This paper introduces a novel programming model for safe and incremental construction of live applications. We capture a verified style of agile development that spans over the whole development life cycle, from specification and prototyping to maintenance and evolution. This approach proposes a step forward with relation to the traditional code-compile-deploy cycle, allowing for both code and data updates to be safely applied during execution. We pro- pose a language-based development and runtime system to evolve data-centric applications. Our approach is presented as a core typed imperative calculus with a reactive semantics. The associated type discipline ensures a correct interleaving of interaction and construction of systems. The soundness of our calculus is supported by standard progress, type preservation and convergence results.