This paper develops the steel-house design interface that helps make building design effectively done by constructing building materials as components and assembling them as patterns. It also aims to develop the Flexible Building Design System that supports a technology which makes change of constructing easy and reduces the cost effectively through a simulation of building design like design, analysis, change information, etc., by grafting the virtual building technology into the process of building. It especially is possible for a designer and user to change a building easily by using patterns and according to the change, pattern information of materials needed and the plans of the building are made automatically. Kin-search that can be happened through personal proficiency or knowledge visualization is also the reason why the technology should be embodied.