Why is my EPC rating so low?
Why is My EPC Rating So Low? Common EPC Issues Explained by a Certified Assessor
By Darren - Agents Aperture - 1st November 2025
Below are some of the more common reason why Domestic Energy Certificates are often lower than expected.
1) When comparing a previous EPC to a newly lodged one, confusion often arises. Even if nothing in the property has changed, factors such as the age of the boiler are reassessed — so a system that was new ten years ago is now considered older and less efficient, which can lower the overall rating.
2) Following on from this, not only has the boiler aged by another ten years if the previous EPC has expired, but the assessment criteria may also have changed — meaning it could now be more difficult to achieve the same high rating as before.
3) In June 2025, the EPC data-gathering requirements were expanded to include additional details that were not previously assessed — such as measuring all windows individually and accounting for extractor fans. These changes mean that even without any physical alterations to the property, it may now be harder to achieve the same high rating as before.
4) The previous EPC may also have been entered incorrectly — and unfortunately, this happens far more often than you might think. Not all assessors have the same level of experience or attention to detail, and fully understanding the criteria needed for accurate lodgement takes years of practice and knowledge. Many training providers focus more on their bottom line than on producing well-trained energy assessors, which has led to a growing number of qualified but under-skilled assessors lodging inaccurate EPCs.
5) No access! This is one of the most common reasons for lower EPC ratings. EPCs are based on visual inspections, which means the assessor must be able to see — or have legal documentation to confirm — insulation levels and other key details. There are strict rules around this. For example, if more than 25% of the loft insulation is not visible, the assessor is legally required to enter the default assumption based on the building regulations in place when the property was originally constructed.
If the property is Victorian, there were no central government building regulations at the time, and therefore no legal requirement for insulation. In this situation, the software will assume there is no insulation present in the loft space.
This can easily lead to disputes, especially when homeowners have added insulation themselves and then boarded over it. If more than a quarter of the loft is covered or cluttered — often with storage boxes or Christmas decorations — the assessor cannot record what they cannot see, and the EPC may state “no insulation (assumed).” Naturally, this can be frustrating for homeowners who know the insulation is there but hidden from view.
Top Tip: Always have any insulation certificates — such as those for cavity wall or loft insulation — ready to show your assessor during the visit. Providing documented evidence allows them to record the correct insulation levels, which can make a significant difference to your EPC rating.
Common Reasons Why EPCs Are Sometimes Considered “Wrong”
Q. The windows are double glazed, but the EPC says their rating is poor. Why?
A. In 2025 the EPC methodology was significantly updated. Previously, most double-glazed windows were given a reasonable rating by default. Under the newer standards, only high-performance double glazing or triple glazing typically scores well.
If the assessor cannot verify the installation date (for example via a FENSA certificate or manufacturer documentation), the software will default to an assumed performance level — which may result in a “poor” rating even if the windows are double glazed.
This isn’t the assessor’s opinion — it’s how the EPC algorithm works based on available evidence.
Q. There is a lot of insulation in the loft, but the EPC doesn’t show this. Why?
A. EPC rules state that the assessor must visually confirm at least 75% of the loft insulation at joist level.
If the loft is boarded, covered with stored items, or otherwise inaccessible, we must rely on software assumptions based on the property’s age and building regulations at that time.
For example, a 1930s property with a boarded loft and no documentation will be assumed to have no insulation — even if the homeowner is certain insulation is present.
This is not within the assessor’s control. To ensure an accurate rating, please make insulation visible or provide formal documentary evidence (such as installation invoices or guarantees) during the visit.
Q. The EPC says not all lighting is low energy, but every bulb in the house is energy-efficient. Why?
A. The EPC only considers fixed lighting, not plug-in or portable lamps.
Halogen spotlights are not classified as low-energy lighting, even though many people mistakenly assume they are. We check every fixed light fitting during the assessment. If any halogen fittings remain, the system will not record the property as having 100% low-energy lighting.
Other Points to Consider Before Making a Complaint
Visual Survey Only
An EPC is based on a visual inspection only. This means that insulation or energy-efficient features may exist but cannot be recorded unless seen or proven. If we are unable to verify insulation, the EPC software will apply a default assumption based on the building regulations at the time the property was built.
For example, Victorian homes had no requirement for floor insulation, so unless floorboards are lifted or formal evidence is provided, the system will assume no floor insulation. The same principle applies to cavity walls, lofts, and roofs — if we cannot see it or you cannot provide official documentation, we must record the assumption set by the government software.
Assessors Do Not Set the Rules
EPC assessors do not write the standards — the rules are set by the government. EPCs are legal documents, and assessors must follow strict guidance and audit processes.
If an assessor records information without proof, they risk being struck off and permanently banned from producing EPCs. For this reason, we must adhere to the evidence-based rules, even if a homeowner believes otherwise.
The Rating Is Not the Assessor’s Opinion
An assessor's responsibility is to produce an accurate, compliant EPC, not to influence the rating. The rating is a by-product of the evidence gathered and the software’s calculations.
If you are unhappy with the rating, you can use the recommendations section to plan improvements — but the assessor must be unbiased and cannot adjust the score without evidence.
EPCs Can Be Incorrect - But That Doesn’t Mean the Assessor Is Wrong
In some situations, EPCs may appear to be “wrong” simply because hidden upgrades cannot be confirmed. Unless insulation or other improvements are visible or backed by formal documentation, it cannot legally be recorded.
This can lead to discrepancies between the EPC and what a homeowner believes is present — however, this is a limitation of the EPC rules, not assessor error.
How involved is an EPC?
Very! Accurately lodging Domestic Energy Certificates requires far more than just ticking boxes. Assessors must complete a formal City & Guilds qualification and undertake extensive study to fully understand how buildings work.
Domestic Energy Assessors (DEAs) need a strong grasp of construction methods, heating systems, property age and type, and health and safety procedures. While some training courses are better than others, even the best programmes can’t teach experience. It takes time — often years — for an assessor to build the knowledge and confidence needed to accurately and consistently lodge EPCs across all property types, from converted barns to period homes.
Are Listed Properties Exempt?
Not all properties are legally required to have an EPC. Some listed buildings are effectively exempt, as they would be at a significant disadvantage under standard assessment criteria. The reason is that an EPC might recommend improvements — such as adding insulation — that could harm the building’s architectural or historical integrity.
In general, if it’s determined that meeting EPC requirements would “unacceptably alter” the building’s character or appearance, then the property is considered exempt. However, there are some exceptions, so it’s always worth seeking professional advice before assuming a listed property doesn’t require an EPC.
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