France expands AI aerial checks to spot undeclared garden sheds

French tax authorities are using AI and aerial images to track down undeclared garden sheds and verandas, following similar searches for swimming pools.

France’s public finance and tax administration, the Direction générale des Finances publiques (DGFiP), is reportedly focusing its efforts on garden sheds measuring between 10m² and 15m².

The method was initially tested in nine departments in 2022 before being rolled out across metropolitan France in 2023. That first experiment found more than 20,000 undeclared swimming pools and generated almost €10million in additional revenue for local authorities.

Wider checks had then generated around €40million.

The DGFiP uses AI algorithms to identify the outlines of buildings and swimming pools from publically-available aerial images from the IGN (the French National Geographic Institute). 

The software then compares what it has found with tax declarations for the property, flagging anomalies which are then investigated further by a tax official.

If the tax official suspects the presence of undeclared structures on a property the owner is contacted.

While there has been no official confirmation from the DGFiP of the wider roll-out of the aerial checks for undeclared sheds, an earlier parliamentary report based on DGFiP answers confirmed building detection was being carried out in about 30 departments.

A 2023 DGFiP assessment estimated that between 1% and 10% of residential extensions could be undeclared, according to Le Parisien. 

Which structures must be declared?

All new constructions and extensions that affect a property's taxable value (which is used to calculate the taxe foncière and taxe d’habitation on second homes) must be declared within 90 days of completion. This includes garden sheds, verandas, garages and swimming pools.

This obligation applies even when planning authorisation is not needed for the construction.

For example, a garden shed of up to 5m² does not normally require planning permission, while one measuring more than 5m² and up to 20m² usually requires a prior declaration.

Larger structures generally require a building permit, although local planning rules and protected areas can affect these thresholds.

The information is used to update the property's cadastral rental value, which helps determine the amount of taxe foncière (property tax) that has to be paid.

Owners who declare qualifying works within 90 days may also be eligible for a full or partial exemption from property tax for two years on the new construction or extension. 

How to declare an extension or garden shed

Owners can declare new constructions and extensions through their personal account on the impots.gouv.fr website, by selecting the 'Biens immobiliers' section.

Those who have already completed work without declaring it can also update their property’s details through the same section. If this cannot be done online, they can use the H1 (for a new construction of a detached building) or IL (for a change in the characteristics or intended use of an existing building) paper form and send it to their local property tax office. 

Owners can also check the current information about their property in the 'Biens immobiliers' section of their account.