Build a house or buy one - financing possible?

My wife spoke on the phone with the tax office in D, with the gentleman responsible for cross-border commuters. There, she mentioned my basic salary (the one from the initial post) because all my allowances are tax-free. I only receive the basic salary if I don’t work at all in a month; with work, I earn an average of around 5800 euros, sometimes only 5000 euros in winter, but we have actually never had problems with our 2000-euro rent, even then there was always some left over. I would pay a maximum of 800 euros in taxes; currently, I pay 550 euros in taxes in Switzerland. However, health insurance in D is much cheaper; we would save 100 euros per month, and the car insurance would save another 50 euros per month. I do not have additional fuel costs; the distance is the same.

At the moment, after all expenses and fixed costs, we have about 1700 euros left per month, which we could theoretically also save.

I am more concerned about unforeseen costs with a prefab house! What do you think would be a reasonable margin to allow for? My wife has now looked at some, and with 300,000 euros we would be fine. The plot costs a maximum of 80,000 euros, rather less.

@shism :My wife is a housewife and receives childcare allowance + family allowance + child benefit (1300 euros). Since this is limited in time, I did not include it, it also does not count for the tax office. After that, she will remain a housewife, so the taxes are probably that low.

Thanks to the great German tax law, you will get 100 different opinions from 100 people. The only way I see for you is to go to a tax advisor who specializes in cross-border commuters. Because even normal tax advisors don't understand this. After all, it's about two countries that aren't even in the same trading zone, and yet have the same currency.

Before you sign anything, you must have 100% certainty about how your salary will improve/deteriorate.

We would have had a tax advantage as cross-border commuters to France and would have saved about €5000 annually. But unfortunately, there were no suitable properties in Alsace.
So get detailed information before taking this step. And also calculate additional fuel costs. Also remember that you will spend most of the time in Switzerland, and therefore have different costs than going to the cafeteria in Germany at noon. Such things add up too. But as someone who now works in Switzerland, you know this better than I do anyway. I was just shocked when I was supposed to pay €20 for my lunch recently in the cafeteria in Zurich :)

We will definitely consult a tax advisor, there are so many things you can deduct from taxes. We just don't know enough about it. But at least we now know the "worst case."
Luckily, I don't have to go to the cafeteria either, my employer provides my meals. I have no expenses during my working hours.
We gave up eating out in Switzerland a long time ago ;)

Oh, I actually always go to Tibits :) In Winterthur and Zurich... :) Everything else is not included in the expense report :D

I am not quite sure whether you earn 4600, 5000, or 5800€ or where which deductions/taxes etc. have already been taken off...

With 5000€ you would have to pay about 800€ tax in Germany with tax class 3. Additionally, unemployment and pension contributions are due in Switzerland (I do not know the exact amounts).

Where you end up exactly or what you additionally spend on health insurance, you have to calculate based on your specific numbers.