For a general sense of a farmhouse in France:
* Ferme française (most literal translation)
* Maison de campagne française (literally "French country house")
* Gîte rural français (literally "French rural cottage")
For a more specific type of farmhouse:
* Ferme provençale (Provençal farmhouse)
* Ferme bretonne (Breton farmhouse)
* Ferme normande (Norman farmhouse)
For a charming, rustic feel:
* Maison de caractère en campagne (literally "House with character in the countryside")
* Ferme authentique (literally "Authentic farmhouse")
The best option will depend on the specific situation and what you want to emphasize.