Negative demographic trends have actualized the search for Russia's demographic development reserves. The article defines the concept of “demographic development reserve” and highlights the authors' approach to studying multigenerational households as an institution providing potential reserves for marriage and family relations reinforcement. The purpose of the study is to identify the prevalence of multigenerational households in Russia, the nature of interaction between generations in families under the conditions of nuclearization and the role of generational solidarity as a demographic development reserve. The authors' conclusions are based on the data of All-Russian population censuses (1989, 2002, 2010 and 2020), as well as the results of author's sample studies (international sociological research “Family and Family Policy: A Generational Perspective”, All-Russian sociological research “Demographic well-being of Russia. National demographic report”, regional sociological research “The life of a multigenerational household in conditions of self-isolation”, All-Russian sociological research “Student family of Russia”). As a result of the structure of households’ analysis based on population census data were identified several trends. Firstly, increasing number of people living alone. Secondly, reduction in number of families consisting of several married couples. Thirdly, reduction in the number of children in the family along with an increase in the proportion of large families, etc. According to authors' sociological research the proportion of those who believe that young families (as well as children over 18 years old) should live separately from their parents is increasing. Most of parents help their children who have already created families and live separately. They help in raising grandchildren, paying for housing, buying things, providing financial assistance, etc. Children growing up and their parents to retiring change the situation. Adult children begin to provide assistance to their elderly parents (care during illness, household help, monetary assistance, etc.). It can be stated that solidarity (a stable relationship and the connection of several generations of a family) is an instrument for supporting the economic and social well–being of Russian families regardless of residence type (separately in one or different localities or jointly).