This paper presents a framework to support the assessment of urban design projects through Urban Living Labs (ULLs). The framework is based on the Tactical Urbanism (TU) practices and involves the use of Mobile Urban Elements (MUE) in uncertain and potentially confusing conditions (e.g., the COVID-19 context). The methodology includes the application of the Four-Phase Model (problem and ideation; development; implementation, testing and assessment; final proposal) and a quantitative and qualitative assessment. The proposed assessment criteria were developed through an evaluation according to three aspects: (1) feasibility impact; (2) social impact; and (3) spatial impact. The methodology was applied to Furnish, an urban design project based on a ULL and prototyping, which was recently developed in five European cities. The empirical results, obtained using the impact analysis, indicate that the prototypes developed in the project are transferable to other cities and generate social interaction in public spaces. The applied research showed that the Four-Phase Model may be used as a new and improved iterative design process: the LOOP Scheme. The application of this assessment methodology to ULLs may provide valuable information for the future planning of urban interventions in public spaces.