Quick answer
RdSAP 10 is the methodology producing your EPC right now. HEM (the Home Energy Model) is what eventually replaces it - but not for a while yet, following an official delay from October 2026 to the second half of 2027. For a landlord, this isn't really a "which is better" question - it's a timing and strategy question: RdSAP 10 offers a real, government-sanctioned route to lock in compliance now (grandfather rights) that simply won't exist once HEM takes over. This guide compares the two methodologies specifically through that lens: what changes, and what it means for the decisions you make about your properties between now and 2030.
The Comparison, Side by Side
| RdSAP 10 (today) | HEM (from H2 2027) | |
|---|---|---|
| In use now? | Yes - this is what's producing EPCs today | No - delayed from October 2026 to the second half of 2027 |
| Calculation method | 12 monthly averages across a year | 17,520 half-hourly simulations across a year |
| What you're shown | A single Energy Efficiency Rating (EER), A–G | Four separate metrics: fabric performance, heating system, smart readiness, energy cost - plus EER retained temporarily alongside them |
| Route to PRS MEES compliance | Reach EPC C before 1 October 2029 → compliant until that EPC expires (up to 10 years) | Meet the fabric standard + your choice of smart readiness or heating system standard |
| Gas boiler outcome | Can still reach EPC C with enough fabric and heating efficiency improvements | Reportedly cannot reach a C on the heating system metric specifically, regardless of efficiency - the smart readiness route exists precisely because of this |
| Heat pump/solar credit | Calculated using fixed seasonal averages - often under-credits real performance | Calculated dynamically against real half-hourly conditions - generally more accurate and often more favourable |
| Air permeability testing | Optional; a measured test can improve on a conservative default | Reportedly a required input, not optional (moderate confidence - not independently confirmed against GOV.UK) |
| £10,000 cost cap / 2030 deadline | Applies regardless of methodology | Applies regardless of methodology |
Why the Methodology Switch Actually Matters to Your Strategy
For a landlord, it's more consequential than that, because RdSAP 10 and HEM can genuinely produce different verdicts on the same physical building. A property that comfortably reaches EPC C today isn't guaranteed to sail through the new fabric-plus-secondary-metric standard once HEM assesses it - the two systems measure different things, weight them differently, and (per the most consistent reporting available) treat heating technology quite differently.
That's what turns this into a genuine timing decision rather than background information: reaching EPC C on today's RdSAP 10-based EPC before 1 October 2029 locks in compliance with the 2030 standard for as long as that EPC stays valid - up to 10 years - without your property ever being assessed under HEM's rules at all. Miss that window, and your property will be judged against the new metrics instead, on thresholds that (as of this writing) haven't been finally confirmed by government. For a landlord trying to plan spend and timing, "which system judges my property" is a real, practical choice, not a technicality.
What This Means for Specific Situations
If your property has a gas, oil, or LPG boiler. Under RdSAP 10 today, a well-insulated property with an efficient, well-maintained gas boiler can still reach EPC C - plenty of currently-compliant rented homes do exactly this. Under HEM, the heating system metric is reported to cap fossil-fuel heating below a C regardless of efficiency, because it factors in carbon intensity, not just running performance. This is precisely why PRS MEES lets a landlord meet the secondary standard through the smart readiness route instead - you're not forced to replace a working boiler. But it does mean a gas-heated property's path to compliance looks structurally different depending on which system is judging it. See our companion guide, Can a Gas Boiler Still Reach EPC C Under HEM?, for the full detail.
If you've already invested in a heat pump or solar PV. RdSAP 10's fixed seasonal averages tend to under-credit these technologies compared to how they actually perform. HEM's half-hourly modelling is generally expected to reward them more accurately - meaning a landlord who's already made this investment may see a fairer, potentially higher score once HEM is actually in use. The catch is timing: that fairer assessment isn't available for well over a year from now, so it isn't something to bank on for near-term compliance planning.
If you own an older or solid-wall property. RdSAP 10's EER is a known, well-understood quantity - you can get an accurate read on where you stand today. HEM's fabric performance metric, and the exact thresholds that will constitute a "C" on it, are things this guide can't yet describe with full confidence, since they weren't finally confirmed at the time of writing. For a property in this position, the known route (pursuing EPC C now) carries a lot less uncertainty than waiting to find out how HEM treats it.
If you're considering an airtightness test. Under RdSAP 10, this is optional and can lift a conservative default assumption today (see our airtightness testing guide). Under HEM, a measured result is reported to become a required input rather than an optional extra. Testing now potentially does double duty: a possible score improvement under the system in use today, and preparation for a requirement that may arrive with HEM regardless.
What Doesn't Change, Whichever Methodology Applies
It's worth being clear that the core PRS MEES policy decisions aren't tied to which methodology happens to be assessing a given property. The £10,000 cost cap, the 1 October 2030 compliance deadline, and the exemptions regime all come from confirmed government policy independent of RdSAP or HEM specifically. The methodology delay changes when reformed EPCs exist and how a property's performance gets measured - it doesn't move the deadline landlords actually need to hit, or reduce what they might need to spend to get there.
A Practical Decision Framework
- Close to EPC C already? Pursue it now, before 1 October 2029. This is the single clearest way to sidestep HEM's uncertainty entirely and lock in compliance for up to a decade.
- A long way from EPC C, and unlikely to get there affordably? There's less urgency to rush specifically under RdSAP 10 - you'll likely need a HEM-based EPC eventually regardless. But note that spend on qualifying measures from 1 October 2025 already counts toward your £10,000 cost cap under either system, so early fabric work isn't wasted even if you end up assessed under HEM later.
- Not sure where you stand? Commissioning a current RdSAP 10 EPC now establishes a real baseline, shows you exactly how far from EPC C you are, and gives you the information needed to decide whether the grandfather rights route is realistic before the clock runs out.
Key Takeaways
- RdSAP 10 is in use today; HEM replaces it from the second half of 2027 at the earliest, following a confirmed delay from the original October 2026 target.
- The two methodologies can genuinely rate the same property differently - this makes "which system assesses me" a real strategic question, not just background detail.
- Gas boiler properties have a clearer, currently-achievable route to EPC C under RdSAP 10 than they're expected to have on HEM's heating system metric specifically.
- Heat pump and solar investments are generally expected to be credited more fairly under HEM - but that assessment isn't available yet.
- The £10,000 cost cap and 1 October 2030 deadline apply regardless of which methodology ends up assessing your property.
- For most landlords, the practical move is the same one repeated throughout this guide: pursue EPC C before 1 October 2029 where realistically achievable, rather than waiting to find out how HEM treats your property.
Sources: the calculation mechanics (monthly vs half-hourly), the four new metrics, the heating system metric's treatment of fossil-fuel heating, and the reported mandatory air permeability input under HEM are drawn from the same sources used in our What Is the Home Energy Model (HEM)? and airtightness testing guides - principally Elmhurst Energy and the HEM Guide's technical explainers, corroborated across multiple independent sources but not fully confirmed against a final published government position on exact thresholds. The HEM launch delay to the second half of 2027 is confirmed directly against HM Government's own HEM:EPC methodology documentation (GOV.UK, updated 17 March 2026). The cost cap, compliance deadline, grandfather rights cutoff, and exemptions regime are drawn from HM Government's "Improving the energy performance of privately rented homes: government response" (GOV.UK, updated 21 January 2026). Where specific claims (particularly the gas boiler/heating system metric point) rest on third-party technical reporting rather than confirmed final government thresholds, this has been flagged in the relevant section - treat this article as current best guidance, not settled law, and revisit once government publishes final HEM:EPC band thresholds.
Frequently Asked Questions
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On this page
- The comparison, side by side
- Why the switch matters to your strategy
- What this means for specific situations
- What doesn't change either way
- A practical decision framework
- Key takeaways
- FAQs
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