Contingency
A Contingency works better than a range. Here is how much.
The graph above showing contingency plotted against the amount of scope that is known came from a RedVector course on estimating.
I like the way the size of contingency is related to the level of scope that has been developed. What I don't like is the amount of contingency that is suggested as being needed. Surely as architects we can come closer than 35% contingency at the end of Design Development.
Contingency is very helpful in preparing construction or project estimates for a client. My experience is that if you give a client a range of potential cost, they immediately forget the higher number and begin acting as though the lower number is a fact.
Including a contingency for "what we don't know yet” in a bottom line tends to work better. In my mind the goal is to prepare the client for reality rather than tell him/her what they want to hear.
It is very rare that a client is unhappy about a project coming in under budget. The opposite almost always creates a serious problem for everyone.
The contingency amounts that I used at the start of various phases are shown in this table:
Contingency Percentage by Phase
I find that explaining the purpose of the contingency and how it becomes lower as the project progresses is accepted as logical. You will want to develop your own list of contingencies for use at different stages of the work.