This new annual reporting obligation has arisen from the abolition of the taxe d'habition, due to the loss of essential information on property types and occupancy. This information is required by officials to reliably determine which of the remaining relevant property taxes to apply (Taxe annuelle sur les logements vacants (TLV), Taxe d'habitation sur les résidences secondaires, Taxe d’habitation sur les logements vacants (THLV)).

The Gérer mes biens immobiliers (GMBI) system is entirely on-line, and requires owners to verify the information contained on each form, and if necessary, correct the data entered by the tax authority.

The information would also be used to provide an accurate database for the planned revision of local rateable values.

The auditors describe the development of the system between 2018 and 2023 as being marked by "poor management and governance", a "misunderstanding of taxpayers" and a "failed pre-launch test phase."

The report states that "the system put into operation in January 2023 was not finalised (...) while the calendar constraint prevented any postponement.”

The project ultimately cost between three and five times more than budgeted, in particular due to the extensive use of "expensive external service providers". In the end, the project cost the State some €57 million.

Lack of training of tax officials, a poorly calibrated timetable, insufficient resources dedicated to supporting users, and poor communication, are some of the problems that made the declaration programme "chaotic", according to the auditors.

Above all, it was the failure of officials to take into account the difficulties of many users with the IT that produced countless errors, without any paper form available for those with difficulties with the internet.

The trades unions had warned many times about the problem, without these alerts having been taken into account. The unions also claim that the main aim of the project was to transfer part of the work of officials to users – a logical consequence of the closure of countless local tax offices.

In the absence of sufficient responses, the tax authorities were forced to postpone the deadline for return of the forms several times, and then to recruit dozens of contract workers in a hurry to deal with the countless complaints and requests for assistance – while the switchboards of the tax offices were totally saturated with calls.

In the end, officials were obliged to urgently publish a paper declaration to allow those owners unable to complete the on-line form to make the return.

The auditors state that officials chose not to send a letter to taxpayers, in order to "reduce postage costs". All communications were therefore only by email or messages on the tax website. Belatedly realising that these emails and messages were not reaching some of the users, officials finally sent a letter, but only 15 days before the deadline for completion of the form, which led to an immediate saturation of services from panicking users.

Between the fact that the computer application was "immature", and that the target audience risked, in part, being handicapped by a fully on-line procedure, "the fiasco was inevitable", state the auditors.

The result was that hundreds of thousands of households received incorrect tax notices, including 15,000 children who should not have received one. Beyond that, more than 900,000 people were unduly taxed for their second home even though their property was a main residence. More than 220,000 people had to pay a tax on vacant housing when the housing was not vacant.

These incorrectly collected taxes represented a total of €1.306 billion, equivalent to 34% of the total of the three main taxes, which were paid by the State to the local authorities concerned, but which in turn had to be reimbursed to taxpayers who had been wrongly charged.

In her 2023 report on the digitalistion of public services the Ombudsman stated that: "It is [now] up to the user to train, to do, to be capable. In order to access their rights, it is up to them to adapt to the conditions of the administration. This is a historic reversal of one of the three principles of public service: adaptability – which has become a quality expected of the user, rather than a requirement that falls to the service."