All enterprises registered in France will obtain a SIRET number at the time of registration. This is a number that you can check to be sure that an enterprise is correctly registered and authorised to work on your property.
An enterprise must be not just registered, but registered to do the kind of work that you are asking them to do. The SIRET number must appear on the devis as well as the final invoices – so you can check before accepting a devis whether the company is authorised.
Officially it should also appear on any publicity from the enterprise, but I think this happens rather less often.
To check the validity of a SIRET number, enter the first nine digits of the SIRET number (called the SIREN number) at the following website, and the name of the company and the type of work they carry out will be shown:
Note that if you employ a UK registered company to do the work in France, you will not benefit from the obligatory 10 year insurance that French artisans carry, you will not be eligible for a reduced VAT rate (reduced from 19.6%), and you will not be eligible to deduct the cost of works from an eventual capital gains calculation in France.
Hence there is very good reason to check the SIRET numbers both exist, and are valid, and to only employ artisans who can provide them.
There is a short period when an enterprise first registers when ‘SIRET in course’ note can appear instead of a valid SIRET number. This should usually only apply for a couple of weeks, and you should make it a condition of proceeding with the work that a valid SIRET number can be produced at that time.