Hm, I'm afraid if you don't lower your demands significantly or find a much better-paid job, it’s not going to work. What you’re doing is trying to justify it to yourself. You’re already calculating in €100 increments to achieve something. That won’t work.
A reputable financial advisor (like mine in Karlsruhe) would have told you long ago that it’s not going to work. What happens if your basement doesn’t cost 35,000 but rather 60,000 (this price is much more realistic; you only get your price if you do the masonry yourself)? And I promise you, if you start building a house... it will be at least €20,000-30,000 more expensive than what’s stated on the house company’s offer. Because often many items are not included in the contract, unforeseeable costs come up... you have no chance with your plan. And if your bank gives you this loan, they’re already counting on the house. Get away from this bank... Also, a 30-year fixed interest rate is nonsense. You simply pay too much interest. We have 20 years and a financing model by which we can comfortably repay the €250,000 within 20 years.
You talk about special repayments and inflation. How do you want to make special repayments if you’re already maxed out at €1,500? Inheritance... Look at the contract, usually you’re only allowed 5% special repayments. You won’t get a KFW loan at 2.04%, not with your equity.
We got 3.4% at the bank for 20 years with an equity of €150,000. And we compared many banks. Everything you read on advertising sites on the internet is misleading. You only get good interest rates if you only have to finance 60% of the total costs. Because exactly these 60% are risk-free for a bank (they will definitely get the money back in a foreclosure). Everything above that is more expensive. By the way, if you have to go to another bank for additional financing, it will be very difficult or only expensive for you to get a loan, because this bank cannot get any security from you, since the first bank registered the land charge.
I have to say something about inflation: Unfortunately, it does not affect the debt sum... the bank is the last to make concessions. Your debts will not become less. Only your money, which you have to live on, will lose value. Gas prices, cost of living, etc. will take up more and more % of your income.
So honestly: Tear down the house on the property, sell the property, split it half and half with your brother, and buy yourselves a cheap plot further out if necessary. Forget the basement, and forget the gas prices. By the time you’ve paid €50,000 more for gas, we won’t be driving on gas anymore :) With 30 km more travel each way, you can easily fuel the extra demand for 15 years.