an air pressure test or a thermal bridging calculation for work on an existing dwelling, so SAP falls back to its default assumptions for both. The defaults are deliberately pessimistic: far leakier and lossier than a typical tested new build. Providing a test or a junction calculation replaces the default with your real figure, and the test is usually the cheaper of the two.
No. A new dwelling has to meet three targets, and one of them, fabric energy efficiency, ignores the heating system entirely. A heat pump can carry a design through the carbon target while it still fails on fabric. Only insulation, airtightness and junction detailing move that third target.
Yes, in two separate ways. The Home Energy Model replaces SAP as the calculation engine behind certificates, and the single A to G rating is being replaced by four metrics: fabric performance, heating system, smart readiness and energy cost. Reformed certificates are targeted for the second half of 2027, with the existing A to G rating continuing alongside them until at least October 2029.
Related reading: EPC ratings explained for existing homes (with its own RdSAP explorer), RdSAP vs SAP EPC ratings, SAP calculations for new builds, SAP calculations for extensions, EPC changes explained, and our sister site on how EPCs are changing.