This paper explores design as a catalyst of urban transformation and as an epistemic instrument within contemporary planning culture. Using New York’s High Line and its global reinterpretations — including The Bentway in Toronto, Cheonggyecheon in Seoul, Nordhavn in Copenhagen, and King’s Cross in London — the study traces a paradigmatic shift from design as aestheticization to design as mediation and co-production. These cases demonstrate how urban design operates not only through form-making but through iterative processes of governance, observation, learning, and negotiation. The empirical part situates these ideas within the Polish context through the case of Kraków’s Wesoła district — the example where statutory and strategic planning instruments (MPZP and Masterplan) were intentionally interwoven through participatory design. The Wesoła Masterplan (2024) translates global principles of critical design and operational urbanism into local practice, creating a “living laboratory” of adaptive governance, affective mapping, and co-creation. The paper concludes that the most transformative potential of design today lies not in producing new forms but in cultivating the city’s capacity to design itself — a planning culture based on reflection, experimentation, and shared knowledge.