Virtual Early Prototyping addresses the concept phase of a product development that is a critical issue for complex engineering products. Best specifications shall be defined through multiple iterations by different designers. The designers shall cooperate to quickly set up and discuss architecture proposals starting from a 3D representation of the product. VEP opportunities are subordinated to well defined requirements and new metaphors providing adapted solutions. The deployment of a cooperative system suffers mainly from the client-server approach that is inefficient in many ways and attributes a leading role to a server site. Moreover, requiring a specific quality from the under-laying communications restricts openness and usability of the solution. This paper defines Virtual Early Prototyping requirements. It shows that the distributed building site metaphor provides a good solution. That metaphor enables a real-time 3D cooperative design and parallel work within virtual shared worlds while preserving the consistency of the structure and system design that a number of designers are working on simultaneously. It is a fully distributed approach without any centralized control. The solution does not require a specific quality of services from the under-laying network.