Sell an apartment from a two-family house immediately after completion

Maybe he just expressed himself poorly? More like in the direction of "wants to afford." If he can get the two-family house project off the ground (meaning the bank says yes), then he can "afford" it. And it's certainly economically sensible not to build just a single-family house on the expensive plot. Maybe he also thinks that if he bears the construction risk and sells one apartment upon completion, his own apartment will be cheaper because there will be a small markup on the construction costs. I don't find any of that objectionable.

Don’t always immediately pull out the "you can’t afford it, just forget it" card.

Planning it as a two-family house isn't that stupid after all. That way you have different options for what to do with it. As a luxury single-family house, as an owner-occupied apartment of the two units, or as a landlord of one apartment … or a sauna club (just kidding).

Man, man, man… there's a knot in the head - what you can't afford, you don't build.

Renting is the magic word. But I agree with the predecessors. You have to be able to afford it. When the air no longer whistles out of your pants, you just have to tighten your belt and buy an apartment

I am confused by the wording. Sounds like you want to sell a granny flat.

What do you want to build now? A house with a granny flat? Two semi-detached houses? Something else?

Is the property going to be divided? How much % of the garden does each get, or do you actually only want to sell the apartment and create a condominium ownership community or whatever it’s called?

Note that after sale and resale/renting to outsiders, unwanted people can live there after a very short time. That seems important to you.

But the idea would be to sell 2 condominiums, the lower one including the garden, and only keep the upper one for myself. Then I can pay off my apartment quite quickly.