Base extension is a critical operation in RNS implementations of asymmetric cryptosystems. In this paper, we propose a new way to perform base extensions using a hierarchical approach for computing the Chinese remainder theorem. For well chosen parameters, it significantly reduces the computational cost and still ensures a high level of internal parallelism. We illustrate the interest of the proposed approach on the cost of typical arithmetic primitives used in asymmetric cryptography. We also demonstrate improvements in FPGA implementations of base extensions on typical elliptic curve cryptography field sizes using high-level synthesis tools.