My father donated a property to me in full ownership. Today, I want to sell it, but the notary has informed me that I need the signature of my brothers and sisters, even though I'm the only one who owns the property. Is this formality compulsory?

Yes. Article 924-4 of the French Civil Code stipulates that heirs with reserved rights may bring an action for reduction or revendication against third parties holding real estate that was donated and subsequently disposed of by the recipient. In practical terms, if your father's gift encroaches on your brothers' and sisters' shares of the hereditary reserve, they are entitled to claim compensation from you. If you are unable to pay, they can recover all or part of the property from your purchaser (this is known as reduction in kind). On the other hand, if they give their consent to the sale, they waive their right to bring an action for reduction against the buyer, thus ensuring the legal security of the deed of sale.