Costs for partial extension and flat roof with roof terrace, any experiences?

If this is going to be a single-family house, life usually takes place in the garden or on the terrace in our region. Roof terraces are more common for condominiums. Or is this supposed to be a multi-family house? The roof terrace will probably be rentable for 8 or 4 euros instead.

Is the sketch from the architect?

Certainly from the architect, but a very long time ago.
The plot is cheaper to purchase due to its peculiarities than would usually be the case in a very good area,
this is because there is no garden and the back house has a right of way, meaning there is a house behind the back house.
There is only one parking space and a strip about 2m wide leading to the path that goes to the back house.

I want to use it myself and can live well with about 100 sqm living space, which will hardly be visible.
I would prefer a garden as well, but apparently I cannot afford that in the region where I live.
I would be very happy if it works out as planned, really happy.

The property is apparently adjacent on one side, and the floor plan states "office". Have you confirmed that a residential conversion would be permitted?

The usage is of course already clarified.

If you have the approval in writing, you can go to a carpentry shop with the plan and ask what it costs. Without a plan, no calculation is possible.

Here is also a study for a floor plan for an unusually long and narrow rear building.

In my opinion unnecessarily continued here: