C-DRONE GUIDE · 4 SEPTEMBER 2026

Substandard and unfit housing: what a drone survey documents for a case file, price

A substandard-housing case file stands or falls on the quality of its findings. Since French ordinance n° 2020-1144 of 16 September 2020, in force from 1 January 2021, a single administrative police power over the safety and health fitness of buildings has replaced around ten procedures previously scattered between the public health code and the building and housing code. The health-fitness strand — the unfitness treatment order, issued by the prefect after a report from the regional health agency or the municipal hygiene and health service — rests on defects that are observed first and foremost inside the dwelling, where a drone has no business being. Yet a decisive share of the underlying causes sits outside, often out of reach: a holed roof, a gutted chimney stack, blocked vents, a blind courtyard never maintained, cracked asbestos-cement sheets. A dated, georeferenced aerial survey documents those elements, and only those. Here is what it genuinely adds to a building-police case file, what it cannot legally do, and the prices charged in 2026.

Published on 4 September 2026, reviewed on 4 September 2026 — regulations in force as of September 2026.

Health fitness and safety: one police power, two strands, two authorities

Ordinance n° 2020-1144 of 16 September 2020, completed by decree n° 2020-1711 of 24 December 2020, merged on 1 January 2021 a set of previously scattered procedures — seven from the public health code, three from the building and housing code — into a single administrative police power. Article L.511-1 of the building and housing code states the principle: the police power over the safety and health fitness of buildings, premises and installations is exercised under the conditions set by that chapter. Article L.511-2 covers dangers to the health or safety of occupants or neighbours.

The procedure is unified, but the competent authority depends on the defect found, under article L.511-4: the mayor — or the head of the inter-municipal body where the power has been transferred — for structural soundness, malfunctioning shared equipment and storage of hazardous materials; the prefect for unfitness in the public health code sense, defined at articles L.1331-22 and following. In practice, a building at risk of collapse falls under the mayor's safety order — the subject of our guide to dangerous buildings and emergency safety orders — whereas a damp, poorly ventilated dwelling unfit for habitation falls under the unfitness treatment order issued by the prefect, after a visit and report by the regional health agency or the municipal hygiene and health service.

The distinction is not academic: it determines who investigates, who signs, and which documents carry evidential weight. A single building can trigger both strands — a collapsed roof undermines soundness and feeds damp in the dwellings below — which often justifies a shared external survey serving both departments. Law n° 2024-322 of 9 April 2024 on renovating degraded housing, in force since 11 April 2024, reinforced this toolkit: broader powers for elected officials, tougher sanctions against slum landlords, and the option for a co-ownership association to approve by simple majority a loan financing works needed to preserve the building and protect occupants' health and safety. The ministry's press pack of 27 March 2024 put at around 1.5 million the number of dwellings located in fragile or degraded co-ownership buildings.

What an aerial survey evidences from the outside

The external causes of unfitness are often invisible from the street and unreachable without scaffolding or a cherry picker — precisely the sticking point for a department that must act quickly, on a building that may be vacant or whose owner cannot be traced. A flight combining vertical and oblique photography, at a resolution of a few millimetres per pixel, documents in a single morning:

• the condition of the roof covering: missing or displaced tiles, slipped slates, holed flashing, failing valleys and abutments — the primary source of the water ingress that feeds the damp recorded inside the dwellings;
• the chimney stacks: torn-off cowls, dislocated masonry, blocked or diverted flues, a subject detailed in our guide to drone inspection of chimney stacks and flues — a sensitive point where the file raises a carbon monoxide poisoning risk;
• the ventilation outlets on the roof: dead extractors, blocked ducts, vents sealed off during earlier works;
• the courtyards, light wells and blind yards that cannot be reached from the communal areas: clutter, standing water, severed rainwater downpipes, absence of natural light;
• the defects on courtyard-facing facades: blown render, cracks, degraded window sills, read using the method set out in our guide to drone facade diagnosis before renovation;
• the likely presence of asbestos-cement sheeting on roofs or outbuilding cladding, to be confirmed by a regulated survey.

On that last point, a study published in 2020 in Remote Sensing by Małgorzata Krówczyńska, Edwin Raczko and co-authors at the University of Warsaw showed that high-resolution aerial imagery processed by convolutional neural networks can identify asbestos-cement roofing across a whole territory (see the study on Google Scholar). Such an aerial reading remains an indication, never a diagnosis: only the pre-works asbestos survey required by the French labour code, carried out by a certified operator, has regulatory standing — the method detailed in our guide to drone asbestos roof surveys before works. More broadly, a study by Caroline Gevaert, Claudio Persello, Richard Sliuzas and George Vosselman published in 2017 in the ISPRS Journal of Photogrammetry and Remote Sensing showed that combining point-cloud and image features from drone data allows automatic classification of buildings, vegetation, terrain and clutter, in dense and irregular urban fabric, despite the heterogeneity of roofing materials that makes classification harder (see the study on Google Scholar).

The limits: what a drone must not do in an unfitness case

This is the most important point in this guide, and it deserves to be stated plainly: a drone records nothing inside a dwelling. Unfitness is characterised by criteria observed within the inhabited rooms — damp and mould, ceiling height, natural light, ventilation, sanitary and electrical installations, degraded lead-bearing coatings, overcrowding. None of these can be assessed from the air. The report by the regional health agency or the municipal hygiene service, drawn up after an on-site visit, remains the founding document of the unfitness treatment order. The right of entry is itself regulated: article L.511-7 of the building and housing code allows visits to dwellings between 6 a.m. and 9 p.m., and requires judicial authorisation where the occupant objects. An overflight does not circumvent that requirement — it cannot be used to obtain from the air what the law subjects to a judge's authorisation on the ground.

Three safeguards therefore apply to every mission:

• No capture through openings. Article 226-1 of the French criminal code punishes with one year's imprisonment and a €45,000 fine the recording or transmission, without consent, of the image of a person in a private place. Filming through a window or rooflight, even incidentally, exposes both the pilot and the client. The mission must plan camera angles aimed at built surfaces, and include an image review before delivery.
• The GDPR applies. The French data protection authority stresses that camera- and sensor-equipped drones fall under it: an identified legal basis — for a local authority, the public-interest task of building policing — information of the persons concerned, data minimisation, a defined retention period, blurring of faces and number plates, and distribution restricted to the investigating departments alone. An unfitness report is not a communications document.
• The survey alone does not ground the order. It feeds the file as material findings, alongside the health report. The adversarial procedure required by article L.511-10 of the building and housing code assumes the owner can contest these documents: dated, georeferenced, unretouched images accompanied by flight parameters are, on that count, more robust than an isolated photograph taken from the street.

Finally, operations remain subject to the aviation rules applicable to flying over built-up areas, with the corresponding authorisations and aviation liability insurance: in a contested case, the lawfulness of the flight itself can be challenged.

Where the survey fits: municipality, inter-municipal body, renovation operator, landlord

Several actors use this kind of survey, at different points in the chain.

The municipal hygiene and health service, or an inter-municipal housing department, uses it upstream, at the reporting stage: when a tenant reports water ingress, an overflight establishes within the hour whether the roof is genuinely the cause, before arranging a visit and referring the case to the health agency. On a vacant or bricked-up building, it is often the only way to evidence the roof's condition without an access procedure.

A housing-renovation programme operator uses it at block scale: a single flight produces an orthophoto of every roof within the perimeter, allowing buildings to be ranked for priority inspection and a works budget to be estimated before individual diagnostics begin. The same deliverable then serves as a baseline for measuring rehabilitation progress.

A municipality that has introduced a rental permit scheme finds in it contextual evidence when processing a prior declaration or authorisation to let, on a building already flagged.

Lastly, a social landlord or managing agent running an older stock uses it to anticipate: spotting the roofs that will tip into pathology before the defect reaches the dwellings and triggers a procedure, the logic developed in our guide to drone-based social housing stock audits. On a building already under order, photographic follow-up documents completion of the prescribed works — a direct stake when article L.511-15 of the building and housing code allows a penalty capped at €1,000 per day of delay and, failing completion, works carried out by the authority at the owner's expense.

Method and 2026 prices

A typical mission runs in three stages. Preparation first: defining the exact perimeter with the investigating department, checking airspace constraints over the built-up area, giving prior notice to occupants and neighbours, and setting down in writing what will be captured — and what will not. Then the flight: vertical coverage for the orthophoto, an oblique series for courtyard facades and chimney stacks, close-ups on specific defects, all time-stamped and georeferenced. Finally delivery: an annotated photographic set, a roof orthophoto, and a note of factual findings — with no legal characterisation, which belongs to the investigating department.

Ranges observed in France in 2026, excl. VAT:

These amounts cover the flight, image processing and the findings report; they include neither the regulated asbestos survey, nor a structural study, nor the health inspection. Our guide to how much a drone service costs places these prices within the wider French market.

One final and decisive reminder: a drone is a tool for external observation; it replaces neither the technician's visit, nor the health agency's report, nor the regulated diagnosis by a certified operator. It documents, at a certain date, what is visible from the air — nothing more, and that is already a great deal in an adversarial file. The regulatory information on this page reflects the rules in force in September 2026; the exact terms of the procedure should be checked with the authority's housing department, the regional health agency or the departmental housing information agency. For a municipality, an inter-municipal body, an operator or a landlord, request a quote stating the number of buildings, the perimeter and the deadline for the case.

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