Energy Performance Certificates (EPCs) are one of the most widely used tools for measuring the energy efficiency of homes in the UK. They are required whenever a property is built, sold, or rented, and provide a rating from A (most efficient) to G (least efficient).
But how EPCs are produced, and how much we can rely on them, depends on whether you are looking at a new build or an existing property.
For new homes, EPCs are based on detailed SAP (Standard Assessment Procedure) calculations carried out by accredited assessors. These calculations are informed by:
This combination of design, evidence, and testing should give a fairly accurate reflection of the building’s theoretical performance as it was built, but in reality there is quite often a performance gap between the theoretical and actual performance.
By contrast, EPCs for existing homes are usually produced using RdSAP (Reduced Data SAP). This is much less detailed and often based on:
Crucially, there is no air test, and very little evidential checking of insulation or building fabric. As a result, EPCs for existing stock can be quite approximate.
Here’s the interesting part: the certificates looks the same.
An EPC issued for a carefully modelled, tested new build home and one produced for a 100-year-old terrace house based on a visual inspection will both appear identical to a buyer or tenant. Both show an A–G rating, estimated running costs, and recommendations for improvement.
However, this does not mean either rating reflects how the property will actually perform once lived in. EPCs are based on theoretical models, not post-occupancy data. Even with the introduction of photographic evidence requirements for new builds since 2022, which has improved accuracy, EPCs still do not measure real-world energy use or comfort.
This is the essence of the performance gap: the difference between what an EPC suggests “on paper” and how a property actually performs in use.
That’s why, when considering a retrofit, it’s important not to take the EPC rating as the sole baseline for design. Instead, the real performance of the building should be established, for example, through thermographic surveys, airtightness tests, or detailed fabric analysis — and retrofit measures should be designed accordingly.
EPCs take into account both:
At present:
As the UK grid continues to decarbonise and reforms to EPC methodology are introduced (from 2025/26 onwards), electric heating may appear more efficient than it does today.
This could have the unintended consequence of making properties with basic direct-electric panel heaters seem to perform well in EPC terms, even if in practice they are expensive to run and provide poor comfort levels.
It’s vital to remember that EPCs measure energy performance only. They do not take into account:
As we highlighted in our earlier articles on retrofit, measures designed simply to improve an EPC score, such as adding insulation or sealing a building, may achieve a better rating but also create new problems if not done carefully, such as damp, condensation, or poor indoor air quality.
That’s why an EPC should never be confused with an assessment of overall building quality.
The government has already introduced photographic evidence requirements for new builds, and the next version of the assessment method (SAP 10 / RdSAP 10) will improve data accuracy for existing homes.
Future reforms are also expected to bring:
EPCs are an important tool, but they are not all created equal. A new build EPC is backed by detailed modelling, photographic evidence, and airtightness testing. An EPC for an existing property may rely on assumptions and defaults. Yet both are presented in the same format, which can mislead owners, buyers, and tenants about how efficient a property really is.
At BB Partnership, we understand these limitations. That’s why we go beyond EPC ratings, using tools such as thermographic surveys and Passive House principles to assess and design for true building performance, improving efficiency while also protecting occupant health and comfort.
If you are a developer, landlord, or homeowner looking to improve energy performance, we can help you understand the real picture, and ensure your property is designed or upgraded to perform in practice, not just on paper.
BA [Hons], Dip Arc, RIBA
Director
BA [Hons], Dip Arc, RIBA
Director
BA [Hons], Dip Arc, RIBA
Director