Living in and renting out DH: Please share opinions/experiences

On the phone, the lady from Okal Haus quoted €330K for a single-family house at the turnkey completion stage

I once visited the Okal Haus page: the base plate is included. But there is no turnkey completion, only ready to move in…
Then I also came across:

I’m not assuming you want to build only 100 sqm. If I assume 160 sqm, that means prices of about €2200 per sqm – in Bavaria that definitely is not enough. Except maybe in the Hof region.

That would result in a 130 sqm house in premium quality ;), at least that’s how Okal Haus advertises it.
Plan A, given the income bracket, does not include a granny flat or a rental house or anything similar.

I will take a look at the topic of short selling over the weekend.

In my estimation, you won't need that here, since this is a different scheme – as I said, the keyword "real estate agent" makes me think of another model: suppose you fell for the proposal (though there would still have to be a sly loan broker around the corner who does a bit of risk cosmetic towards the bank). Then you buy the plot of land and order the duplex, which unfortunately turns out to be more expensive than expected. You want to provide additional funding; the bank reassesses the whole thing more thoroughly and unfortunately has to say no. The dream shatters; you then have an unfinished house, which would fetch correspondingly little at the auction. I would almost bet that the real estate agent then knows someone who has a solution to "save" you ;-)

On the phone, the nice lady from Okal Haus said 330K€ for a single-family house turnkey finish. I somehow knew that was a teaser offer, but plus 150K€ ... tense as a bowstring.

But all that was already listed here: purchase incidental costs, additional construction costs (earthworks, for example), garage, outdoor area (terrace, driveway), kitchen, the step from turnkey to ready-to-move-in (floors, painting), upgrades for fancier bathroom furniture and tiles... all not included and costs extra. And yes, very quickly a six-figure sum is added on top of your 330K for the house.