In fact it needs to be 'negotiated'. It must be thoroughly discussed so that you know what had to be done, how and why it was not foreseen, what the materials and time involved were and demand it in writing. Even a 'comprehensive' devis may not be entirely correct and it is possible for one piece of work to lead to another unforeseen thing. Given the €300 is actually quite little in terms of working time plus materials, it hardly seems worth the effort charging that much extra. If he can't justify it or itemise extra materials then he may well just walk away.
I agree with Gerald in effect but find that Anthony's attitude is totally bizarre. We know very well that just about every try on is done to French and has nothing to do with 'Brits being walked on' whatsoever. Because we are having quotes for work we cannot do ourselves at present, we are sometimes fighting an uphill battle to get everything included in the estimates. The people coming in forget things, so we are expecting them to have extras if we sign a devis. We know full well that a devis is an estimate and that it is a commitment to use the artisan once signed and agreed but it is not a formal contract. Properly defined it is in two parts:
Le devis descriptif, qui indique en détail la position et les dimensions générales de la construction, les distributions et les dimensions particulières, la nature, la qualité, la quantité et le mode d'emploi des matériaux, et enfin le mode d'exécution des travaux;
Le devis estimatif, qui énonce le prix détaillé de tous les matériaux et de la main-d'œuvre, et se termine par un résumé exprimant le prix total de la construction, si elle est faite à forfait. Après avoir établi les plans et devis, le propriétaire et l'architecte dressent le marché. C'est un acte contenant les clauses et conditions générales, suivant lesquelles le propriétaire et l'entrepreneur qui sera choisi s'engageront, chacun de son côté, à exécuter les travaux conformément aux plans et devis qui demeureront annexés au marché. Les devis et le marché, réunis ensemble, constituent ce qu'on appelle le cahier des charges.
The second part of that legally correct definition specifically explains the fact it is an estimate, forget some of the details like architect, and put it in context. If it is explained and your builder can put it on paper then he could easily take the claim to law and ultimately cost you more. If he is local, he runs the risk that if he is playing silly buggers that you might warn other people, thus cost him work and reputation. It takes a real fool to wade in it up to their neck without doing themselves damage. Thus far on work we have had done we have two things that have been done badly for which we have withheld payment plus a devis at present that is 'loaded' so we cannot trust. All of them non-French artisans (only one is a Brit) and because we use local people who sometimes start out costing a bit more than others not so local, we have had only good quality work to a good standard and have actually experienced the bill cut because the artisan did less days than estimated. So, no the French are not 'like that'.