In practice, budget overruns are far more common than we would like.

The first reason concerns the quality of quotations prepared by contractors. Many initial estimates do not take into account all the technical constraints that will appear during construction. Some expenses are simply discovered as the work progresses.

The second reason is insufficient analysis of the project. Even when a detailed design exists, some contractors do not study it carefully enough before preparing their quotation. Work specified in the plans can therefore be forgotten or underestimated.

There is also a commercial reality: some companies try to present a more attractive price than their competitors. Part of the real cost then appears later, once construction has already begun.

Finally, renovation always contains an element of uncertainty, particularly in older properties. Opening walls, floors or ceilings can reveal technical problems that were impossible to identify beforehand.

A detailed project and a carefully prepared quotation cannot eliminate every unforeseen expense, but they make it possible to identify more costs before work begins and considerably reduce the risk of unpleasant surprises.