Formal planning instruments and procedures have often been unpopular and ineffective for solving complex spatial issues, such as urban sprawl or transport congestion. As a result, such conflicts turn into complex planning tasks that usually exceed the provisioned time and funding, especially when faced with adversarial interests of actors from different organisations, sectors or social groups. Hence, informal planning, as a non-binding supplement to official planning instruments, is often considered highly effective. In its broadest sense, informal planning includes the principles of collaborative dialogue, diverse networks, trustful relationships and tailor-made processes among interested parties. Consequently, informal planning processes foster sound decision-making delivering a spectrum of problem-oriented solutions and increasing public consensus, while enacting experimentation, learning, change, and the creation of shared meanings among stakeholders. However, informal planning cannot be taken for granted – it is strongly interwoven with the planning culture influenced by the historical and political background, and the current socio-economic conditions. This paper revolves around several pillars. After an introductory section, a brief historical overview firstly identifies the place of informal planning in various planning models that have appeared since the 1960s. More specifically, informal planning is analysed against the theoretical concept of collaborative rationality. Finally, the paper focuses on a specific informal planning procedure called the ‘test planning method’, being analysed against the previously elaborated theoretical background. As this instrument links both formal and informal planning, its comparison and interrelation with the theoretical background of collaborative rationality contributes to elucidating the following attributes of adaptive (collaborative) urban governance: 1) flexible and agile institutional arrangements supportive to various kinds of urban planning mechanisms (not only official tools), 2) proactive and imaginative planners ready to accept solutions created outside the technical domain of instrumental rationality, and 3) inclusion of numerous stakeholders to exchange various information and different types of knowledge, i.e. expert and experiential knowledge. Observed through the example of the test planning method, the article finally highlights the successful aspects of informal planning, however, pointing also to its shortcomings, which could be expected in the societies with a lack of key democratic elements