TLDR: If you are a foreign diplomatic or consular agent established in France, the exemption generally applies from January 1 of the year following your establishment. However, when a tax assessment concerns the year of your establishment, a tax relief may be granted, for reasons of international courtesy, from the date you officially take up your duties. This possibility is not automatic and does not concern people who had their permanent residence in France on that date.
The ordinary rule
In principle, you may benefit from the exemption from January 1 of the year following the year of your establishment in France. This rule results from the principle of annuality of the housing tax: the date on which you take up your duties does not normally change the year of taxation.
The exception based on international courtesy
If a tax assessment is issued for the year of your establishment, the administration traditionally accepts, for reasons of international courtesy, not applying the annuality rule.
In this case, tax relief may be granted to you from the day you officially take up your duties. The tax relief reduces or cancels the tax assessment already established. However, you should not consider this measure automatic: the BOFiP presents it as a possibility granted under the conditions indicated.
The exception does not apply if you had your permanent residence in France when you took up your duties.
Residence concerned for diplomatic agents
For ambassadors and other diplomatic agents of foreign nationality, the exemption concerns the municipality of your official residence and that residence only. It is subject to reciprocity: the State you represent must grant similar benefits to French ambassadors and diplomatic agents.
Rules applicable to consuls
For consuls and consular agents, you must refer to the conventions concluded with the represented State. In all cases, the exemption may concern only the municipality of your official residence and that residence only.
The BOFiP does not specify, in the elements examined, the practical procedure, deadline or channel to use to request the tax relief.