Why did my new build EPC rating drop once RdSAP replaced my SAP certificate? In almost every case, the house itself has not changed. What has changed is the assessment method. Your on construction certificate was based on design stage SAP, calculated from the architect’s exact specification. Once that certificate expires, it has to be replaced with a full RdSAP assessment on the finished home, a standardised method built on visual inspection, conventions and defaults rather than the original design data.
Homeowners and landlords across Milton Keynes, Northampton, Towcester, Daventry, Buckingham and Brackley get in touch, confused and sometimes worried, when a new build marketed as a B rating shows up as a C on the official register. It can knock confidence in a recent purchase, and for landlords it can matter for meeting minimum energy efficiency standards. The reassuring part is that the drop is almost always explained by the change in methodology rather than by anything wrong with the building, and part of it can often be corrected.
Design stage SAP and RdSAP are not the same assessment
SAP and RdSAP both produce an Energy Performance Certificate, and both use the same rating scale from A to G. But they are built from completely different sources of information, which is why they can score an identical building differently.
SAP: built from the architect’s specification
Design stage SAP, used for new build EPCs, is calculated from the full construction specification: the exact U-values of the walls, roof and floor, the precise glazing being installed, and often a measured air pressure test result taken once the shell is complete. Because it is data rich and specific to that plot, it tends to produce an accurate, and often favourable, rating for a home built to current standards.
RdSAP: built from what an assessor can see on the day
RdSAP is the method used for existing dwellings, and it takes over once your on construction SAP certificate has expired. It is a non invasive survey: the assessor cannot open up a cavity wall or verify what insulation is hidden inside a floor void. Where the exact specification cannot be confirmed with evidence, RdSAP falls back on standard conventions and default values based on the Building Regulations standards that applied when the property was built, rather than the specific, calculated figures used in the original SAP assessment. For a refresher on what an EPC assessment actually involves, we have covered the basics separately.
This caution is deliberate, and it works in your favour. RdSAP conventions exist so that a rating cannot be inflated on the strength of a claim alone: if a wall, a window or an air test cannot be evidenced, the certificate has to assume the more modest figure rather than take a builder’s word for it. That protects anyone relying on the EPC, whether for a mortgage, a rental, or a purchase decision, from a rating that overstates the truth.
Why a new build EPC rating drop happens even when nothing has changed
Three areas typically account for most of the gap between a design stage SAP score and the RdSAP score that follows it, and in each case it is because RdSAP requires proof rather than assuming the best.
Air permeability reverts to an age based default
At the end of construction, most new builds have an air pressure test, and the measured result is entered directly into the SAP calculation. RdSAP does not carry that tested figure forward automatically. Unless the assessor is given the original pressure test certificate as documentary evidence, RdSAP applies a standard default for air permeability that varies with the property’s age band and construction type, rather than the property’s own measured performance. That default is generally more cautious than what a well built new home can actually achieve when tested. Elmhurst Energy, the accreditation body, has described this missing test result as one of the most common reasons a new build’s RdSAP EPC differs from its original SAP EPC.
Glazing falls back to an age band without evidence
A design stage SAP certificate can record precisely what glazing is being installed, including higher performance units. RdSAP can also record triple glazing or high performance double glazing, but only where there is documentary evidence to support it, such as a FENSA, CERTASS or building control certificate, or the manufacturer’s specification. Without that evidence, the RdSAP conventions require the assessor to place the glazing into a standard age band instead, even though the glass itself has not changed.
Walls, roof and floor fall back to the Building Regulations standard for the property’s age
The same pattern applies to the building fabric. SAP uses calculated U-values for the actual construction as designed. RdSAP records what the assessor can identify on a visual inspection, for example a cavity wall assumed to be insulated, and applies the Building Regulations standard for the property’s age rather than the specific calculated figure. The result is often recorded as good rather than the very good rating the same fabric achieved under the original SAP calculation.
A real example from a recent assessment
This is not a theoretical difference. On a recent job, we compared the on construction SAP certificate for a property we assessed with the RdSAP assessment we carried out on the same home once that SAP certificate had expired.
The original SAP certificate rated the home 83 points, a solid B, with primary energy use of 86 kWh per square metre per year. Walls, roof and windows were all recorded as very good, and a tested air permeability result of 4.9 supported a good air tightness score.
Our RdSAP assessment of the same home recorded 79 points, still a good result, but enough to move it from Band B into Band C. Primary energy use rose to 114 kWh per square metre per year. The walls and roof were recorded as good rather than very good against the Building Regulations standard for the property’s age, the windows moved from high performance glazing to an average rated double glazing category, and no air pressure test evidence was available to override the age based default for air permeability.
Nothing about the property had changed between the two assessments. What changed was the amount of verified information each method was allowed to use.
Does a lower RdSAP score mean your home is less efficient than promised?
Usually not. A drop from B to C on paper does not mean the walls have got thinner or the boiler has got worse. It means the later assessment could not verify, or was not given evidence for, details that the original design stage calculation already knew. The building’s actual performance is likely to sit somewhere between the two certificates, closer to the SAP figures if the build quality matched the specification.
That said, RdSAP conventions exist for good reason. Some new builds do not achieve their design stage air test on every plot, and site changes during construction are common. A lower RdSAP score is a prompt to check, not proof that you have been misled. If you are comparing your certificate with a similar looking property, our guide to why your EPC can differ from a similar looking home covers the same evidence based logic.
What you can do if your RdSAP score is lower than your SAP certificate
Since the RdSAP 10 changes went live in 2025, assessors have been able to override the default air permeability assumption where a genuine pressure test certificate is available, rather than being restricted to the standard age based default. If you still have your original air pressure test certificate, hand a copy to whoever carries out your RdSAP assessment.
For the rest of the building fabric, the evidence that carries weight is the original accredited SAP assessment calculated for that specific property, not a generic as built specification sheet. If you, your developer, or the original assessor can trace a copy of the on construction SAP calculation, your RdSAP assessor can use it to override some of the standard defaults, bringing the certificate closer to the building’s actual performance. Bear in mind that your EPC is only reassessed and renewed once the current certificate has expired. If you are also weighing up who should carry out that reassessment, our guide to choosing the right EPC assessor covers what to look for.
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