Article R145-20 of the French Commercial code
The request for a rent review provided for in article L. 145-37 is made by extrajudicial act or by registered letter with acknowledgement of receipt. It shall specify, under penalty of nullity, the amount of rent requested or offered. Failing agreement, the request is judged under the conditions provided for in articles L. 145-56 to L. 145-60. The new price is due from the date of the request.