I have also thought about an alternative because flat roofs (except for the garage) are not allowed according to our development plan. What I wonder is whether you can get approval for your flat roof as a large roof terrace and only plan a small upper floor with, for example, a hipped roof (see attachment). As far as I know, a flat roof is no longer considered a flat roof once a railing of about 90-100 cm is installed all around. Then it is considered a roof terrace and would therefore be allowed, provided that the roof required by the development plan is also present.
In my attached design, a shallow hipped roof with about 15° (< pitch angle allowed for us) and a small roof overhang (20 cm) is planned on the 65 sqm upper floor. Above that, there is the 60 sqm L-shaped roof terrace with a built-up "railing".
For us, these are just initial ideas and financially probably not feasible. But maybe it's a good approach to still achieve a fairly modern house with cubic elements and some flat roof character, maybe it can serve as an inspiration
