A practical guide for landlords and portfolio owners: reaching EPC C on high-D rental stock through evidence-led RdSAP 10 reassessment and a verified air permeability test. Most landlords have now been told the same thing: get every let property to EPC C, and the way to do it is retrofit, meaning insulation, heating upgrades and solar. The assumed cost sits somewhere between £8,000 and £27,000 per property. Across a portfolio of any size, that quickly becomes a capital programme running into seven figures. But a meaningful share of the properties sitting in the high D band do not actually need that programme. Many can move from EPC D to C on evidence alone, because they are already performing better than their certificate suggests. The certificate simply has not caught up. This piece explains why that gap exists, how a property can be moved from D to C legitimately under the current methodology, what the evidence looks like across five real properties, and, just as importantly, where the approach does not work and should not be attempted.
How the EPC D to C route works
For the right property, an EPC can move from D to C by combining two things: an evidence-led RdSAP 10 reassessment that replaces default assumptions with confirmed, inspected values, and a verified air permeability test that replaces the conservative default with a measured figure. No scaffolding, no decant, no fabric works. Where the property genuinely supports a C, a new certificate is lodged. Where it does not, the property is redirected to a proper retrofit route before any money is spent.
RdSAP 10 and the air permeability test: what changed
RdSAP 10 became the mandatory methodology for existing dwelling EPCs on 15 June 2025. For the first time, it introduced the option to use a measured air permeability value in place of the assumed default that earlier versions of RdSAP applied automatically. That default matters. Where no measured value is available, RdSAP works out an assumed infiltration figure from the age and construction of the property, which for a typical older home is equivalent to roughly 15 cubic metres per hour per square metre at 50 Pascals, and sometimes worse. Modern, well-maintained homes typically test somewhere between 5 and 13, well inside that figure. The five properties discussed below all came in below the default, most of them considerably so. In other words, the methodology now allows a property to be credited for airtightness it genuinely has, rather than penalised on an assumption. That single change is often enough to tip a high-D property over the line, but only when it is combined with fixing the other things the certificate got wrong.
Why your EPC may under-record the property
An EPC is only as good as what the assessor could see and verify on the day. Where a feature cannot be inspected (loft insulation hidden under boarding, cavity fill with no surviving paperwork, retrofit measures installed by a previous owner), the RdSAP convention is to assume the worst case. That produces a very common pattern in long-held rental stock. Over the years a property has cavity fill added, the loft topped up, LED lighting fitted throughout, thermostatic radiator valves (TRVs) retrofitted to the heating system. None of it appears on the certificate, because the certificate was produced before the work, or by an assessor who defaulted to worst case in the absence of evidence, or simply has not been redone in a decade. A reassessment with proper inspection (loft hatch lifted and depth measured, cavity borescoped, drill patterns under the windows checked for signs of past cavity work, retrofit paperwork requested) frequently corrects several of these defaults in a single visit. The measured air test then adds a verified figure on top. That combination is the route. The reassessment recovers what the property has actually got; the air test adds a real measurement. For a property starting in the high D band, that is often enough.
Is this a legitimate way to improve an EPC rating?
It is a fair question, and worth answering directly. This is not a shortcut or a trick. It is the use of measured evidence exactly where the methodology now permits measured evidence to replace a default assumption. Two things keep it honest. It only works on a narrow archetype. The property has to genuinely be better than its certificate says. If the fabric really is uninsulated, or the heating system really is the problem, no amount of reassessment will invent performance that is not there. Screening, not optimism, is what makes the difference. It has to survive inspection. The corrections are backed by physical checks and documentation, not assertion. If the on-site evidence shows the original certificate was actually right, the property drops out of the candidate set. Where a property performs better than the old EPC suggests, the certificate should reflect that. Where it does not, or where the real issue is the heating, the hot water, or poor fabric, it should be sent down a genuine retrofit route instead.
Case studies: five rentals moved from EPC D to C
The five properties below were tested over a two-month window. All five reached C. The average starting score was 63 (D); the average finish was 70 (C); the average gain was 7 SAP points. They fall into two clean archetypes. Addresses and certificate references are omitted here; the point is the mechanism, not the specific homes.
Archetype 1: multiple under-recorded defaults
The strongest profile. The property has had insulation work over the years, but the certificate never picked it up. A single visit recovers several defaults at once. Example A: mid-terrace, about 81 square metres, mains gas. SAP 61 to 69. The loft had been assumed to have no insulation; inspection confirmed 200 mm. The cavity had been assumed empty; it was confirmed insulated. Air permeability moved from the default to a measured figure comfortably inside it. An 8 point gain from correcting the record alone. Example B: mid-terrace, about 75 square metres, mains gas. SAP 61 to 69. The original certificate was thirteen years old and had expired. In the intervening years the property had gained LED lighting throughout, TRVs, and additional loft insulation, none of it on the certificate, simply because it had never been redone. Walls were reclassified from assumed uninsulated solid brick to confirmed insulated cavity, loft from 200 mm to 225 mm, lighting from very poor to very good, controls from average to good, and the air test replaced the default. An 8 point gain. Example C: mid-terrace, about 73 square metres, mains gas. SAP 68 to 69. This one only needed a single point, and shows the value of the route right at the top of the D band. Correcting the cavity record plus the measured air test delivered exactly one point. Narrow, but enough to carry it over the C threshold. The lesson: even a one point lift can be worth paying for if it is the point that changes the band. A note on bills that seem to move the wrong way. On one of these properties the estimated annual bill appeared to rise between the old and new certificate. That is a change in the price base the methodology uses (an older price year versus a current one), not a drop in efficiency. Primary energy use per square metre actually fell. Always compare the efficiency metric, not the headline bill across different price years.
Archetype 2: a single under-recorded default doing the work
Narrower, but very clean. The certificate is broadly right on everything except one significant item, usually the cavity wall. Correcting that plus the air test does it. Example D: top-floor flat, about 78 square metres, mains gas. SAP 63 to 72. The cavity had been recorded as built with no insulation (assumed), and the original certificate had even recommended cavity fill, a strong sign the assessor never verified that it was already done. Confirming the insulation plus the air test produced a 9 point lift. This profile is common where a previous owner had cavity fill done under a grant scheme and the paperwork was lost at sale. Example E: mid-terrace, about 103 square metres, mains gas. SAP 61 to 71, the largest gain of the group. The original certificate carried three significant defaults (cavity, roof, heating controls), all correctable against confirmed evidence, with the air test on top. Notably, this property tested less airtight than the others, the equivalent of about 12.85 at 50 Pascals, still inside the typical assumed default but only rated average. It cleared C comfortably anyway, because the other three corrections carried it. The takeaway: a property does not have to test exceptionally tight to benefit. What matters is the combination of measured airtightness and the under-recorded fabric it is sitting on.
When this EPC route does not work
This is the part that keeps the whole thing credible, so it is included deliberately. One property in the same batch was declined before any test, and it is a useful illustration of the limits.
Declined: mid-terrace bungalow, about 40 square metres. SAP 46 (E). It failed the screen on three counts at once:
Starting score too low. At E46 it sat 23 points below C. The air test route, on its own, delivers roughly 1 to 3 points. Nothing about default corrections and airtightness closes a 23 point gap. Heating is the dominant penalty. The property ran on gas room heaters, with no central heating boiler and no thermostatic control. The certificate's own recommendations confirmed it: replacing the room heaters with a condensing boiler was, by itself, the single biggest available improvement. Hot water compounds it. Electric immersion on a standard tariff rates very poor and is a substantial penalty in its own right. The honest recommendation here was a genuine retrofit, realistically a grant-funded air source heat pump (via the Boiler Upgrade Scheme) to replace both the room heaters and the immersion, plus a cavity correction and a loft top-up. That combination would clear C comfortably and probably reach a low B. This is exactly the kind of property that should not be pushed through a certificate correction route, because the problem is not under-recorded evidence, it is a physical system that needs replacing.
Which properties qualify: the four-point screen
Deciding which properties to put forward comes down to four checks:
Starting SAP score. Focus on the high D band, roughly SAP 63 to 68. Above 68 the property is nearly C already and the spend may be unnecessary; below 63 the gap is usually too large for this route alone, unless the certificate carries several substantial under-recorded defaults. Indicators of under-recorded fabric. Review the existing certificate for assumed defaults, especially cavity wall, loft, and air permeability. The more assumed entries, the more recovery is potentially available. A certificate already full of measured values has little room left to correct. Physical inspection. Where the certificate suggests a default may be wrong, verify it on site: loft depth measured, cavity borescoped, drill patterns checked, retrofit paperwork requested. Inspection can just as easily remove a property from the set if the original assumptions turn out to be right. Heating and hot water sanity check. A modern condensing gas boiler with TRVs and a programmer is the baseline. Old non-condensing systems, electric heating without storage, room heaters, or solid fuel generally need a separate route first. Profiles that are typically declined include: mid-D properties (SAP 55 to 62) whose certificates already have all major fabric values measured; anything running on electric storage or room heaters as the main heat source; F and G rated stock, which is not lettable under current MEES rules regardless of methodology; and properties whose certificate is recent and already produced under RdSAP 10, where there is rarely additional gain to recover.
EPC reassessment cost vs retrofit cost
The financial case is simply the gap between correcting a certificate and rebuilding a property's fabric. For one of the properties above, the certificate's own recommended path to C ran to internal wall insulation, suspended floor insulation, solar water heating and rooftop solar PV, a total in the region of £12,000 to £27,000. The reassessment and test route reached the same C for £500.
| Route | Typical cost | When it is the right answer |
|---|---|---|
| Reassessment plus verified air test | £500 | High D properties with under-recorded insulation, heating controls or air permeability |
| Internal wall insulation | £4,000 to £14,000 | Solid wall properties where insulation is genuinely needed |
| Solar PV | £3,500 to £5,500 | Properties suited to PV where the capital spend is justified |
| Heating system upgrade | £5,000 to £12,000+ | Poor, old, electric or room heater based systems |
| Full retrofit package | £12,000 to £27,000+ | Lower rated homes that cannot reach C through reassessment |
The purpose is not to dodge necessary retrofit. It is to stop unnecessary retrofit spend where the current certificate is simply out of date or missing evidence, so the capital stays available for the properties that genuinely need it.
Frequently asked questions
Is RdSAP 10 mandatory for EPCs?
Will every D-rated property reach EPC C this way?
What happens after ten years?
Isn't this too good to be true?
The bottom line for landlords
A large slice of high D rental stock is being lined up for retrofit it does not need. Those homes have quietly improved over the years (cavity fill, loft top-ups, lighting, heating controls), but the paperwork never followed. RdSAP 10 now lets a certificate reflect what a property genuinely is, including a measured airtightness figure in place of a pessimistic default. Where that combined evidence clears the C threshold, a new ten year certificate can be lodged for a fraction of the cost of fabric works. Where it does not, the property is sent down the retrofit route it actually needs before a penny is committed. The approach is narrow, evidence-led, and honest about its own limits, and for the right property, it is a faster and dramatically cheaper way of getting the certificate to tell the truth.