What loan amount is realistic for house construction?

If I understood correctly, he wants to officially or on paper, for the bank, notary, mortgage, and co., to go ahead only with his net salary of 2xxx €? But he plans to secretly include his girlfriend's salary in the form of a home savings contract and whatever else.

With his salary alone, it won't work out; with both salaries, I find it tight in this size and constellation.

And what happens if children come and the woman/girlfriend works only part-time or not at all anymore?

So first sort out family circumstances, and then approach the house construction topic somewhat more modestly.

Again, if the girlfriend pays but is not in the land register, she is stupid.

Again, if the girlfriend pays but is not listed in the land register, she is stupid

I'm not denying that at all.

That the girlfriend does not go on the land register but helps with the loan is something I would urgently want to avoid. Otherwise, later on, she will be in the same position as her parents. It will take a long time until the loan is paid off - and even then, an owner-occupied house is protected. Please make sure to inform yourselves beforehand about what assets are protected regarding [Elternunterhalt] - she does not need to panic about owning anything!

why, she can also make a corresponding financial contribution for her right to live there. Otherwise, she would have to pay rent and she wouldn't get into the land register either. In return, however, she also has no financial obligations if the loan goes wrong. Just speculative, in case the bank approves the project (if not today, then maybe later with a higher salary and equity). How high this financial contribution is and how to do it correctly for tax purposes is another question. But banks generally like a second borrower, even if that person earns nothing (it was the same for us when I had no income but still had to be included in the building financing). Nevertheless, in this case I would probably start smaller, with a condominium or an existing house. You can rent it out later or just sell it.

The plan is a single-family house of 150 sqm with a spacious garage of 70-80 sqm with an attic as storage space - no basement.

We have already had garages here that were almost the footprint size of the main building. Here it is similar... you really have to see whether the garage is supposed to be subordinate according to the development plan. At least the idea is quite daring when you hardly have any equity and everything else is also rather tight.

If you have debts, you have no assets. You should first consult a lawyer to see what makes sense. As a woman, I certainly wouldn’t take out a loan without a land charge registration and invest it in the house.

That fits roughly if about 1400 are deducted from my 2350 – then I would have to get a loan of almost 1000 euros alone – and my girlfriend is supposed to stay out – if the house were ever legally counted, it is her asset and she would then possibly have to partially give it up – we never want to consider/come into that situation.

Regarding your calculation:
2350 - 1400 living expenses results in a surplus of 950, not 1000. Then you would have 950 for the loan installment, but still no 400€ for additional costs.
If your plot costs 100,000, your house 320,000, you have incidental construction costs of 30,000 and outdoor facilities of 20,000, you are at
= 470,000.
– equity 20,000
Remaining: 450,000€!
But for 950€ you won’t get financed for 450,000€.
Period.

For the bank, a salary of about 2400€ is barely good for a loan of 200,000€ or so.
With a >400,000€ case, they definitely won’t accept one borrower.

Do you find the project feasible together with my girlfriend?

Probably not.

I realized for myself 4 years ago: building a house doesn’t work below 4000€ salary. Exceptions due to equity, housing allowance, and others prove the rule.

We have chased him away