I checked the french regs very carefully about constructing a small greenhouse (just under 5m2 footprint) using recycled materials from a porch (the porch demolition is part of a wider scope of works to the house for which we had to obtain full permissions).
I was advised that sticking within the 5m2 square footprint would not add to our calculable area for tax.
I went to the Mairie, and had a very helpful discussion with the administrator, who advised me that although I could get away with it, it was much better to be open about it, lodge a “déclaration préalable” and wait a month for deemed permission.
The catch 22 mistake that I am told that many make it seems is to fill in the last section, entitled “Déclaration des éléments nécessaires au calcul des impositions pour les déclarations préalables”. If you put a changed area into that part of the form, you are likely to be pursued later for additional tax.
I double checked this at the Mairie when I took the forms back in. So for the first page of the “calcul des impositions” I left the area statements 1.2 unfilled, completed: section 1.2.2 about receipt of grant aid; 1.4 about no archaeological works; 1.5 about no particular circumstances; and signed and dated at the bottom.
Clearly if it is a structure that actually does add to the overall area for calculating tax obligations it should be declared under 1.2!
The bit that I think is more involved (and hassle for such a small structure, in the middle of our plot - screened by an orchard,) was preparing the: location plan (DP1); dimensioned plan including heights (DP2); a contextual image (DP6); and marked up photos from each neighbours viewpoint and the road - I took photos from each boundary, printed them and marked them up with the sketched in structure, showing on the same page the viewpoint on plan, the before image, and the sketched in image (DP7).