From top to bottom
Can you also approach prefabricated house companies for something like this?
Yes
Who would be particularly suitable?
Anyone who feels capable of building you a house will also build you a house that can be used as a dental practice.
Does modular construction make sense?
Possibly
With what costs per sqm must one reckon?
With the usual ones. Whether apartment or commercial, heating and running water as well as sockets and flooring must be included.
Does it make sense to possibly build two stories?
Oops, there I would – as with the idea of commercial use – first refer to the development plan: what is allowed? Is commercial use permitted? How many units are allowed on the property? Can it be divided? Is a two-story building permitted?
Does the business case improve by two rented condominiums on top on the second floor?
Counter question: are you sacrificing your property and the value of your house so that the business case works out?
For the apartments you could presumably set 10€ per sqm, for the practice 14. Can this currently be realized so that it pays off?
Certainly. Pure/fine calculation.
What should one especially keep in mind during planning?
has nicely listed that. I wouldn’t want my dream of living, which once existed, to be spoiled by a business case.
Commercial use on your own property including the disadvantages – you really must have had bad luck to have to accept such a deal.