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.

Some EPCs are unfortunately issued containing mistakes and it's possible for the EPC rating to be skewed one way or another as a result.

Ideally, assessment methodology would be applied equally to a single building by any energy assessor, and result in a pretty consistent rating, and to support this ideal, the occasional auditing of EPCs was introduced in 2018 by the Government approved accreditation schemes.

Mistakes however can happen even at the best of times. Many can't be spotted from just looking at the EPC certificate, but this page contains some examples that you might be able to spot by checking the Features list on an existing EPC.

If you suspect a mistake due to a difference in the rating between a previous EPC and a new one, then there may be other reasons contributing too. Some methodology and Conventions updates can result in  changes to EPC rating, and it's important to bear this in mind too.

Each of the following parts of a property need to have a construction dateband assigned to them when an EPC assessment is conducted:

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

A number of pre-defined datebands are available for an energy assessor to choose from, and these vary slightly between nations in the UK due to building regulations updates occuring at slightly different times.

For England & Wales, which is where I'm based, these datebands are currently:

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 based on assumed minimum thermal performance values determined by the dateband chosen. These performance values became ever more tighter (better) over the years as formal building regulations were introduced in 1965 (E&W) and made tighter in the years thereafter.

If the wrong date band is assigned during the assessment, the system may 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.

This can apply to several parts of the property, but the most common way of noticing this is with how cavity walls are decribed on the EPC features  table.

Up until 1976, houses were almost entirely built without cavity wall insulation as they did not need it in order to meet building regulations. For date ranges prior to 1976, in the absence of any visual indication of retrofitted cavity wall insulation, an energy assessor will record the wall as 'As built', and on the EPC Features table it will be described as:

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

From 1976 to 1982, with slightly tighter requirements for thermal performance, it was more usual for builders to partially insulate cavities. For this date range, in the absence of any visual indication of retrofitted cavity wall insulation, an energy assessor will record the wall as 'As built', and on the EPC Features table it will be described as:

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

Then from 1983 onwards it was much more normal for cavities to be insulated at the construction stage due to further tightening of the thermal performance requirements. For this date range, in the absence of any visual indication of retrofitted cavity wall insulation, an energy assessor will record the wall as 'As built', and on the EPC Features table it will be described as:

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

If you review the Features table of your EPC, this distinction between the different descriptions of a cavity wall might give some insight to the dateband assigned to that building part.

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

In the case where a newer dateband has been assigned, and an elevated EPC rating results, it can lead to disappointment when a new EPC is produced 10 years later with the correct dateband applied.

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.

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

You could for example include a copy of the Land Registry Title Register for the property (this often, but not always, contains information that can date the age of construction) - if you dont have a copy of this from your conveyancer when you purchased the property, you could download a copy from the Land Registry website (now costing £7).

Other evidence could include copies of any Building Control completion letters for extensions of building works done etc. This would be particularly important for Rooms 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.

There is a position towards the top of Energy Performance Certificates where the internal floor area of the property is indicated.

The floor area is calculated by the assessment software summing up the floor areas of all the separate building parts. The value should reflect fairly accurately the real floor area of the property.

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.

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 EPC

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.

As a quick guide, RDSAP 9.94 Convention 2.06 describes the inclusion of 'Roof Rooms' as:

• Include when accessed via a permanent fixed staircase such that one is able to walk downwards facing forwards.
• Does not necessarily contain habitable rooms.
• For a roof room to be classed as such and not a separate storey, the height of the common wall must be less than 1.8 m for at least 50% of the common wall (excluding gable ends or party walls). The common wall is a vertical continuation of the external wall of the storey below.

There are further instructions for when the area of dormer windows account for more than 20% of the floor area, and also for overiding the default insulation elements, but the common wall measurement mentioned above is the best known indicator.

In our photo above, the pitched roof slopes all the way down to the floor level of those roof rooms, leaving a common wall height of zero. Therefore, even without considering the rear elevation and looking only at the front elevation as shown in the photo, we have already reached the threshold of the 50% rule for Room In Roof.

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 has been refurbished to newer building regs).

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.

Let's compare them and see what we find:

This is the Features List from the EPC for the property on the left:

There is no mention of 'Room In Roof', and the Roof section indicates a standard pitched roof, likely with no access available, because it says 'Insulated (assumed)'.

In comparion, this is the Features List from the EPC for the property on the right:

This looks much more like what you'd expect. There is reference to 'Room in Room' in one of the Roof entries, and the mention of 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.

In summary, it would concern me that the Room In Roof on the left hand property may well have been included as a standard storey in it's EPC. 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.

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.

It's possible to enquire with the energy assessor if you are concerned that an EPC might be incorrect. If you are still not happy after contacting them, you can contact the accreditaton scheme they are a member of (Elmhurt, ECMK, Quidos etc.).

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