The problem is the Plumbers usually have no say in the design.
Its usually a case of "Here are the comics, follow them!"
I have worked on contracts, where it was obvious things would not work out, but been told to carry on, until a variation order came through. Then the company of course could get paid for taking out the faulty installation and putting in a new one.
Apparently its the same in the navy ship yards, as technology progresses, they update the specification all the time.
Also I often wonder who does the estimating nowadays. The Scottish Parliament building was only supposed to cost about £200 million it came out nearer a Billion.
You could probably go on and on.
But failing to get to within about 5% of final cost does seem to bring into question of how good planning and estimating is. To a certain degree you can understand on a navy ship, how can you estimate for a job when you don't know what it may be?
But a building should be easy.