Single-family house: Is the rate realistic? How much house can we afford?

Cheers boss. :)
(After a quick calculation, 2500€ is definitely too much for us. 3000€ for you I still find very doable, and 2500€ even more so.)

Do you already have the equity or does it still need to be saved up?

Everything is so vague with you. Starting with the Grundstück

The only risk is the development of the building area. Normally, the municipality buys all the plots, plans and builds the roads and canals, and then sells the building plots individually. Those who sold a plot beforehand are allowed to choose a building plot first. Development costs are offset against the sale price. Since the municipality has to pre-finance this, there is a risk that it will take a little longer. Especially if there are enough other building plots available. A senior civil servant, however, gets everything financed.

I don't see that you consider falling income due to parental leave and subsequent part-time work. Both full-time with children is something you have to want and very few follow through

I have thought about it - however, the above plan already assumes a 30-hour week, which my fiancée also wants to resume. Nothing will change with my full-time job, simply because I don’t have fixed working hours anyway and am overall maximally flexible.

Honestly, I had thought that the maybe 2 years of lower income for my fiancée wouldn't weigh that heavily when you consider the 25-30 years of repayment. Sort of along the lines of: "Then we just don’t go on vacation during that phase and the like - we’ll manage!"

But maybe I’m wrong. Is there actually the possibility to change installments afterward? For example, I could well imagine starting with €2,500, dropping to €2,000 during the "birth phase" of the two children, and then going back up again when my wife returns to work (and I also have further fixed salary increases coming).

Is there actually a possibility to change installments afterwards?

It is stipulated in the contract, but yes, you can have something like that offered to you (meaning, if you consider it foreseeable, you should make sure to also request this with the bank). The keyword there is "free repayment rate change," I believe. Of course, they still expect a minimum repayment, and often it is limited to a few times.

It is specified in the contract, but yes, you can have something like that offered to you

Yep, that's correct.

The keyword here is "free repayment rate change"

The repayment rate change is not free. You pay for it through an interest surcharge over the entire fixed interest period. So it is actually a very expensive repayment rate change. A €500,000 loan with a surcharge of 0.05% (can also be somewhat less or significantly more) over 20 years fixed interest is over €2,000 price for this option. With some banks directly included, with others separately stated. But never free ;).

Expect a minimum repayment

Most take 2% as a lower limit, very occasionally (countable on one hand) it goes down to 1%.