Our concern is the modular development of a certified static analyzer in Coq: we extend a certified abstract domain of convex poly-hedra with a linearization procedure approximating polynomial expressions. In order to help such a development, we propose a proof framework, embedded in Coq, that implements a refinement calculus. It allows to hide for proofs several low-level aspects of the computations on abstract domains. Moreover, refinement proofs are naturally simplified thanks to computations of weakest preconditions.