EPC Assessor Default Assumptions

The Rule That Explains Almost Everything: No Evidence, No Credit

The governing principle across RdSAP, the methodology used to calculate EPCs, is that assessors must not guess. Where a feature of the property can't be confirmed - either by direct visual inspection or by documentary evidence - the software applies a standardised default tied to the property's age and construction type, rather than crediting the property for something that can't be proven. This is a deliberate safeguard, not an oversight: without it, there would be nothing stopping an owner from simply claiming improvements that were never actually carried out or verified - a landlord asserting "there's 100mm of PIR insulation in that flat roof" with nothing to back it up, for instance. Requiring evidence protects the integrity of the whole EPC system.

The trade-off is exactly what the title of this guide describes: genuine improvements without paperwork get treated, for scoring purposes, as if they were never done. That's not a flaw specific to your property - it's how the system is built to work everywhere. But it does mean the gap between "how the property actually performs" and "how the EPC says it performs" is very often a paperwork gap, not a performance gap.

Walls and Cavity Insulation

Where wall construction or cavity insulation can't be verified, RdSAP falls back on age-band-linked defaults rather than an assessment of the specific building. Historically, this has worked out roughly as follows (exact figures shift slightly between RdSAP versions, so treat these as illustrative of the principle rather than a guaranteed current number for your specific property):

• Solid brick walls have used fixed, age-related default U-values applied consistently across a run of age bands, rather than reflecting variation in individual buildings of that period.
• Stone walls are handled with a formula based on wall thickness and stone type (sandstone/limestone versus granite/whinstone), with an adjustment where internal dry-lining or lath-and-plaster finish is present.
• System-built properties (non-traditional construction types) have historically used a single catch-all default, regardless of how much the actual construction varies within that category.

Glazing: "No Certificate, No Credit"

This is one of the clearest and most commonly misunderstood examples. Without documentary evidence proving when double glazing was installed, assessors are required to assume the least efficient reasonable option for that window category - not because the windows look old, but because they can't be proven to be anything better. Roughly:

• Unknown installation date → typically rated "poor," regardless of appearance.
• Installed before roughly 2002 → often rated "poor" by default.
• Installed roughly 2002–2022 → needs evidence to avoid a conservative default; with proper evidence, often rated "average" to "good."
• Installed after roughly 2022 → typically rated "good."

Critically, "looks new" or general visual condition does not count as evidence for this purpose - an assessor isn't permitted to use their own judgement to override the documentation requirement. What does count: a FENSA certificate, Building Control sign-off, an installer's invoice or paperwork, or manufacturer documentation confirming the specification and installation date.

Boilers and Heating Systems

The same logic applies to heating. If a boiler's make and model can't be identified - a worn or missing data plate, no accessible documentation - the assessor has to apply a cautious default rather than the actual efficiency rating of that specific unit, even if it's a genuinely efficient, modern boiler. A current Gas Safety Certificate (which typically identifies the boiler model), the original installation certificate, or manufacturer documentation are the kinds of evidence that let an assessor record the real performance rather than a fallback assumption.

Renewables: Solar Panels and Heat Pumps

Installed renewable technology only gets credited where it's properly documented. A solar PV array or heat pump without an MCS certificate or identifiable model information may not receive full - or in some cases any - credit for its actual output, even though it's physically present and working. This is particularly worth checking for landlords who bought a property with renewables already installed by a previous owner, where the original paperwork may not have been passed on.

Construction Details and Measurements

Beyond insulation and equipment, basic data about the building itself - floor area, room count, construction type - feeds directly into the calculation. Extensions and loft conversions are a common source of discrepancy here: if an assessor works from outdated or incomplete information about the property's actual layout, the resulting figures can be wrong in ways that have nothing to do with insulation defaults at all, and everything to do with the underlying data being incorrect.

What Actually Counts as Evidence

Across all of the categories above, the kinds of documentation that let an assessor move away from a default and record your property's real performance generally include:

Two conditions generally apply across all of these: the documentation has to clearly relate to the actual property being assessed, and it has to demonstrate the work was actually carried out - evidence of an intention to install something, on its own, isn't sufficient. Where there's genuine evidence that work was both intended and either overseen by Building Control or visually confirmed as completed, that combination can be enough even without a formal certificate.

A Practical Checklist Before Your Next EPC Assessment

Gathering the following before an assessor visits - rather than trying to track it down afterwards - can be the difference between a default-driven score and one that reflects your property's real performance:

• FENSA certificates (or equivalent) for any replacement windows
• Building Control completion certificates for extensions, loft conversions, or insulation work
• Cavity wall insulation guarantee documentation, if applicable
• Current Gas Safety Certificate and any boiler installation paperwork
• MCS certificates for solar panels, solar thermal, or heat pump systems
• Any U-value calculations or specialist reports commissioned for the property

Why This Matters Even More Under the 2030 Standard

Under the confirmed 2030 standard, fabric performance is the primary metric a property must meet before the secondary standard even comes into play. That makes this evidence gap directly relevant to cost, not just accuracy: an assessor working from conservative defaults might recommend (and count toward your £10,000 cost cap) fabric measures that, in reality, your property may already have - just without the paperwork to prove it. A documentation review before committing spend is, in effect, free due diligence: it costs nothing but time, and it can only move your score in your favour or leave it unchanged, since a genuine default only exists where evidence is missing.

It also connects directly to grandfather rights: if better-evidenced defaults are what push a property to an EER of C on today's EPC before 1 October 2029, that's compliance secured - for up to 10 years - without installing a single additional measure.

Key Takeaways

• Assessors don't guess - where something can't be verified, the software applies a conservative, age-based default rather than assuming good performance.
• This exists to prevent unverified or false claims of improvement, protecting the integrity of every EPC, but it means genuine, undocumented improvements score as if they don't exist.
• Glazing is the clearest example: no evidence of installation date means an automatic "poor" rating, regardless of how the windows actually look or perform.
• Even proven cavity wall insulation can be capped at a standard-material credit, rather than reflecting genuinely superior insulation.
• The evidence that actually counts is specific: certificates, Building Control sign-off, FENSA documentation, Gas Safety Certificates, MCS certificates - not visual condition or stated intent.
• Reviewing and gathering this paperwork before an assessment is free, low-effort, and can only improve or maintain your score - never worsen it.

Frequently Asked Questions