Before you start discussing your plans with artisan builders you need to be absolutely clear yourself about your plans. Study them until you are familiar with every detail. Whenever possible you should follow the plans in every detail.
This is not because I think the local planning authorities will demand that your property be demolished because you have put a sink in the wrong place, rather it is because as soon as you allow one change that can escalate into misunderstandings between the different workers involved, disputes, and an uncontrollable increase in your costs.
Overly dramatic? Assume that the plumber has a good idea about using a wider shower tray than you had planned, and at the same price. Why not, you may think, and agree with him. As an absolute minimum you now need to tell the maÃ§on (the drainage pipes may need relocating before the concrete floor is laid), the plasterer who is constructing internal walls since these will contain the water pipes, the electrician (there are detailed rules about lights and electrical fittings not being permitted within a certain distance of a shower) and the tiler. All to be informed in writing, with copies of the revised plans. There may be other workers to inform as well, this is just an example.
If the mason says no, it is not possible, you now have to retract your change in the same way. With problems of language thrown in to the confusion as well, it can very quickly become a project that is impossibly complicated.
Now you are ready with you pile of plans and a clear mental picture of how the finished building will look. Let the party commence. It is now time to get the estimates (devis as they are known in French). It is usual to get the estimates during the (typically 2-3) months that you are waiting for the planning permission to be approved – this will save a lot of time afterwards.