What Is RdSAP 10?

RdSAP vs SAP: What "Reduced Data" Actually Means

It's worth understanding the relationship between these two terms, because it explains a lot about how EPCs work in practice. SAP - the Standard Assessment Procedure - is the government's core methodology for estimating a home's energy performance, first published in 1993 and periodically updated since (the current full version is SAP 10.3). It's used to demonstrate that new-build homes comply with Part L of the Building Regulations, among other purposes.

RdSAP - Reduced Data SAP - was introduced in 2005 specifically for assessing homes that already exist, rather than new builds. The "reduced data" part is the important bit: rather than requiring the exhaustive data collection a full SAP assessment would need, RdSAP is deliberately built around a set of standardised assumptions about the dwelling, cutting down how much an assessor has to physically measure and verify on-site. This is precisely why default assumptions exist at all in the EPC system, and why the evidence-versus-defaults dynamic covered in our assessor default assumptions guide is baked into the methodology's design, not an accident of any particular version.

A Brief History: How We Got to Version 10

RdSAP hasn't been static since 2005 - it's gone through a series of incremental updates, each one usually tracking an update to the underlying SAP methodology or responding to changes in the housing stock and heating technology available. The version history:

The pattern is worth noting: RdSAP has always evolved incrementally, version by version, rather than being rebuilt from the ground up. RdSAP 10 breaks that pattern somewhat - it's described as the biggest jump in more than ten years, largely because so much changed in UK housing and heating technology between 2019's version 9.94 and 2025: heat pump adoption accelerated, domestic solar and battery storage became mainstream, and smart heating controls went from niche to common. HEM, covered separately, is a further step beyond even RdSAP 10 - not another incremental version, but a genuinely different calculation approach.

What Actually Changed in RdSAP 10

For a landlord, the practical changes fall into a few clear categories:

• Measured data over assumptions, wherever it's available. This is the core shift - where a property's actual construction, insulation, or performance can be evidenced, RdSAP 10 is built to use that real data rather than falling back on the standardised defaults described in our assessor assumptions guide.
• Photographic evidence requirements for material elements of the property, to support auditability of what's actually been recorded.
• More precise data collection generally - windows recorded with frame type, wall insulation thickness and type, and roofing construction detail, rather than broader categorisations.
• Significantly better recognition of modern technology - heat pumps, solar PV, battery storage, and PV diverters are now properly included in the calculation, alongside smart heating controls.
• Improved treatment of ventilation systems and air pressure test results - this is the change that underpins our airtightness testing guide, and it didn't exist in this form under 9.94.
• Better handling of heated basements, a previously under-modelled category.

What This Already Meant for Landlords Who Got Reassessed

This isn't speculation about a future system - RdSAP 10 has been live for over a year at the time of writing, and its real-world effect on landlords is already documented. The direction of change for any individual property has been genuinely mixed, and consistently described as depending entirely on what the earlier assessment had assumed. Landlords who'd already invested in heat pumps, solar, or battery storage were widely reported as clear winners, since the previous methodology had systematically under-credited that technology. Conversely, landlords whose earlier rating rested heavily on generous, unverified age-based defaults saw those ratings correct downward once genuinely measured under the new rules - a D resting on generous assumptions could reassess to an E.

Does My Existing EPC Need to Be Redone?

No. An EPC issued under any earlier RdSAP version remains fully valid for its normal 10-year lifespan, regardless of the June 2025 methodology change. RdSAP 10 only comes into play when a new EPC assessment is actually commissioned - whether that's for re-letting, a sale, or a voluntary reassessment. There's no requirement to proactively replace a still-valid EPC just because the calculation method behind it has moved on.

What Landlords Should Actually Do

1. Gather documentation now, before your next assessment is due. Since RdSAP 10's central shift is measured evidence over defaults, having FENSA certificates, insulation guarantees, Gas Safety Certificates, and MCS certificates ready means your next EPC - whenever it's commissioned - reflects your property's real performance rather than a conservative fallback.
2. If you've installed a heat pump, solar, or battery storage since your last EPC, consider getting reassessed sooner rather than later. If that EPC pre-dates June 2025, you may be sitting on a materially better rating than your current certificate shows, given how much more favourably RdSAP 10 treats this technology.
3. If your current EPC pre-dates June 2025 and you suspect it rests on generous assumptions, be realistic about what a fresh assessment might show. That's not a reason to avoid reassessment - evidence gaps are fixable - but it's a reason to gather supporting paperwork first rather than being surprised at the point you need to re-let.

Key Takeaways

• RdSAP 10 has been the live methodology behind every EPC in England and Wales since 15 June 2025 - it's the current system, not a future one.
• RdSAP itself (any version) is a "reduced data" simplification of full SAP, built around standardised assumptions when evidence isn't available - that's the origin of the defaults landlords need to actively override with evidence.
• RdSAP 10 is the biggest single update in over a decade: measured data over assumptions, photographic evidence, and much better treatment of heat pumps, solar, batteries, and air permeability testing.
• Real-world outcomes for landlords reassessed under it have been mixed - technology-heavy properties tended to improve, evidence-light properties resting on old defaults tended to correct downward.
• Existing valid EPCs don't need to be redone - RdSAP 10 only applies to newly commissioned assessments.
• RdSAP 10 is entirely separate from HEM, which is a different, not-yet-live system delayed to the second half of 2027.

Frequently Asked Questions