You had your loft insulated in 2015, replaced your windows with double glazing and installed a condensing boiler. Yet on the day of the visit, the assessor finds no invoice: neither yours, nor the one filed at the company. Without proof, they are not allowed to take these works into account. The calculation method then forces them to use "default" values, pegged to the building's construction period — meaning your wall is treated as if it were not insulated, and your window as single glazing.
This is not a question of bad faith: it is a rule. Since the 2021 reform, the DPE (Diagnostic de Performance Énergétique — the French Energy Performance Certificate, or EPC) is no longer calculated from your energy bills but from the dwelling's technical characteristics, and each of those characteristics must be justified. A well-prepared file does not "inflate" your rating: it stops it being unfairly downgraded. Here, item by item, is what to gather before the visit — and why each document counts.
A wall whose insulation is not proven is counted as uninsulated in any building from before 1975, and an unjustified window is reduced to single glazing (Ug coefficient of 5.8 W/(m².K)). Two of the most penalising assumptions in the 3CL method — and they only unlock with proof.
What this article covers
Why the DPE since July 2021 rests on data that must each be justified, how the "default values" mechanism that penalises the absence of proof works, and above all the full checklist of documents to gather before the assessor's visit: envelope and insulation, heating and hot-water systems, ventilation, administrative documents, the dwelling information record, and the condominium-specific items held by the managing agent. Plus the mistakes that downgrade a rating and the economic stakes of an accurate DPE.
Why your supporting documents determine your DPE rating
To understand why preparation matters, you have to go back to what the DPE became after the reform that came into force on 1 July 2021. Before that date, a DPE could be established "on bills", from actual energy consumption. That method was abolished for dwellings: the certificate is now calculated by a single conventional method, 3CL-DPE 2021, which models the building's performance from its physical characteristics (areas, insulation, glazing, systems), independently of occupant behaviour. This method is set by the order of 31 March 2021 on the methods and procedures applicable to the DPE.
A second major change: since 1 July 2021, the DPE is binding (opposable). It is no longer merely informational — its content engages the assessor's liability, and a buyer or tenant can seek redress if it is wrong. This binding nature has a direct consequence for preparation: for a piece of data to be entered, it must be justifiable. The assessor's on-site visit is mandatory; a DPE established without a real visit is exposed to rejection.
It is annex 2 of the order of 31 March 2021 ("how data are obtained") that sets the rules of the game. A characteristic of the dwelling can be obtained in four ways: by the assessor's direct measurement or observation, on the basis of a supporting document provided by the owner, from public data online, or by indirect observation (probing, thickness measurement). The text is unambiguous about verbal declarations:
⚠️ Warning: "No input data may be obtained on the mere declaration of the property owner, if it is not confirmed by one of the listed supporting documents" (annex 2, order of 31 March 2021). The Ministry's DPE Guide confirms it: it is forbidden to use the owner's information without proof, "even if a declaration is signed".
In other words: the assessor cannot take your word for it. Without a document, they have no choice — they apply the conventional value set by the method. And these default values are, in the very words of the Ministry's official notice, "by definition penalising". One notable exception: the building's year of construction may be estimated by the assessor in the absence of proof; every other piece of data must be justified.
The default-values mechanism: why the absence of proof costs classes
The heart of the matter lies in a hierarchy of data access, set out by the official DPE Guide: priority 1, visual inspection; priority 2, a supporting document attributable to the property; priority 3, the default value. The default value is therefore not a choice: it is the last resort when neither the assessor's eye nor your paperwork can establish a characteristic. And because these values are pegged to the least favourable assumption for the construction period, the absence of proof mechanically translates into overestimated consumption and a downgraded rating.
Without proof, the default value applies
The general rule is stated as follows in the method: "If the value of an input cannot be obtained by measurement, direct or indirect observation, on the basis of a supporting document or from public data online, the default value proposed in the conventional method is used." For insulation, this default depends on the year of construction: "deperditive walls whose insulation status is not known are considered uninsulated for buildings from before 1975". Beyond that date, the wall is assumed to be insulated to the regulatory level of its era — often far from the reality of an unproven recent renovation.
Profile: a 1960s house renovated in 2015 (internal wall insulation, double glazing, condensing boiler) but whose owner has lost every invoice. Without proof, the assessor calculates it as a 1960 house never renovated: uninsulated walls, single glazing, standard boiler. The gap with reality can amount to several classes.
The characteristics where proof changes the result
Every part of the envelope and of the systems has its default value. The table below sums up the main penalising assumptions that your supporting documents let you replace with reality. The figures shown come from the official method; the size of the final gain depends on the property and remains at the assessor's discretion.
| Characteristic | Default value (penalising) | What proof allows |
|---|---|---|
| Wall insulation | Before 1975: wall "uninsulated" | Enter the real U-value (insulant R-value and thickness) instead of a wall assumed bare |
| Roof / loft, floors | Same "unknown = uninsulated" rule | Value the loft insulation (≈ 25 to 30% of heat loss) and the floors |
| Glazing / windows | Single glazing, Ug = 5.8 W/(m².K) | Enter the real double or triple glazing (date, Uw), far more efficient |
| Heating | Standard boiler of the construction year; pilot light for pre-2003 buildings | Enter the real efficiency: condensing boiler, heat pump (the model's COP/ETAS) |
| Domestic hot water (DHW) | Tank of standard capacity, unfavourable energy if unknown | Enter the real type (thermodynamic tank, solar water heater…) |
| Ventilation | Natural ventilation / lack of airtightness (most penalising case) | Enter the real mechanical ventilation (single-flow, humidity-controlled, double-flow) |
The logic is always the same: in the absence of proof, the method takes the low assumption. This is exactly what the Ministry's notice criticised when it justified the 2021 easing (order of 8 October 2021) by "a heavy use of default values, by definition penalising in the performance assessment". Conversely, proof of wall or window insulation can also unlock a better air-permeability assumption on older buildings — an indirect gain that is often overlooked.
Note: your energy bills (electricity, gas, oil, wood) no longer serve to calculate the rating — the DPE has been conventional since 2021. They remain useful to the assessor to check the result's consistency and identify equipment and energy sources. Do not confuse them with technical supporting documents, which are decisive.
The reference area (SREF), not the Carrez law
One last technical point matters for preparation. Since the order of 25 March 2024 (in force on 1 July 2024), the DPE no longer relies on living area alone but on a reference area (SREF — surface de référence) — living area plus certain heated rooms and conservatories of sufficient ceiling height. This area directly drives the kWh/m² ratios and therefore the rating. It is not the "Carrez law" area used for selling a condominium lot, which follows different rules. Having the existing living-area certificate (loi Boutin) and up-to-date plans helps the assessor make this measurement reliable.
The checklist of documents to gather before the visit
The Ministry actually provides an official document, "Préparer mon DPE" (Prepare my DPE), whose message is blunt: "Before your assessor's visit, you must provide them with as many documents as possible. This will let you get better results on your DPE." Here, organised by item, is the — non-exhaustive — list of papers to gather and hand over, ideally ahead of the visit so the assessor has time to use them.
Envelope and insulation
To gather for the envelope: invoices and certificates of insulation works on the walls (external ETICS / internal ITI), the roof / loft and the floors, stating where possible the thermal resistance R and the insulant thickness; the contractor's RGE certification (recognised environmental guarantor); invoices for windows / glazing (glazing type, installation date) and for the front door; proof of a façade renovation with external insulation; site photos; a wall-probing report by a professional.
These are the most profitable supporting documents: they are the ones that unlock the real insulation coefficients in place of the "uninsulated" assumptions. A detailed quote stating the R-value and thickness can serve as a complement when the invoice is vague — but it must come with proof that the works were carried out. A materials-purchase invoice alone, without proof of installation, is not enough.
Systems: heating, hot water, ventilation
To gather for the systems: invoices and manuals for the boiler (make, model, year — to distinguish a condensing boiler); the heat pump's technical sheet (make/model, to retrieve the COP/ETAS) and its maintenance certificate; the invoice for the stove or insert (wood, pellets); the invoice for the hot-water tank (stating whether it is thermodynamic or solar); the mechanical ventilation documents (single-flow, humidity-controlled, double-flow); the air-conditioning invoice; annual maintenance contracts; and, for consistency, your energy bills for the last three years.
The appliance's rating plate is an accepted supporting document, as is the technical manual of the installed model — including the one the manufacturer makes publicly available. Where the model is not specified, the assessor will take the least favourable case. For heating, the gap is considerable between a proven condensing boiler and the standard boiler assumed by default.
Building and administrative
To gather on the administrative side: the building permit or proof of the construction year (notarial deed, property-tax notice); for a recent dwelling, the RT2012 / RE2020 compliance certificate and the standardised thermal-study summary; technical description, plans and sections; the living-area certificate (loi Boutin); title deed; the previous DPE or energy audit; where relevant, the dwelling information record.
The thermal study of a new dwelling is valuable: the method allows its data to be reused rather than applying penalising default values to an otherwise high-performing building. As for the dwelling information record (CIL — carnet d'information du logement), created by the Climate and Resilience Act (articles L.126-35-2 and following of the French Construction and Housing Code), it has been mandatory since 1 January 2023 for new dwellings and for renovations affecting energy performance. By legal design, it gathers exactly the plans, equipment manuals, material characteristics (including insulant R-value and thickness) and documents attesting energy performance: it is the safe where most of your DPE supporting documents are already stored. If it exists, start there.
In a condominium: the documents held by the managing agent
If your home is in a condominium, some of the data — notably collective heating and hot water, or façade insulation — does not depend on you but on the building. This information is held by the managing agent (syndic), and its absence directly penalises your DPE. The Ministry says so plainly: where there is collective heating or hot water, "if no information is provided, a default calculation will be made, which will penalise the DPE result".
To request from the managing agent: the building's collective DPE; the condominium's maintenance log (works history, maintenance contracts for collective heating and ventilation); the condominium's summary sheet; the condominium regulations (description of the collective heating); the general-meeting minutes mentioning works (façade insulation, renovation, replacement of the collective boiler); the collective heating operating contract.
The collective DPE is being rolled out: mandatory since 2024 for condominiums of more than 200 lots, since 2025 for those of 50 to 200 lots, and since 1 January 2026 for condominiums of fewer than 50 lots (Climate and Resilience Act). Where it exists, it feeds part of your individual DPE's data, without exempting you from it: your lot's DPE remains mandatory to sell or let, because it includes elements specific to your home (floor level, orientation, exposed surfaces, replaced windows). To better understand how the two relate, see our comparison of collective and individual EPC in a condominium.
Case study: what a box of invoices can be worth
Take a 110 m² house from 1968 in a suburb, valued at €320,000 in a market where a house rated D trades around that price. The owner did insulate the walls and fit double glazing, but cannot find any proof. Without it, the assessor applies the default values: uninsulated walls (pre-1975 building), single glazing. The house comes out in class G.
Scenario — the gap between the default rating and the justified rating
According to the Notaires de France green-value study (2024 transactions), a house rated G sells for around 25% less than an equivalent rated D. On a €320,000 property, the value gap between the two extremes of this scenario therefore runs into tens of thousands of euros — not to mention the letting ban that already hits homes rated G. A reclassification is never guaranteed: it depends on the actual building and remains at the assessor's discretion. But finding the insulation and window invoices, or asking the company and the administration for them again, radically changes the calculation's starting point. It is one of the best-paid trade-offs in the whole preparation of a sale. To quantify the impact of a class on the price, our DPE discount calculator draws on the same Notaires study.
The mistakes that penalise your DPE
Mistake no. 1 — Relying on a verbal declaration
"I insulated it in 2008" has no value in the calculation. The assessor cannot take a piece of data on your word alone, nor even on a declaration you would sign. Without an invoice, manual, rating plate or thermal study, the characteristic is treated as unknown — and the default value applies.
Mistake no. 2 — Blocking access to the loft, cellar or boiler room
The assessor must be able to observe the building and the equipment. A sealed loft hatch, a locked cellar, an inaccessible plant room: all are characteristics they cannot inspect visually, and which will fall back on default values. On the day of the visit, open everything: loft, cellar, boiler room, and give access to the meters and the electrical panel.
Mistake no. 3 — Confusing the areas
Carrez area (sale of a condominium lot), living area (loi Boutin, letting) and the DPE reference area are not calculated the same way. Presenting the wrong area distorts the kWh/m² ratios and can shift the rating. Provide the existing area diagnostics and let the assessor establish the reference area.
⚠️ Warning: a heated conservatory, an extension or a converted loft that has not been declared distorts both the area and the envelope taken into account. A DPE established on a wrong area is not only inaccurate but challengeable — to the seller's detriment in a dispute.
Mistake no. 4 — Presenting a purchase invoice without proof of installation
An invoice for insulating materials alone does not prove that the insulant was actually fitted in the diagnosed dwelling. The strongest supporting documents tie the document to the property: a works invoice stating the address or your name, a subsidy certificate (MaPrimeRénov', CEE) obtained for those works, site photos, or a probing report. It is this traceability that makes the data usable.
Prepare your personalised checklist
Our tool generates, from your address and property type, the personalised list of documents to gather before the visit — envelope, systems, administrative, condominium — and indicates for each what it justifies in the calculation. For a flat, it cross-references the DPE of the other homes in the same building to pre-fill the shared characteristics (collective heating, façade insulation, construction year) from the most recent and majority certificates.
Going further: once the DPE is in hand, check its consistency with the OneDpe DPE verification tool.
Conclusion
Preparing for a DPE inspection is not about trying to influence the assessor: it is about giving them the means to describe your home as it really is, rather than as the method assumes it to be in the absence of proof. Since 2021, every unjustified piece of data falls back on a penalising default value — an insulated wall becomes a bare wall, a high-performance boiler a standard one. Gathering your invoices, manuals, subsidy certificates and, in a condominium, the managing agent's documents, is the best-paid few hours of work before a sale or a letting.
The OneDpe DPE visit checklist stops you forgetting a document and tells you, for each one, what it changes in the calculation. If, despite good preparation, the result seems inconsistent, bear in mind that because the DPE is binding, an erroneous certificate opens up remedies against the assessor — all the more reason to keep all your proof carefully.
