Quick answer
A property's EPC rating is calculated using the methodology and information available at the time of assessment. If the methodology changes, the same property can potentially produce a different result when assessed again. The important question is what your current rating is actually based on.
Where an EPC relies on assumptions, defaults or incomplete information, a future assessment could produce a lower rating if those assumptions are no longer appropriate. Conversely, where a property has genuine energy-efficient features or improvements that were not properly recorded on the existing EPC, a future assessment could potentially produce a better result.
The Precedent That Already Happened to Landlords
Before looking ahead to HEM, it's worth looking at what happened the last time the underlying EPC methodology changed for landlords specifically - because it's a genuine dry run for the same dynamic at a larger scale. RdSAP 10, phased in from June 2025, moved the methodology toward requiring measured data over age-based defaults, introduced photographic evidence requirements for material elements, and improved how low-carbon technologies like heat pumps, solar panels, and batteries were modelled.
The effect on individual rental properties was genuinely mixed - there was no single direction, and plenty of landlords found out which way their properties moved only when a new EPC was next commissioned, typically at the point of re-letting. As one technical explainer put it plainly: it depends what the old assessment assumed. A rental property whose earlier rating had rested on generous, unverified defaults could re-assess lower once genuinely measured - a D resting on generous assumptions could become an E, which for a landlord isn't just a number, it's the difference between being above or below the current legal minimum to let. Meanwhile, landlords who'd already invested in heat pumps, solar installations, or batteries were among "the clearest winners," since the previous methodology had systematically under-credited that technology. Landlords with real, undocumented insulation or heating upgrades that could now be properly measured also tended to see their properties improve.
That's the template for what HEM is likely to do again to your portfolio - except HEM isn't a version update to RdSAP, it's a genuinely different calculation approach (covered in full in our RdSAP 10 vs HEM guide), so the scale of the shift for some rental properties could be larger still.
What Government's Own Accuracy Research Means for Your Rental Stock
Government-commissioned research into EPC accuracy, reported in mid-2026, compared modelled EPC energy predictions against actual metered energy use and found that modelled energy use generally runs higher than what's actually used - meaning current EPCs tend to be somewhat conservative relative to real-world performance, rather than flattering properties.
That's worth flagging specifically if any part of your portfolio has electric heating - a growing share of newer-build lettings and converted flats - since it suggests those properties carry more uncertainty in how their current rating compares to real performance than a comparable gas-heated property, and potentially more movement once reassessed.
The research's own conclusion is directly relevant to how you manage evidence across a portfolio: it found that measured evidence - airtightness test results, proper U-value calculations, documented retrofit work - meaningfully closes this accuracy gap, and it's cited as part of the evidence base supporting the move to HEM's multi-metric approach in the first place. In other words, government's own data backs up exactly the point made throughout this guide: the biggest single driver of an inaccurate rating on a rental property is missing paperwork, not necessarily poor genuine performance - which is good news, because paperwork is fixable without touching the building at all.
Who's Likely to See a Rise, and Who's at Risk - by Portfolio Type
Based on the RdSAP 10 precedent and the direction HEM's methodology is taking, here's how to think about it across a mixed portfolio:
- Rentals with a heat pump, solar PV, or battery already installed - previously under-credited by calculations based on fixed seasonal averages rather than real dynamic performance. If you've already spent on this technology in one or two properties, those are worth prioritising for reassessment once HEM arrives, rather than assuming they're locked at their current rating.
- Rentals with genuine improvements a previous owner or you carried out that were never documented - once evidence exists (see our assessor default assumptions guide), those improvements can finally be credited.
- Well-insulated rentals currently held back by an older heating system dragging down a single blended score - HEM's separated fabric performance metric shows that fabric quality on its own merits, which matters if you're weighing whether a property is genuinely close to compliant or not.
- Rentals whose current EER leans heavily on generous age-band defaults rather than actual evidence - exactly the properties RdSAP 10 already exposed once measured data was required for landlords letting through 2025.
- Gas, oil, or LPG heated rentals specifically on the new heating system metric, which is reported to cap fossil-fuel heating below a C regardless of efficiency (see our RdSAP 10 vs HEM guide for the full detail, including why this doesn't force you to replace a working boiler).
- Rentals with claimed improvements - by you or a previous landlord - that have no supporting paperwork trail at all, which is common in older portfolios that have changed hands.
Genuinely uncertain: most properties, honestly. Exact HEM band thresholds weren't finally confirmed by government at the time of writing, so treat any specific prediction - including the ones above - as informed direction for planning purposes rather than a guarantee for a specific rental.
How to Protect the Rating You Have - and Manage the Timing Around Tenancies
Given that uncertainty, the practical response for a landlord isn't to guess at exact HEM outcomes property by property - it's to take the moves that protect your position regardless of which way any individual metric eventually swings, timed sensibly around your lettings.
- Reach EPC C under RdSAP 10 before 1 October 2029, if realistically achievable - and prioritise the properties closest to it first. This is the single most reliable protection available. It locks in PRS MEES compliance for up to 10 years under the system in use today, sidestepping HEM's uncertainty for that property entirely, regardless of whether it would have risen or fallen under the new metrics. Across a portfolio, this becomes a triage exercise: identify which properties are one or two measures away from C now, and get those over the line first, rather than spreading effort evenly.
- Build your evidence file for every property in the portfolio now, independent of timing. FENSA certificates, Building Control completion certificates, cavity wall insulation guarantees, Gas Safety Certificates, MCS certificates for renewables - all of this protects a rental's rating whichever methodology eventually assesses it. This is especially worth doing for properties you didn't originally buy new-build or from a developer, where a chain of previous owners means paperwork is most likely to have gone missing.
- Time reassessments around void periods, not mid-tenancy, where you have the choice. An EPC reassessment that reveals a lower rating than expected is far easier to absorb - and to act on with physical works - during a natural void between tenancies than while a tenant is in place. If a property's current EPC is nearing expiry and you suspect it's resting on generous old assumptions, consider getting ahead of it during a scheduled void rather than being surprised at the point you need to re-let.
- Consider an airtightness test on properties where you have reason to believe they perform better than a conservative default. It can help under RdSAP 10 today, and is reported to become a required input rather than optional under HEM - see our airtightness testing guide for the full detail. Prioritise this on properties close to a band boundary, where it does the most good.
- Don't panic-spend across the portfolio against unconfirmed HEM specifics. Genuine fabric measures - insulation, glazing, draught-proofing - improve your position under both systems and count toward your £10,000 per-property cost cap regardless of methodology. Chasing a specific reported HEM rule that hasn't been finally confirmed is a weaker bet, across many properties, than improving fundamentals that help every one of them either way.
Key Takeaways
- Whether a rental's rating rises or falls under HEM depends on what its current rating is actually built on - evidence and genuine performance, or generous unverified defaults.
- This exact pattern already played out for landlords with RdSAP 10 in 2025: some ratings rose, some fell, and plenty found out only when re-letting triggered a new EPC.
- Government's own 2026 accuracy research found current EPCs run conservative relative to real performance on average, and that missing evidence - not poor genuine performance - is the biggest single driver of inaccuracy.
- Portfolio landlords should treat heat pump/solar/battery properties as likely winners worth prioritising for reassessment, and gas-heated or poorly-evidenced properties as the ones needing attention first.
- Time reassessments around void periods where possible, so a lower-than-expected result doesn't land mid-tenancy.
- The most reliable protection available today is reaching EPC C before 1 October 2029 on the properties closest to it - it removes the uncertainty entirely for that property, for up to 10 years.
Sources: the RdSAP 9→10 rating-change precedent is drawn from Energy Assessors UK's RdSAP 10 explainer. The government EPC accuracy research findings (modelled-vs-metered energy use gaps, and the case for measured evidence) are drawn from Elmhurst Energy's coverage of the government's EPC accuracy research, published June 2026. Neither source is landlord-specific in its original framing - the rental/portfolio implications drawn from them here are our interpretation, not claims made directly by those sources. The HEM-specific claims about the heating system metric and air permeability testing are drawn from the same sources cited in our What Is the Home Energy Model (HEM)? and RdSAP 10 vs HEM guides, and carry the same moderate-confidence caveat: exact HEM band thresholds were not confirmed as finalised by government at the time of writing. The cost cap, compliance deadline, and grandfather rights cutoff are drawn from HM Government's "Improving the energy performance of privately rented homes: government response" (GOV.UK, updated 21 January 2026).
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On this page
- The precedent that already happened
- Government's own accuracy research
- Who's at risk, by portfolio type
- How to protect the rating you have
- Key takeaways
- FAQs
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