How much does it cost to buy or build a house?

The formula doesn’t work. It may be true for low wages, but 25% is not enough for high earners.

Just because you have a great salary doesn’t mean you are "forced" to build/buy pompously and spend 40% of your net income.

That is a misconception. We pay exactly 18% with 6k net.

The finances could fit so well. But ultimately, the bank decides.

I would also think carefully about the situation with your girlfriend. Is she really the woman for the future or are you still very uncertain about it? Always keep in mind (including her) that she gets absolutely nothing if the house belongs to you, she becomes pregnant, and for example, you pass away. Drastic example, but you should think about things like that if you are heading towards family planning. Look at it this way: She goes into the loan with you. That saves you money over a certain period. She pays off part of it. If then the relationship breaks down, you buy her share from her and ultimately pay (roughly, as the conditions certainly won’t be as good) what you would have paid without her anyway.

just because you have a great salary doesn't mean you are "forced" to build/buy pompously and spend 40% of your net income.

That is a misconception. We pay exactly 18% on 6k net

But then you don't live in southern Germany, because there we're talking about about 1000 euros. With that you can't even get an apartment here.

You don't live in southern Germany then, because there we're talking about about 1000 euros. You can't even get an apartment here for that.

Stop always acting as if the south is the be-all and end-all.

I live in the Rhine-Main area, we also have absurd prices here. A 60 sqm shoebox for 1400 euros rent including utilities.

I live about 60 km away with great connections. Many who build or buy here are commuters. It used to be said: Money is earned in Frankfurt, affordable building is done at home. Meanwhile, prices are high here too because everyone wants to move out. Still, with a good 6k net income, you don't have to build a 650k house. You can, but you don't have to.

We have a new apartment and allow ourselves four vacations a year.

The formula doesn't work. That might be true for low wages, but for high earners, 25% is not enough.

So the absolute expenses for the house are roughly proportional to the square of the net income? Whoa, bold claim. I rather don't believe that. We pay quite a bit less than 25% of our net income for the house. You can also buy/build a half-million house and land without becoming depressed afterwards from too many compromises.

The OP can certainly spend more than 25% of their income to service a loan – because they are single and can plan. With the available equity, the project will probably be realizable with about 400k in loans, so around 1,500 euros in installments. However, that would be too large a portion of my salary, but that depends on personal priorities. Priorities can change, though, e.g. due to partnership and children, so the OP should at least carefully consider what the family planning might look like.

You have to consider two things. First, the basic expenses are always similar, which means that all income above that is freely available. Usually, building is done differently then.

Second, the house payment also includes some savings, which are lost when renting. Therefore, only interest plus maintenance should be seen as lost.

And for a single-family house in the south, you pay between 1500 and 2500 cold rent. A house payment of 6,000 net with 25% would be very tight there.