Relevant problems of the practice of application of law and permanent process of improvement of the Ukrainian legislation are examined in this article. In particular authors stress on contradiction between process of the permanent improvement of the legislation and sustainability, predictability and legal certainty as principles of legal system. On the one hand, constant reforming of the legislation of Ukraine does not give possibility for subjects of law to create stable legal relations, which are regulated in a predictable way. On the other hand, legislation of Ukraine must be actual and even few “step ahead” the actual situation in Ukraine. It must give necessary methods, tools and legal instruments, which regulate relations between subjects of law. Described judicial and notarial practical cases show in what way flexibility of the legislation could be provided in conditions of the ongoing changes. Authors consider that qualified and high-quality application of the current legislation by the subjects of application of law gives possibility to provide flexibility of the legislation. Such application of law is done by subjects applying the law, although their actions are not directly provided for by the legislation, but they are not prohibited either. Essential principle of such application of law must be the whole tendency to create those legal consequences of the application of law which will not lead to the direct infringement of somebodies rights and interests within the process of application of law. Illustrated lawsuit and case of notarial practice show how exactly interpretative mechanism and analogy as a tool of the subject applying the law help to avoid infringements of rights and interests of heirs of the deceased person.