Documentary Evidence

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 10. A significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2026. In addition, government consultations indicate future changes to the format of EPCs and minimum rating levels required for private rental properties. Methods for improving EPC ratings will therefore change in the future.

As your chosen assessor makes their way around your existing dwelling, you'll notice they gather lots of information about your property and record this in their assessment software, to generate the EPC.

There are many rules, called conventions, which they must follow and the methodology operates in the way it has been designed, using the information entered, to calculate the final rating, and produce the EPC.

Despite un-educated opinions expressed on social media, this is far more than just ticking boxes, and the rating is actually a very complex 'guess' indeed.

I understand for example there are two pages of code just to work out the contribution of lighting to the final rating.

Anyhow, for some elements of the assessment it may be necessary for additional information to be provided to the assessor to fully recognise certain features of the property.

This is referred to as 'documentary evidence', and this will be required for example when elements might not be easily accessible or recognisable.

We'll look at some of these situations here and what may be required.

If you are able to gather this type of information in advance, and provide this information to your assessor they will be grateful, and it can help to get a more accurate rating.

One of the classic misunderstandings expressed in social media posts is that EPCs are full of assumptions.

Well, there is in fact only really one big assumption, and it's that an existing property was built to the minimum thermal requirements required by building regulations in force at the time.

This would be from 1966 onwards for example in England & Wales, and prior to this the assumption is that property features were built using the most prevalent methods used at the time.

Considerable research into this has been conducted by the Building Research Establichment (BRE).

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EPC assessments of 'existing dwellings' are non-invasive as standard, which means the assessor is not permitted to drill holes in walls or ceilings to confirm what materials are present.

This type of work would not be covered by the standard insurance cover provided by their accreditation scheme.

If they did, and they were qualified to interpret what they saw, in most cases they would probably only be able to confirm the point mentioned above, that the building (particularly the walls, roof and floor) either met the minimum thermal performance required by building regulations at the time (1966 onwards) or was built in line with common practices at the time.

Buildings however are updated by their owners from time to time and the assessment methodology provides options for including additional insulation that has been retrofitted, ie fitted since the building was originally constructed.

In the case of floor insulation, flat roof/sloping ceiling insulation, room-in-roof insulation and internal wall insulation for example, it is easy to realise that an energy assessor would not ordinarily be able to see that such insulation had been retrofitted.

When such insulation has been retrofitted, 'documentary evidence' is required in order for it to be included in the assessment.

Many people ask why, and it is because the chances are high that some property owners would otherwise say 'Oh yes we put 100mm PIR insulation in that flat roof last year' when in reality they didn't.

If the assessor was bound to accept whatever the property owner told them, this insulation would then be included in the assessment, and a rating produced that was too generous for the property.

To all those people who express ignorant disparagement that EPC methodology is a 'scam', well you'd all be bitterly complaining if assessors had to simply accept whatever a property owner told them. EPC ratings would be entirely unreliable in that scenario.

So, essentially it's a bit like why we get the pub quiz answer sheet marked by one of the other teams at the end of the night.

It's to provide a level of transparency and certainty.

RDSAP Convention 9.02 gives guidance regarding Documentary evidence.

For England, Wales & Northern Ireland:
 

Acceptable documentary evidence includes, but is not limited to, official correspondence from the applicable Registered Social Landlord (RSL) or certificates, warranties or guarantees or any documents verifying that work has been carried out. The assessor must be confident, and able to demonstrate, that any documentation relates to the actual property being assessed, and/or the work has been carried out, and that there is no physical or other documentary evidence to the contrary.

Evidence of intent to install does not on its own qualify as acceptable documentary evidence.

Evidence of intent to install, supported by evidence of subsequent Building Control oversight, or visual 
evidence that such an upgrade has been undertaken does qualify as acceptable documentary evidence.

a) If it can be demonstrated that there was both an intention to upgrade the element (in planning documents for example) and Building Control oversight of the work, the element can be treated as having been upgraded as indicated in the planning documents.

b) If it can only be demonstrated that either there was an intention to upgrade the element (in planning documents for example) or that there was Building Control oversight but it can also be  seen that an upgrade has occurred, the element can be treated as upgraded using the minimum 
selectable upgrade of the relevant type defined in RdSAP. (If upgraded from ‘as-built’ to ‘insulated’ using the lowest selectable insulation thickness* that is better than the as-built assumption).

*For roof insulation between joists use 100 mm as the lowest selectable insulation thickness

For Scotland:

Acceptable documentary evidence includes, but is not limited to, official correspondence from the applicable Registered Social Landlord (RSL) or certificates, warranties, guarantees. The assessor must be confident, and able to demonstrate, that any documentation relates to the actual property being assessed and that there is no physical evidence to the contrary.

Evidence of intent to install does not qualify as acceptable documentary evidence

EPCs are required whenever a property is sold, rented or initially constructed.

In the case of a new build property, a full SAP EPC is conducted.

If the developer had the property built with significantly better energy performance than that required by building regulations in place at the time, then that advantage will be lost after 10 years when a new EPC assessment is carried out using RDSAP rather than SAP.

This is because RDSAP assumes the minimum thermal performance used by building regs at the time.

If you can get hold of that original engineering information that would have been supplied to the On-Construction energy assessor for the original SAP EPC then your RDSP energy assessor will be able to use that information to override the U values in the RDSAP assessment 10 years afterwards and improve your EPC rating.

The likelihood of you getting hold of that engineering information is slim, and I have not come across a client yet who has had it available. That however does not stop you from being the first one.

We mentioned previously how the RDSAP methodology by default makes only 'one big assumption' around the thermal performance of certain elements of building fabric.

In some cases however, a property may be built with thermal performance better than required by building regulations at the time.

This is usually by intention with Passivhaus buildings a classic example.

In these cases, an owner should be interested in getting an assessment conducted using accurate thermal performance information.

This would result in a more 'accurate' assessment and better EPC rating, thus providing justice to the building.

By convention, building fabric should be recorded 'As Built' if it is still in the same condition as originally constructed, but the methodology provides a mechanism to manually override the u-values it would otherwise use with ones entered by the assessor.

This of course requires qualified documentary evidence, and this is described in Convention 3.08 as follows:

U-value entry (walls, roofs, floors)

The U-values of existing elements (walls/roofs/floors, etc.) must be the RdSAP default values (e.g. entered “as built”) and must not be overwritten unless specific documentary evidence of the thermal conductivity of individual materials of the building element of the property being assessed is provided and was undertaken in accordance with BR 443 “Conventions for U-value calculations” (BRE, 2006).

The U-value is that of the whole element, including any added insulation.

Documentary evidence applicable to the property being assessed (see convention 9.02) must be provided and recorded if overwriting any default U-value. This evidence shall be either:

- relevant building control approval, which both correctly defines the construction in question and states the calculated U-value; or
- a U-value calculation produced or verified by a person with suitable expertise and experience.

Evidence of suitable expertise and experience can be demonstrated by, but is not limited to, membership of a recognised U-value calculation competency scheme or OCDEA1 or Level 4 non-domestic energy assessor membership, or any other process recognised by Accreditation Schemes/Approved Organisations and Government.

Where it is known that only part of an element has been insulated use the alternative wall if possible 
for the insulated part, or use extensions

In addition, the overriding of any u-value, apart from windows and doors, results in a Smart Audit trigger, meaning the resulting EPC is highly likely to be audited by the assessor's accreditation scheme.

An energy assessor therefore will want to ensure any evidence meets the qualifying requirements before they lodge the final EPC.

Similar to themes above, EPC assessment methodology has a mechanism for obtaining performance information of systems such as boilers, heat pumps etc.

There is a Product Characteristics Database (PCDB).

Manufacturers pay to have their equipment tested and included in this database.

Despite occasional posts on social media suggesting EPCs should use more technical onsite testing of installed equipment like boilers, the methodology assumes this type of equipment is functional and working to the performance as verified by the original testing.

There's therefore no need to take a boiler off your wall and have it tested by an independent third party test lab to measure its performance criteria, 

There are a couple of situations to be aware of however.

Occasionally it's not possible to positively identify the model of boiler in a property.

There are some models where there are no external labels or marks identifying the model.

in these cases an assessor would need some sort of dumentary evidence of the boiler and this could include the latest gas safety certificate.

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