This paper focuses on ``15-min cities'', a perspective that argues that residential, work, and leisure activities should be accessible within a short time. Its purpose is firstly to explore the basic principles of the 15-min city concept, and its contribution to the sustainability of a city, and then to develop a methodology modeled according to the 15-min city principles. The aim is to reconnect people with the neighborhood and to encourage the localization of different urban functions at the neighborhood level. The methodology was put together based on a review of planning strategies from international examples and subsequently modified accordingly. Specifically, the division of the study area, which is the urban complex of Thessaloniki, into smaller units was carried out to examine whether the conditions required to classify each subdivided area as autonomous are met. The areas that meet the conditions for the formation of 15-min cities as well as those that show deficiencies in amenities/facilities are depicted on a heatmap together with the number of these deficiencies. This methodology combines the advantages of sustainable development, smart mobility, and resource management. With an emphasis on the participatory process, the methodology proposes practical solutions for creating cities that enhance community cohesion, improve quality of life, and reduce the impact. The paper highlights the importance of applying such principles in contemporary city planning to achieve a sustainable, accessible, and thriving environment for residents.