The article is devoted to one of the most acute problems of modern Russian inheritance and notarial law — the admissibility and limits of a notary’s initiative allocation of the surviving spouse’s marital share in the absence of his/her application. On the basis of a comprehensive analysis of the norms of the Civil Code of the Russian Federation, the Family Code of the Russian Federation, the Fundamentals of Legislation on Notaries, the contradictory judicial practice of 2024–2025, and the positions of domestic legal doctrine, the author examines the collision between the principle of dispositiveness and the public-law obligation of the notary to ensure the legality of the composition of the hereditary mass. Particular attention is paid to a comparative legal study of the French model, where the notary is traditionally vested with active powers to determine the marital property regime. As a result, two opposing approaches in practice and doctrine are identified. Specific measures for improving the legislation are proposed, aimed at strengthening the preventive function of the notariate and reducing the number of judicial disputes.