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.
August 2026 update: RdSAP 10 launched on 15 June 2025 and is now the official UK methodology for assessing existing homes, replacing RdSAP 2012 (v9.94).
In this blog you’ll learn:
- When RdSAP 10 launched and what happened to certificates produced under the old methodology
- What actually changed – from individually measured windows to a new age band and recognition for battery storage
- Why RdSAP 10 now requires documented evidence for past upgrades, and what that means if you can’t provide it
- How air tightness testing now feeds directly into an EPC under RdSAP 10
- What landlords pursuing MEES compliance need to do differently before booking an assessment
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 is the latest version of that methodology, and periodic updates like this one are how the tool keeps pace with modern building technology, changing environmental priorities and evolving standards.
RdSAP 10 isn’t the end of the road, either: from October 2026, the single A-G rating it produces is due to be replaced by a four-metric system calculated under the Home Energy Model. See our guide to the confirmed EPC reform for what’s changing next.
When Did RdSAP 10 Launch?
RdSAP 10 launched on 15 June 2025, replacing RdSAP 2012 (v9.94) as the official UK methodology for assessing existing home energy performance. EPCs produced before that date under RdSAP 2012 remain valid for their original ten-year lifespan – RdSAP 10 applies to assessments carried out from the launch date onwards, not retrospectively.
What’s New in RdSAP 10?
- Individually measured windows – assessors now measure every window rather than relying on typical glazing area assumptions for a property’s age, making the assessment more reflective of the actual building.
- More detail on roof rooms – additional measurements capture different construction types for common walls and gable walls in roof rooms.
- A new age band – covering properties built from 2023 onwards, to reflect improved U-values in the newest stock.
- Isle of Man added as a region – alongside England, Wales, Scotland and Northern Ireland, making RdSAP assessments possible there for the first time.
- PV diverters and battery storage recognised – assessors can now account for these where present.
- Heat pumps recommended more often – as the government’s preferred low-carbon heating solution, subject to the property being reasonably well insulated.
- Updated energy factors – bringing the calculations in line with the latest data underpinning EPC ratings.
- Closer alignment with Building Regulations – making it easier to assess compliance alongside an EPC.
- Air tightness data can now be input directly – DESNZ reversed its earlier decision to exclude air pressure test results from RdSAP 10, so a measured result can now override the default assumption. More on what this means below.
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.
Why These Changes Matter
For homeowners and prospective buyers, 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. For a fuller picture of the MEES 2030 deadlines and cost caps this evidence feeds into, see our guide to the confirmed roadmap.
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 retrofit pathways where further work is needed to hit your target rating.
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