Cost planning requires an approximation of construction costs broken down for each design stage. In dealing with building design involving basically the same type of units, such as is done for telephone offices, the work can readily be separated into functional subdivisions. This is based on the assumption that a statistical distribution phenomena pertains, even if there are some requirement differences or design individualities involued. These statistics are based upon multiples of some specific "in-place" element cost, involving the unit quantity of the "in place" element, which becomes evident as design stages develop. Based upon sufficient experience and past cost data, along with cost-increase index information, we can obtain a more accurate planning stage cost estimate than would be obtained from a one-time lump sum costruction cost estimate, by carefully and systematically taking into consideration the design of each element. When we use statistical data based on graphs, dates and costs, some cost deviation is inevitible for individually identified in-place element unit costs. However, we have devised and applied the "Theory of Deviation Distribution" to show the influence of individual inplace element costs upon the overall work picture, in order to more efficiently obtain a sufficiently occurate cost factor. We have shown the accuracy of our methode through comparison of results both theoretical and actual, based upon data made available from 25 telephone offices located all over the country. These results show an effective rationalionalization of cost estimation in the cost planning stages of telephone office construction planning.