My Charente-Maritime notary has to come to Paris to sign a deed of sale. As he doesn't have any premises in Paris, can this signature take place in my buyer's professional warehouses?

No. Article 12.1 of the national regulations governing notaries stipulates that "In the interests of the dignity and independence of his functions, the notary may, save in exceptional circumstances, only receive clients and deeds in his office, in ancillary premises and in one or more ancillary offices or in the premises of a colleague, at the domicile, residence or registered office of one of the parties, in the premises of an administration, a town hall, the courts, hospital establishments or the premises of professional bodies".