RdSAP is the tool that produces an EPC for almost every existing home in the UK – so when it changes, every rating built on it changes too. RdSAP 10 replaced the previous version, RdSAP 2012 (v9.94), bringing more detailed data collection and, for the first time, a direct route for measured air tightness results to feed into the certificate itself. Here’s what changed, when it happened and what it means for a rating today.
In this blog you’ll learn:
- What RdSAP 10 is, and when it replaced the previous methodology
- The handful of changes that actually matter, out of the full list
- Why documented evidence now matters more than ever for a good rating
- How air tightness testing now feeds directly into an EPC
- What landlords pursuing MEES compliance need to do differently
What is RdSAP 10?
RdSAP – the Reduced Data Standard Assessment Procedure – is the simplified, reduced-data version of SAP used across the UK to assess the energy efficiency of existing residential properties. It calculates an EPC, rating a property from A (most efficient) to G (least efficient), based on data such as property age and construction type, heating system and energy source, insulation levels, and windows and ventilation details. RdSAP 10 launched on 15 June 2025, replacing RdSAP 2012 (v9.94) as the official methodology. EPCs produced before that date remain valid for their original ten-year lifespan; RdSAP 10 applies to assessments carried out from the launch date onwards, not retrospectively.
What’s Actually New in RdSAP 10?
The change that matters most: a genuine air tightness test result can now be entered directly into an EPC, rather than always defaulting to an age-and-type assumption. More on this below.
Changes that meaningfully affect accuracy:
- Individually measured windows – assessors now measure every window rather than relying on typical glazing area assumptions for a property’s age.
- More detail on roof rooms – additional measurements capture different construction types for common walls and gable walls.
- Documented evidence is now required for claimed upgrades – covered in full below.
Smaller additions:
- A new age band covering properties built from 2023 onwards.
- Isle of Man added as a region alongside England, Wales, Scotland and Northern Ireland.
- PV diverters and battery storage can now be accounted for where present.
- Heat pumps are recommended more often, as the government’s preferred low-carbon heating solution.
- Updated energy factors and closer alignment with Building Regulations.
The “Evidence First” Requirement
RdSAP 10 puts documented evidence ahead of default assumptions wherever an upgrade is being claimed. If a wall has been insulated, or the glazing has been replaced, the assessor needs to see something to prove it – a receipt, a spec sheet, an installer’s invoice – before it can be entered as an improvement. Without that paper trail, the assessment defaults to a conservative value for the property’s age and type, which can hold a score down even where real work has genuinely been done. If you’ve had energy efficiency upgrades carried out and don’t already have the documentation to hand, it’s worth tracking it down before an assessor visits rather than after.
What This Means for Flats and Mid-Terrace Homes
One practical effect of RdSAP 10’s more refined heat-loss calculations is better treatment for flats and mid-terrace properties. Under the old default assumptions, these dwelling types could be penalised more heavily than their actual exposed wall area justified – a mid-terrace house losing heat through two party walls was sometimes modelled closer to a semi-detached property’s heat loss than its own. RdSAP 10’s more granular calculation reduces that mismatch, so flats and terraces are less likely to be marked down for heat loss they don’t actually experience.
RdSAP 10 and Air Tightness Testing
Before RdSAP 10, air permeability in an EPC was always a default assumption based on a property’s age and type – never a measured figure. That’s changed: a genuine air tightness test result can now be entered directly into the assessment. It’s worth being clear that this doesn’t automatically improve a rating – if a building performs worse than the default assumption, which is common in older, undraught-sealed homes, the measured figure can bring the score down rather than up. We’ve covered what counts as a good result for EPC purposes, and how to judge whether testing is likely to help before booking one, in our companion guide.
What Are the RdSAP 10 Conventions?
“Conventions” in this context refers to the official RdSAP 10 Conventions document – the detailed rulebook assessors follow for handling situations RdSAP’s core methodology doesn’t explicitly cover: unusual constructions, missing data, edge cases in extensions or room-in-roof conversions. It’s a separate, more technical reference from the headline changes covered above, and it’s updated periodically as new edge cases are identified. If you’re commissioning an assessment on a property with anything unusual about its construction or history, this is the document your assessor is actually working from when the standard rules don’t quite fit.
Why These Changes Matter
RdSAP 10 means a more reliable, comprehensive picture of how a home actually uses energy – supporting better-informed decisions on efficiency upgrades, rather than a rating built on generic assumptions. For energy assessors, it’s a more precise, data-driven process: individually measured windows, more detailed roof room data, and now the option to input real air tightness results all push toward EPCs that reflect the building in front of the assessor, not a typical example of its age and type.
The RdSAP 10 update is another step in the right direction regarding the UK’s commitment to energy efficiency and sustainability. These updates are not just about refining the assessment process; they represent a significant shift towards a more accurate and sustainable approach to home energy assessments.
— Sam Speed, Building Sustainability Surveyor at BUilding Energy Experts
What Landlords Need to Do Differently
The evidence-first requirement matters most for landlords working toward MEES compliance, where the target rating and the deadline are both fixed. If a property has had insulation, glazing or heating upgrades over the years without the paperwork being kept, RdSAP 10 won’t credit those improvements – the assessment defaults to conservative assumptions instead, which can leave a property scoring lower than its actual condition supports and further from the EPC C target than it needs to be. Before booking a RdSAP 10 assessment, it’s worth compiling whatever evidence exists for past retrofit work: installer invoices, product spec sheets, building control sign-off, anything that documents what was actually done.
Where a property needs further work to close the gap to EPC C, that’s a retrofit coordination question as much as an assessment one – sequencing which measures to prioritise, and making sure new work is documented properly from the outset so it counts under RdSAP 10 the first time.
What’s Next?
RdSAP 10 is now the live, official methodology behind every new EPC for an existing home – and with evidence requirements tightened and air tightness data able to feed directly into the result, an accurate assessment increasingly depends on real building data and documentation, not default assumptions. Whether you need help pulling together evidence for past upgrades, a RdSAP 10 compliant assessment, or a retrofit pathway to close the gap to your target rating, that’s what we deliver.
We carry out PAS 2035 retrofit assessments and domestic EPC assessments under RdSAP 10, working from real fittings, fabric, construction and evidence data rather than defaulted assumptions – and can advise on a retrofit strategy where further work is needed to hit your target rating.
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