Build a new house, is the financial framework possible?

"Overall, we did not want to exceed €400,000 for the plot, house, ancillary building costs, etc. However, we would like KFW40/40+ with an air/water heat pump."

That was good

They are just starting, so give them a chance. How strong is the relationship (duration), and is there any family planning?

400,000 euros for land and a KFW40 house is not possible. If you can do a lot yourself, you do get closer to it, but it's still hardly conceivable. You don't have the most expensive plots where you want to build (Jüchen / Grevenbroich), but 100,000 euros cost 350-400m² already. (Highly variable)

400,000 minus 42,000 equity is 358,000 financing amount. We earn exactly the same as you (4,000 euros net) and finance 320,000 euros with a 1,250 euro installment at 2.5% over 20 years. That is the absolute minimum, I think.

I would ignore KfW......then you would get closer to it.

I have an offer from a major prefabricated house provider here:

145 sqm 1 1/2 floors, KFW40 including base slab (30cm excavation), move-in ready:

215,000€ + 11,000€ flat rate for sample selection
+ 9,500€ single garage
+ 33,500€ plot (570 sqm)
+ 40,000€ incidental costs (outdoor area only 10,000€, the essentials in EL)

==
309,000€ - 4,500€ Bafa subsidy

I have informed myself in a corresponding builders' group and they all basically came more or less to these prices, of course the earthworks can still hit hard additionally, but that risk always exists...

I would at least not call it impossible to build a KFW40 house for 400,000€.

But we are talking about a region where the sqm costs at best 200 euros, realistically rather 250 euros, and if the location is really nice (not top!), then from 300+ euros. The plot alone then costs not 33 TEUR (+ENK) but suddenly 110k (500sqm×200) or 190k (570sqm×300).

And you should educate yourself on what move-in ready means (which incidentally is not clearly defined). 40k additional costs including exterior facilities (just the essentials) is also rather on the lower end of what is realistic.

But we are talking about a region where the price per sqm is at best 200 euros, realistically rather 250 euros, and if the location is really nice (not top!), then from 300+ euros. Just the land then costs not 33k (+ENK) but suddenly 110k (500sqm×200) or 190k (570sqm×300).

And you should educate yourself about what move-in ready means (which is not clearly defined, by the way). 40k incidental costs including outdoor facilities (just the essentials) is also rather on the lower end of what is realistic.

At this provider, move-in ready really means move-in ready, including floor coverings, painting, and so on. Of course, the base price only covers laminate, and partially even carpet.

But you can google all the surcharge costs beforehand. The furnishing catalogs are freely accessible online.

I consider 40k ancillary construction costs realistic after my research. If you add 9.5k for the garage and 11k for the furnishing flat fee, it's almost 60k...

Sure, our plot at 60€/sqm is already cheap, but in general I was annoyed by the blanket statement...

I would at least not call it impossible to build a KFW40 house for €400,000.

I also consider that realistic, but of course it also depends on the plot of land. In principle, you could say, buy an inexpensive plot under €100k and you'll make it...