Winter garden as a balcony on the roof

We have bought a house, into which we will move in about a year. Now the question is that we want to build a balcony as a conservatory (~3x3 meters) on the roof, which is in front of the 2 windows that are next to each other, so everything made of glass, which should be able to open wide. The question now is, how is this possible? Do you need a building permit for this? What would it approximately cost? Thank you very much

Regarding the building permit: it varies depending on the federal state, just give the responsible building regulations office a call. That is then also binding! And it's not just a conservatory alone: it's about the floor (insulation), possibly heating, etc. A conservatory of that size, good quality and depending on the equipment costs around 40k. Shading is also an issue and possibly a building permit. We lack the information for that.