Communes are units of territorial self-government responsible for carrying out the most public services, for which purpose they need to seek sufficient funds. The income of communal self-governments consists in: own income, general subsidies and targeted grants from the state treasury. Supplementary income of communal budgets includes funds other than own income. Communal budgets are tied to the central budget by transfer of funds through general subsidies and targeted grants. The substantial share of supplementary income within the total income structure makes communes dependent on the state. The need for self-governments to obtain other sources of income opens up a rent-seeking possibility for communes. These territorial units of self-government become interest groups striving to achieve the best possible conditions for their own functioning. The aim of this paper is to answer the question whether communal self-governments engage in rent-seeking as regards supplementary income. The paper describes the specificity of supplementary income in Polish communal budgets, their functioning including appropriate legal regulations, and their application in practice. General subsidies and targeted grants from the state treasury are also characterized. The paper also analyses the relative shares of the two types of supplementary income in the communal budget structure and presents the resulting conclusions. The structure of the paper and the analysis contained herein have mandated the conclusion that communal self-governments engage in rent-seeking as regards supplementary income.