Build the ground floor now - upper floor possibly later

As a complete layman in construction, I asked myself what additional costs would arise if only the ground floor is built initially and the upper floor is retrofitted later.

The situation is as follows: A couple wants to build but does not know whether they want children or if having children is even possible. The idea is to plan the house completely with the upper floor (static structure, wiring, etc.) but initially build only the ground floor with a flat roof. If it becomes clear that they will have children, an additional floor will be added.

As a layman, I imagine that later the flat roof would be removed and leveled to form the ceiling of the upper floor. Accordingly, the stairwell has to be planned from the start; I see difficulties with adjusting the heating system – it would likely be undersized and would have to be replaced. Wiring and such can be laid in advance since the floor plan for the ground and upper floors already exists.

Is it really as simple as the layman imagines?
What additional costs can roughly be expected compared to building the upper floor directly?

I believe the additional financial expenditure for such an action is disproportionate. Better to build 2 completed floors and rent out 1 upper floor.

A flat roof may not be the first choice. Perhaps a shallow gable roof, then dismantle the roof frame and rebuild it on the upper floor. But everything must be well planned, including the structural calculations, etc.

Probably completely uneconomical just due to the logistical effort. Setting up the construction site twice. The house is uninhabitable during the renovation. All trades have to come twice and charge for it. Permits must certainly be obtained for both stages, including the creation of all necessary documents, including statics and energy certificates. Additionally, the costs for building and dismantling a roof, which alone is certainly 50k.

... A new construction means: obtaining a building permit and having a raw construction site again. Forget the naive idea. You can plan a low-pitched gable roof where two cozy and sufficient rooms and a bathroom fit. You don't have to finish that right away. Or you do it later or turn it immediately into an all-round room.

Thank you very much for the answers.

We would certainly be willing to pay a certain extra price for that. However, I do not want to rent out any part of the house, neither the ground floor (then I would miss the garden or have to share it) nor the upper floor, because then people would be walking over my head again. Apart from that, you never know how tenants will turn out ;)

Yes, two building permits would be required.

I think we will continue with the option you, ypg, suggested, not to expand the upper floor or attic. Unless the BU can give us a relatively precise estimate of the additional costs and the amount suits us.