The building analogy works well. It is particularly effective if someone has been involved in a building project; perhaps less so if they haven't.

One of my professors gave this example: He had the good fortune of being able to have an addition built on his house. At one point he went in to inspect it and noticed that the contractor had provided far fewer electrical outlets than had been agreed. By this time the drywall had been put in place, so providing the agree upon functionality involved either using electrical conduit (ugly) or ripping out and the patching drywall. The latter was the result, at significant loss to the contractor.

This is a great illustration because a) it's a good example of how a seeminly small detail can cause a lot of problems and b) it puts a money concept to the problem; patching after the fact can be expensive. This is something that managers need if they're going to approve time for "insignificant" things like planning...