MDSE is a discipline that only recently has begun to be used in mainstream practice. At the Dutch Tax and Customs Administration (DTCA), we use this approach successfully for a major part of the Dutch Social Benefits system, a nationwide online system serving 6 million citizens. We report on our experiences with bringing forth and maintaining this system, and on our future plans for extending this approach to other key systems of our organisation. The DTCA is responsible for (amongst others) collecting taxes and paying social benefits. The supporting software system typically have a lot of functionality and process huge volumes of data. Furthermore, the systems should be easy to adapt within short notice: laws do change often, and once a change to a law has been approved by the parliament, there is only little time to implement the changes. To adapt complex systems quickly, we use Functional Model Driven Development (FMDD), a variant of MDSE. The FMDD approach separates functionality from other aspects of an application, hence its name. We outline the basic way of working by describing the main artifacts and roles. A key feature of our approach is separation of concerns. We organize the specifications and transformations such that different concerns are separated from each other. We detail some of the techniques we applied in the Social Benefits System.