This article assesses on what happens when planning by experiment becomes imperative for strategic city sites such as waterfronts due to the failure of other forms of centralised, top-down, or market-led planning. Through an in-depth case-based analysis of La Marina de València (LMdV) we investigate the potential of experimentation for revitalisation of city sites. To do this, we first review the literature on urban development approaches to identify specific issues that lead to urban planning failure. We then extend the scholarship on urban experimentation by proposing a definition of place-based experimentation as ‘relational process.’ Then, we explore how planning by experiment emerged as a response to planning failures in a broader strategy for revitalisation of LMdV. We propose key processes for planning by experiment through a Placemaking Living Lab based on perception, collaboration, and iteration, which we use to assess experimentation at LMdV. In the conclusion we discuss the potential of this approach to ‘planning by experiment’ to revitalise urban governance and planning processes in cities and their strategic sites.