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€.