Financing check - realistic or nonsense?

Megalomaniac - that's exactly what I'm aiming for, to get a feel for what is megalomaniac and what is not ;)

Two years ago I said: with a salary under €4000, you don’t plan a single-family house, unless you have over €200,000 in equity or you already have land and equity or something similar. It’s always hard to give a flat number. Today I say: with less than €4500 you don’t plan a single-family house.
And that’s roughly accurate.
Exclude a granny flat, 140-160 sqm on two levels (ground floor and basement), then inquire at the bank. The problem with that amount will be individual financing. The bank will require two salaries as security.
May I ask where your wife runs the dog breeding? Outside the home? Is it planned on the large property?

With your income, I would rather look at existing properties.

We did that for half a year, it's actually even worse..

Doesn't suit me at all. Sure, the land is extremely cheap. But that alone will surely cause the project to fail. The banks currently do not seem to view it positively at all. It's supposed to be a luxury house out in the middle of nowhere. A clear negative for resale value. You'll have to search long for a loan, if you even get one – the income is far too low for the project.

Then there's the thing with the two residential units – is there another reason for that besides getting Kfw funding? Even with your optimistic rate of €1700, you're beyond what is financially healthy. And there are still children to come...

We did that for half a year, it's actually even worse..

That is not a significant amount of time at all. You should really plan for 2-3 years.

Megalomaniac - that's exactly what I'm trying to get a feel for, what is megalomaniac and what isn't

I do find it megalomaniac to build such a huge project with your salary. After all, it seems to be an extremely cheap area. €55/m²... wow.

We want a two-family house (single-family house with approx. 40 sqm granny flat for later accommodation, possibly for parents in need of care)

Are the parents really close to needing care? I get the feeling you only want the granny flat so you can get the subsidy a second time. Leave it out and plan more realistically. Unfortunately, your fixed salary does not allow for such playfulness.

By the way, your calculation has the mistake that you claimed the repayment grant twice. Once as equity, then again at the end of the fixed interest period:

usable equity €150,000
KfW repayment grant – 2x KfW 40 Plus €75,000
[..]
Remaining debt at the end of the fixed interest period €65,800 (KfW, after 10 years, after deduction of repayment grant still €28,300)

We did that for half a year, it's even worse..

Do you think a new build is cheaper than an existing property?

We did that for half a year, it's actually even worse..

Half a year is not long. And if the inventory is even worse, that is also a sign to me that the planned costs are too low. As much as I’m sorry. Why the granny flat? Why Kfw 40+? Those are gimmicks, not necessities.

As I said, basement as bedrooms, utility room/technical room, that works perfectly.