Costs for the shell construction of an office building

Only office... IT area. However, I cannot fully understand the argument. According to my planning, there are 3 rooms with 20sqm, one with 10sqm + toilet + small storage. The rest is staircase + corridors. Each floor, or theoretically even each room or quite theoretically coworking spaces themselves could be rented separately. The rental income would basically only be a side income and should never be a substantial part of the financing anyway. To calculate this, however, I need numbers, and that's exactly what I'm interested in here.

Rental income would basically only be a side income and should never be a substantial part of the financing.

However, those of the third floor should stand in economic proportion to its construction, I meant.

According to my plan, there are 3 rooms with 20sqm, one with 10sqm + toilet + small storage. The rest is staircase + corridors.

That and your details on the building volumes sounds like plans already existing at least in sketches (?)

Approximate... I'm still optimizing a bit between the rooms. The one next to the WC would be the technical room for the heating system on the ground floor, it might be a bit smaller, but the back wall of it will probably be thicker than a load-bearing wall, as well as one of the cross walls. But at this stage, I'm not worrying too much about the static structure; first of all, it has to be financially feasible. The 2nd upper floor would be the same... Staircase extended up to the 3rd upper floor (initially a walkable roof terrace). In my first ideas, the staircase also had glass cladding, but I have already noticed that this would drastically increase the costs.

first of all a walkable roof terrace

Forget it, because you also said:

first of all it has to be affordable at all

A walkable sealed flat roof is more expensive than glazing the building fully on all corners. Given the already uneconomical size, I only see the popular shed roof cap.

What makes it so expensive? In the calculation, I had already taken 20cm Ytong ceiling elements anyway, they have to carry the load. What makes a flat roof so much more expensive than a shed roof? What interests me above all are initially as hard numbers as possible... especially those missing in the calculation and driving costs. Experience values from people who have already built with Ytong would be particularly interesting.

What makes it so expensive? In the calculation, I had already taken 20cm Ytong ceiling elements, which have to bear the load anyway. What makes a flat roof so much more expensive than a shed roof?

The significantly more complex sealing and the additional reinforced concrete ceiling above the top occupied floor – a beam ceiling is noticeably cheaper. And if it ever becomes leaky, the fault-finding will be really complicated and therefore more expensive.