I was only trying to expand on the article with an example, the article is quite good in evidencing the common reasons why. In theory every single price is est

I was only trying to expand on the article with an example, the article is quite good in evidencing the common reasons why. In theory every single price is estimated analitically adding to market costs for materials and labor a percentage (usually between 13 and 17 %) and on this total another percentage for the profit of the contractor (usually 10 %)[1]. The quantities are calculated geometrically. So in theory the estimation quantities by price should be exact or within a very small approximation to reality. But in practice this sometimes (often) doesn't happen, the main reasons (cited in the article) are time passed since the estimation, optimism about ground/soil conditions, use of "wrong" price lists (and more generally inexperience of the estimator or Law/bureaucratic restrictions), optimism about weather, optimism about interferences with other infrastructures, change of building code, added features, actual mistakes in the project or in the contract, the list is long, but all the items in it tend to be "negative" ones implying more time and money needed for the construction. Of course there are also many projects that are fine, properly estimated and that are built in time and within budget but essentially all the estimation, even if done in good faith and by experienced technicians, are predictions (and predictions about the future, the most difficult ones). A better, more accurate process during designing and estimating (the 3 to 5% talked in the article) might help, but likely it would be not enough or reduce the incertainty by the same 3-5% that is spent for it. To all the above you add what is a sort of the S.E.P. (someone else's problem) syndrome, the one(s) that make the project and the estimation, by the time the site is ready to start are already elsewhere, in another job or another project, it is not unlike what happens nowadays in many industries, a new CEO is chosen, he does some cuts on costs, changes something in the products, and after two years, by the time the effects of the changes become actually evident he leaves and someone else starts again. [1] it depends on countries, but usually what are used are pre-made price lists that are a (supposed) average for a given work and are either outdated or do not apply to the specific work