The Importance of Planning-1:10:100 rule Everything we make/build goes through three broad stages of Development; conception and planning: building and testing ; delivery and use. In the first stage, we are working with the idea and our milestone is a written plan. In the second stage; we are building and the result is the physical system, tested and working. In the third stage; the system is put into place and then maintained for the length of its useful life.The Importance of Planning It is always least expensive to plan well and resolve all problems in the conception and planning stage. It cost 10 times as much to resolve the same problem in the building and testing stage, and it cost 100 times as much to resolve the problem when the system or build-outs is in production. This rule is the 1:10:100 Rule