Consider EPC Mistakes

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 9.94. This updates to RDSAP 10 on 15th June 2025 and a significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2026. In addition, government consulations indicate future changes to the format of EPCs and minimum rating levels required for priate rental properties. Methods for improving EPC ratings may therefore change in the future.

Most EPCs are completed accurately, but mistakes and missing information do occasionally occur.

In other cases, what appears to be a mistake can actually be normal RdSAP assumptions or a lack of supporting evidence.

This guide explains what to check, how to recognise common issues, and what to do if you think your EPC doesn't accurately reflect your property.

• Incorrect floor area
• Incorrect property construction dateband recorded
• Incorrect wall insulation status
• Loft insulation depth not recorded
• Room-in-roof recorded as a normal storey
• Mansard roof recorded as a normal storey
• Insulation not recorded

The internal floor area of the property is displayed towards the top of the Energy Performance Certificates.

The floor area is calculated by the assessment software summing up the floor areas of all the building partsand it should fairly accurately represent the actual floor area of the property.

The area indicated is the 'Gross Internal Area' (GIA) and includes the footprint of any internal walls, chimneys etc.

As an example, I came across one property where the previous EPC indicated a floor area of 180m2, but after measurement onsite I could only account for 120m2. Clearly the rating of the previous EPC would have been affected in some way in the case of this discrepancy.

The EPC rating is currenty a cost-based metric, referrenced to floor area, so this part of an assessment is important.

If you were interested in checking, you could measure the actual internal floor area of your property and compare this against the value indicated on your previous EPC.

Note that EPC assessments will not include the floor area of:

• Thermally separated conservatories
• Thermally separated porches
• Un-heated garages
• Basements, if they do not qualify for inclusion in the assessment
• Roof rooms, if they do not pass required criteria
• Annexes, if they would require their own separate EPC

Construction datebands are assigned to parts of the property during an EPC assessment, including:

• The main building
• Any real or 'notional' extensions
• Any 'Room In Roof' (where you have a room within a pitched roof)

The datebands are pre-defined, and vary slightly between nations due to changes in building regulations occuring at slightly different times.

For England & Wales, where I'm based, these are:

prior to 1900, 1900-1929, 1930-1949, 1950-1966, 1967-1975, 1976-1982, 1983-1990, 1991-1995, 1996-2002, 2003-2006, 2007-2011, 2012 onwards

Parts of the assessment calculation are usually based on assumed thermal performance values determined by the dateband chosen. This applies particularly to walls, floors and some roofs.

These performance values are tighter for newer datebands, and align with building regulations.

If the wrong date band is assigned, the system could produce an EPC rating for the property that is overly positive or negative as a result, depending whether the dateband is prior to, or after, the actual date of construction of your property.

The clearest indication of this issue is how cavity walls are decribed on the EPC features table.

We cover the textual descriptions on our Cavity Wall Insulation page but if retroffied cavity wall insulation is not evident then the following is displayed:

Wall - Cavity wall, as built, no insulation (assumed) - Prior to 1976

Wall - Cavity wall, as built, partial insulation (assumed) - 1976 to 1982

Wall - Cavity wall, as built, insulated (assumed) - 1983 onwards

If you review the Features table of your EPC, this distinction might give some insight to the dateband assigned to that building part.

Does the description of your cavity wall match your expectation, and more importantly does the corresponding date range indicated above match the construction date of your property?

You can reduce the chances of the wrong dateband being assigned on a new EPC by carrying out some research on your property and providing any evidence to your energy assessor for the age of the building, any extensions and the date of any room-in-roof conversion.

They will usually be happy to receive such evidence because dataing a property is sometimes tricky.

Evidence could include the Land Registry Title Register, copies of any Building Control completion letters for extensions or building works done etc. particularly for Room-In-Roof.

I assessed a property with a cavity wall which had been manually declared on a previous EPC by the energy assessor as having insulation present.

This was indicated in the description of the cavity wall by the word 'filled' and the absence of the word 'assumed' in brackets.

This indicates the assessor has observed a pattern of drill holes on the outside of the wall, or had access to documentary evidence that the wall had retrofitted cavity insulation.

In fact, the property was built in 1973 when cavity wall insulation was not fitted as standard, it was a second floor flat, and there was no visual or documentary evidence that any insulation was present.

On the new EPC, the insulation status of the wall was declared as 'As Built' and the EPC rating reduced as a result, somewhat reducing the effect of the energy improvements that the owner had made elsewhere in the property in the meantime.

The property owner was therefore disappointed.

Does the description of your wall match your understanding of its insulation status?

See our Cavity Wall Insulation page for more information on how the Features table indicates insulation status of cavity walls.

Take the example of this pair of semi-detached properties:

These properties were clearly built at the same time, and both have large dormer windows in the roof, with those first floor rooms entirely built within the sloping structure of the pitched roof.

These first floor storeys are a case of what is called 'Room in Roof' and are handled differently within the EPC assessment compared to a standard storey.

The ceilings in these roof rooms are likely to be sloping (vaulted), possibly with a small area of horizontal ceiling in the middle of the rooms, and possibly with or maybe without a hatch to access the area above that horizontal ceiling.

Normally, it's not possible to visually assess the level of insulation in the sloping parts of the ceiling. Also, if there is no hatch in the horizontal ceiling part, then it will not be possible to assess the depth of insulation in that either.

If there are vertical internal stud walls around the perimieter of the room in roof, and there is no access behind them, then it will not be possible to assess the insulation levels there either.

In the absence of such access, the assessment falls back on minimum building regs that were in place at the dateband that the assessor applies to the Room In Roof.

This dateband is normally the construction date of the rest of the property unless significant documentary evidence is available that proves the Room In Roof complies with more recent regulations (such as if it was contructed or converted since).

Anyhow, you would expect to see a reference to 'Room in Roof' in the Features table of the EPCs for both of these two properties.

Here is the Features List from the EPC for the property on the right:

We can correctly see 'Roof room(s)' mentioned in one of the roof entries.

In addition, reference to a timber frame wall is consistent with a Room In Roof where the floor area under the dormer windows is more than 20% of the floorspace of the whole floor.

For the property on the left however there is no corresponding mention of 'roof room(s)' in its EPC, and the Roof section indicates a standard pitched roof with no access available, because it refers to it as 'Insulated (assumed)'.

If so, it's quite possible that the EPC rating might be higher on that EPC than it would be had the EPC been assessed correctly because for the age of these houses, the heat loss through the room in roof is likely to be greater than it would be as a normal storey with cavity walls.

A Mansard Roof is where the pitched roof of a property is split into two sections of differing slope. Although the origins of the design date back centuries, a number of post Second World War properties in particular were built with them.

See our Mansard Roof page for more information.

The first floor of these were normally built with timber framed external 'walls' and this should be indicated in the EPC Features table.

Many of these properties were also 'System Build' properties, and the main ground floor walls should also be indicated as such in the EPC Features table, even if they have been subsequently 'reparied' by removal of the original concrete panels and replacement with a brick outer skin.

Many however are not quite reperesented correctly.

For a standard pitched roof with a loft, the depth of loft insulation is recorded, and is indicated in the Features table on the EPC.

Check the depth of insulation in your loft, and confirm it matches the depth of indicated on the EPC.

Remember that EPCs are based on an assessment carried out on the day, at a single moment in time, and the situation today may be different from when the EPC was conducted.

Bear in mind that there are some scenarios where it is not possible to record the depth of loft insulation, and in these cases the depth will be 'assumed' depending on the construction dateband of the property:

• There is no loft hatch
• There is a loft hatch but it is locked shut
• There is a loft hatch but it cannot be safely used (obstructions etc)
• The loft is accessible but the depth of insulation cannot be ascertained. A fully boarded loft is one example

If the insulation depth if recorded as 'Unknown', then the EPC features table will indicate an 'assumed' level of insulation, and any recommendation to increase the loft insulation depth will be suppressed.

Evidence could consist of:

• Descriptive comaparison with a neighbour (is the other EPC correct though?) 
• Your existing lodged EPC
• Additional assessment data downloaded from the government website
• A full assessment dataset requested from the energy assessor

It's possible to enquire with the energy assessor if you are concerned that an EPC might be incorrect. their details are listed on the EPC, and this is the first line of approach in case of enquiries.

If you are still not happy after contacting the assessor, you can contact the accreditaton scheme they are a member of (Elmhurt, ECMK, Quidos etc.). Their details are also recorded on the EPC.

Correcting an EPC mistake can sometimes improve a property's rating without carrying out any physical improvements—but only where the existing assessment genuinely doesn't reflect the property.

Click here to return to our 'Improve Your EPC' main page, and see if there is another way you could improve your EPC rating