Another problem is that engineers also tend to estimate only the time for original implementation, not for design, testing, documentation, bug fixes, meetings,

Another problem is that engineers also tend to estimate only the time for original implementation, not for design, testing, documentation, bug fixes, meetings, etc. A long time ago I read about some research that showed that the original implementation makes up about 1/6 of the total time spent on the task. This has been roughly the case in my experience too -- so now I budget for at least 6× estimated implementation cost, more if it's a tricky problem.