To provide a building design device for simply extending a building in accordance with a site form and making effective use of the land.

A floor-plan is defined according to information about points comprising components in floor-plan data 20 and information about the size of the components. Representative floor-plans are selected from past residence sales achievements, for example, and they are standardized to create the floor-plan data 20. The floor-plan data 20 includes an extension area used when extending the floor-plan and its extension rate. The extension area and extension rate are created in advance before a user uses a CAD. Preferably, the extension area and extension rate are created every direction to extend and, for example, an extension area and the extension rate with respect to extension in the depth direction of a building and an extension area and the extension rate with respect to extension in the width direction are defined.