For companies that must provide customised products on demand, it is important that individual customer wants and needs be addressed and incorporated early in the design process. Web-based design interfaces have emerged as useful tools for facilitating customer interactions while providing customer-friendly and cost-effective interfaces. The effectiveness of the interface can be greatly enhanced through the implementation of a strategic customisation process that can proactively react to customer requests. The design process results in a virtual product line that approaches the goal of providing nearly infinite variety at minimal costs. The process provides a basis for a true customisation approach, which is different from the typical mass customisation approach to provide ample but limited product variety a priori for a targeted market. In this paper, we describe a prototypical implementation of a web-based custom product specification system and the underlying strategic design process that is based on a collection of product platforms. The development is illustrated using an example from ongoing research with a company that produces customised valves for the power industry.