French architect

Just mention the two Velux windows by saying "Création de 2 fenêtres de toit, dimensions 78x98(or else)" .

It's definitely 2. (kitchens, bathrooms, corridors aren't considered as main rooms) Aurélie

Will is not completely right when he says that plans are not normally needed for a CU. You need to join a plan but the "plan de cadastre" may be enough, so it's no use paying an architect for this! Anyway, it's a bit strange to ask for a CU while the house is already built on the site. The clause suspensive is usually about obtaining a "Permis de construire" rather than a CU. But anyway, if you need this CU, the DDE has 2 months to grant it. After 4 months without news you're allowed to do a "recours" against French administration. For the "permis de construire", as Will said, you don't need an architect unless the house is bigger than 170m². But, if you want some new ideas and that you don't want to spend too much time on the plans, an architect will be a good solution. But the price of  5000 euros you've been given is not completely right: as for me, I take 3% of the cost of the building works for a new project and 4% for a restoration. Which means that if you want to spend 50 000€ in the restoration of your house, the "permis de construire" will cost you 2000€. And if you want to build a new house of 100 000€ next to the first one, the "permis de construire" will cost you 3000€.

Artisans usually ask clients to pay 30% on prior to commencing work. But, in my job (architect), I always advise my clients to pay each month each artisan for the work he has done. The artisans are used to this, because most architects do so. Anyway we work with artisans that we well know and they trust us as much as we trust them. The law says that the legal up-front payment is 10% for anything (not especially for building).

No, you just need to list the materials that will be seen from outside.

That is not true! The mairie should better re-read the French law! Anyway, you have to do as you're told, because the mairie has the power to give you or not the authorization. Thus, you can make a full "permis de construire" or ask the mairie the right distance between your new garage and your house. Anyway between two buildings on the same site, you must respect a distance of 4 meters at least, sometimes more it depends on what the POS says (plan d'occupation des sols). The documents to join to your demand are about the same (plans, elevations, photos), but the form is not the same. The mairie has 2 months to judge a "permis de construire", and 1 month for a "declaration de travaux", and one more month if the building is located close to a historic monument. So the differences are not so big and I advise you to make a full permis.