To give an optimum plan that can be approved by a user to the user desiring additions and betterments on-line.

A site introducing extension and alteration on a network is provided by using a server, a construction part selection box where the construction part information of a building subjected to extension and alteration is selected and a merchandise selection box where optional equipment merchandise is selected among pieces of equipment merchandise classified in each construction part are given within the site, the sever receives construction part information and equipment merchandise inputted by the user desiring extension and alteration, a site completion predictive image drawing is prepared on the basis of them, construction expenses including selected equipment merchandise pieces and construction expenses are accumulated, and the site completion predictive image drawing and the construction estimated expenses are given to the user on the site.