Estimates are typically a necessary evil in software development. Unfortunately, people tend to assume that writing new software is like building a house or fixing a car, and that as such the contractor or mechanic involved should be perfectly capable of providing a reliable estimate for the work to be done in advance of the customer approving the work. This is pretty attainable by building contractors and auto mechanics, who generally are working with known materials building known things in known ways. ... With custom software, however, a great deal of the system is being built from scratch, and usually how it’s put together, how it ultimately works, and what exactly it’s supposed to do when it’s done are all moving targets.

https://medium.com/@ardalis/the-5-laws-of-software-estimates-fd13af46000b#.94s2f42fz