This paper presents an innovative framework for climate resilience through the development of a co-warning and co-monitoring system rooted in a Coastal City Living Lab (CCLL) approach as a part of the EU project SCORE (Smart control of the climate resilience) with a consortium in European coastal cities. The focus lies in leveraging low-cost sensing technologies and the citizen science approach to foster citizen engagement in environmental monitoring, facilitating hyper-local data collection for eventual integration into early warning systems development. By integrating a network of affordable and easily deployable sensors, the initiative complements existing institutional infrastructure, helping to bridge spatial and temporal data gaps in monitoring extreme weather events and assessing Ecosystem-Based Adaptation (EBA) measures. At the core of this work is a Citizen Science Framework, designed to empower communities with the tools and knowledge needed to build and operate self-sustaining environmental sensors. A dynamic catalogue of low-cost sensing technologies was established and refined through a combination of scientific and grey literature review, with sensor selection processes co-designed in workshops with local stakeholders, citizens, and associations. The onboarding and piloting of these technologies were implemented in the Dún Laoghaire-Rathdown CCLL, Ireland, serving as a testbed for participatory sensor deployment, capacity building, and community training. Citizen-generated data, collected via smartphones and sensor data gathered by both commercially available and home-built instruments, are integrated into a dedicated software platform, enabling real-time data sharing and enhancing the overall resilience of urban coastal areas. The paper discusses the methodology, challenges, and outcomes of sensor onboarding, educational outreach, and community co-creation practices, highlighting the transformative potential of citizen science and smart technologies in climate-adaptive urban planning.