Reminder: All assets acquired during the marriage are presumed to be jointly owned (Article 1401 of the French Civil Code).
To ensure that the new apartment remains your separate property, you must ask your notary to include a reinvestment clause (clause de remploi) in the deed of sale (Article 1434 of the Civil Code). This clause must state that the property is being financed entirely with funds originating from the sale of your separate property.