During the 1920s and 1930s Gdynia transformed from a small fishing village into a dynamically developing city through the construction in its vicinity of the largest seaport of interwar Poland. The city’s first expansion plan (1926, Adam Kuncewicz, Roman Feliński), designed for approximately 100 000 inhabitants, had to be revised already within a few years because development of the port has proved to be faster and the terrain requirements of the port substantially greater than originally predicted. In effect grounds originally planned for general city functions were yielded to the rapidly developing port. It was not until 2003 when the port boundaries shifted significantly as a result of restrictions and changes in the nature of port activity. These changes freed around 53 ha of post-harbour sites for new development, attractively located in the immediate vicinity of the existing city centre and waterfront. In 2015 the port boundaries shifted again adding 3 ha more of post-harbour sites. The changes also spurred investment in the north of downtown Gdynia – in accordance with the intentions of first city planners. Ideas how to make good use of post-harbour areas have been invented and evolved since 1990s. The concept which is currently under development was prepared in 2008 by the City Planning Offi ce of Gdynia. Since 2010 analyses and studies of future development have been conducted using 3D model. The concept is continuously updated and new details are added based on projects obtained through architectural competitions. Since 2015 development areas of the northern part of the city center were rebranded as Gdynia Sea City. In accordance with this concept Gdynia Sea City will be the modern city centre with areas designed for leisure, relaxation and business and will be inhabited by more than 10 thousand people. The area’s new grid is a continuation of the urban grid of historical downtown Gdynia and draws upon unrealized city plans of the interwar period. The scale of new buildings in the area is reminiscent of the historical buildings in the area. Groups of higher buildings are allowed outside of a protected area of the historic center, in areas selected through view and cityscape analysis. The residential and commercial complex Sea Towers together with two other newly constructed tall buildings is currently the dominant in this area. Several new development complexes are under construction. Planning concepts assume public availability of quays around the port basins and maintaining spacious openings towards the sea. Construction of marinas is expected using part of the docks and the ability to expand and reduce existing wharves, movement of pedestrians and cyclists between Fishermen Pier and South Pier will be facilitated through the construction of a bridge or a ferry connection. The planned enlargement into the post-harbour areas will double the current potential of Gdynia downtown, and enlarge the scope of representative areas and change the panorama from the historic city center and from the sea. Attractive downtown sites can provide an answer to the issue of uncontrolled urban spill into peripheral areas of adjacent municipalities.