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I want to practice two extensions on an existing home. One of the wings will arrive last inch on a joint road in joint ownership, a window would be created in the wall overlooking this path. Some companies say that the civil code allows it, others are more reserved... Who can enlighten me on this dilemma?

The regulation on the extensions of existing constructions was modified on January 1st, 2012.
You must make an appointment with the town planning department of your town hall to specify the terms of your future construction and prepare your building permit file. It "carries indeed from 20 to 40 m ² the surface of work for which the building permit is not obligatory, this rule concerning the urban areas.".
In addition, the urban planning code (articles 678 and 679) specifies that a distance of at least 1.90 m from the limit must be respected for a "straight" view (in the axis) and at least 0.60 m for an oblique view (article 679). Not to mention that the boundary on the edge of property is governed by strict regulations (Article R.111-19).
But specific planning rules may exist, different from one municipality to another. I advise you to get closer to the planning department of your town.

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