With an increasing role of computers in the design process, a strong foundation of the basic principles for building CAD systems is extremely necessary. This work describes the architecture of a CAD system from the perspective of the capabilities required to build a human centered CAD system. These capabilities have been reported as task-sharing, communication, transparency, design rationale and cognition. Existing systems for engineering design have been analyzed from the perspective of these capabilities and an agent-based system previously reported in literature has been extended to a new architecture to support three of the aforementioned capabilities. The proposed architecture is an amalgamation of multi-agent systems, the ACT-R cognitive architecture and fuzzy rule based Petri nets. The salient advantage of this architecture is that it focuses on the capabilities of design rationale, transparency and cognition togethersomething which has been missing from all the current works in literature and is unique to the domain of application of the software system, which in this case is engineering design. The other capabilities namely task sharing and communication can be incorporated from lessons learnt from human-computer interaction research. The effectiveness of the proposed architecture is demonstrated by a hypothetical design case along with its implementation.