Design of building owners' association

How about purchasing the property through you, gifting a share to the son (perhaps the repurchase option could also be integrated as a condition for the gift), and then building together? It should then be treated similarly to a WEG.

Alternatively, there is certainly some way of a rental GbR with special business assets. At that point, I am out regarding advantages or disadvantages and taxation.

In any case, this should be thoroughly worked through with a lawyer and tax advisor.
As an entrepreneur, you surely have a tax advisor at hand.

Then, nonsensically, real estate transfer tax is incurred

Quote: "As a rule, it can be said that no real estate transfer tax has to be paid on disposals to persons who are directly related in a straight line. This applies, for example, to sales from parents to children or from grandparents to grandchildren."

Google: "sale parents to child real estate transfer tax," first search result

Buy the property and include the son immediately in the land register? Then build the house and divide it into 2 condominiums (notary)? We buy land and build and then sell one apartment?

The simplest option is actually to buy the land together and to divide the house to be built in advance according to the WEG. Then you do build together, but each party has their own property and their own financing. This is just informally phrased. For a notary/lawyer, however, it is no problem to draft the contracts accordingly.