Here is a cautionary tale which happened to a friend of mine, whose relationship status was akin to yours. He and his partner (a an unmarried and unpacsed couple) had agreed to purchase a house (needing renovation) in France together, to be put into joint names. After they had confirmed this to the estate agent, in writing, she persuaded him that it would be better purchased in her name, giving him plausible-sounding reasons as to why this would be better. In love with her, and believing that she was as besotted with him, he agreed, and the Notaire was duly informed. The purchase was subsequently contracted and concluded in her sole name - the purchase price being declared as paid by her.
He moved into the house (she staying in her existing accommodation until he had made it habitable for them both), and being a builder and skilled craftsman immeditely set about the renovation, putting in many hundreds of working hours and paying for the materials much the same amount as the house had cost to buy. She acknowledged in writing this contribution by him. However, before the renovation work had been fully completed, but at a very advanced stage, they fell out, and never lived in the property together. Neither were they ever reconciled.
Having burned his accommodation bridges elsewhere, he stayed on in the property, but slowed down considerably on the building works.
After some time, she applied, through avocats, to a court, claiming possession of the property and rent for the period during which he had been in occupation thereof. Her lawyers gave no credit for the works he had carried out, nor for the monies he had expended thereon. The court found in her favour. On his avocats’ advice, my friend appealed that decision. Sadly, the appeal court upheld it.
It seems there is no equivalent of the British “palimoney” concept under French law. So be warned.
Therefore you would be well-advised to be a joint purchaser, under whatever regime a French Notaire may advise you is most appropriate to your joint circumstances.
So please think twice before proceeding on the present c ourse. An alternative might be to have a Notaire prepare a legally enforceable binding agreement, signed by you and your partner, to be attached to the acte de vente ensuring that you will have the same rights in the property as if it had been purchased in your joint names.
Or get validly married or pacsed before the acte de vente!