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I am in full period of 7 days of retraction for the acquisition of a house. By doing a counter visit of this house with a handyman friend, by turning a ventilation grid it is seen that there was at the time an extension and that this realization had been done in prefabricated. What to do to change that? And at what cost for a house of 70 m2?

The purchase of a house is a commitment important enough that it always involves a scrupulous verification of the property. What you describe here is not very clear and without going there, it is impossible to give an opinion. I advise you, if you had a crush on this house, to make a new visit with a professional building (architect, entrepreneur...). He will be able to advise you and establish a precise estimate on the work to be undertaken. Noted that "prefabricated" does not necessarily mean "bad quality"...

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