Land and home financing - together or separately?

At the moment, you "only" have 5k net, right? And I would initially go into the race with not much more than that. Who knows what the future holds for you. I wouldn't rely on your income increasing significantly or the house construction prices not rising sharply again or inflation throwing a wrench in your plans or.... I would accordingly adjust the installment (and also the total budget) so that you could cover it with your current income if necessary.

Based on my current experience, I would finance the house and land together and let go of the handbrake when planning the house, especially if you want to include a WI Bank loan and the KfW. Regarding the KfW, note that not all banks offer these components. I can't say anything about reasonable installment rates – but with too many components, the chance is simply greater that something won't work out as planned, and then things get messy because the alternatives are more expensive in the short term... Also, read the conditions of the KfW components carefully; KfW40 with QNG is a really risky tightrope walk – I would only attempt that with turnkey including floors/painting. Do you have family nearby who can help with the kids? Otherwise, it will be tight with your wife's full-time job...

is probably your real contact here :)

How do you want to pay the installments of approx. 3 k?

The bank won't get involved with "in 3 years I'll definitely earn 100k gross" anyway.

The bank won't agree anyway to "in 3 years I'll definitely earn 100k gross."

Exception probably: It is already contractually fixed (e.g. salaried employee with a development plan). Then one can certainly count on that.

Yes, then that may be accurate.

If construction only begins in 2 or 3 years, one must also expect further rising prices, as well as an interest surcharge to extend the interest-free period on the provision.

It can definitely work with the net amount, but with a few square meters less, it would of course be easier.