How much house is in it?

I find your RV with 120 euros very meager. In general, I could imagine that you have forgotten a few points in the expenses. Vacations? Purchases of furniture/appliances? Tax back payments? Car purchase, fuel, repairs?

If I were you, I would set aside the currently calculated surplus for one year. After just a few months, you will realize what is actually realistic and can start planning. That will significantly boost your equity, and you will reach 10-20% by the start of construction, and it gives you the security that you can afford it.
Do not budget house incidental costs and reserves too tight.

With your very high salaries, I would leave 1500-2000 euros unplanned despite secure jobs. Otherwise, it will hit you hard if one of you becomes unemployed or sick or unable to work, or if you are temporarily or long-term partially unable to work due to one child or another.

600,000 I find very decent. In some statements, one can see how far the expectations differ. We built 150 sqm for 270,000 euros (excluding land). Simple and without much fuss, but we do NOT miss ANYTHING, even though we are not undemanding people.
So it is possible, even if many do not want to admit that.

If marriage is soon, will children also follow? Yes, I know the SAP industry. However, even there, sometimes the energy runs out and you have to decide to become self-employed to really earn a lot of money. Partly €1,500 daily rates. So if children and part-time work come, you will have to stretch yourselves very, very, very much. It won't fit then.

The large item of the car is missing: fuel, spare parts, etc. estimated at a flat rate of €500/month. The banks calculate with flat rates per person. A big factor is always the planning of children, which affects income and expenses. If you have not been with the employer for that long, it is also not an advantage. Just make an appointment, that’s what they are there for. You are not signing anything yet, but you can plan a little better. Surely the bank will also suggest saving for 2 years first.

Thank you all.
Yes, the thing with children... Of course the bank assumes the worst case will happen and eventually a child will come. No matter how much you insist that you are not a "people person" when it comes to kids. For us, it’s really like this: all acquaintances are having children and I already get cold sores at the thought that one of them might come up with the idea that I could be a nanny for a weekend.

Does the bank generally calculate 500€/month for the car? Just so you know, I don’t drive a Ferrari and regularly go to the service (so the transmission suddenly breaking is rather unlikely; I have learned), I pay not even 200€ a month for fuel and insurance is under 100€ per quarter. Per quarter. How am I supposed to come up with 500€ per month for reserves? Do they expect me to have a total loss every year? Maybe it makes sense to mention that only I own a car between us. The gentleman takes the train.

And I keep reading that our income is supposed to be very low. Well, I think that 6,000€ household income per month net with both of us in tax class I is by no means little. How do a master painter (who doesn’t exactly own a well-running business) or a simple bank clerk (who doesn’t exactly sell stocks to sheikhs from Dubai) or a plain car mechanic manage? Please don’t get me wrong, I don’t see this as an attack in any way. I’m just very puzzled as to what counts as enough salary to even think about building a house? Especially if you studied for a long time (doctors/lawyers?) or only got a “good” job after some time, you can’t start building a house at age 40. Then you’re still paying it off during retirement. And if you haven’t saved properly by then, it gets really tight. Isn’t that a bigger risk for the bank? Or does it go by the motto “by then I won’t be working as a bank advisor anymore, so it won’t shock me when they declare personal bankruptcy”?

And about the 600,000€ - that was initially just the rough wish we had (including land, house, outdoor facilities). Depending on the cost of the land, maybe less. For the house, we calculated pure construction costs of 350,000€. That’s only so high because I have the dream of the plus-energy house... Originally, it was less about whether we could actually realize that, but more about what amount a bank would roughly grant based on your experience and whether we’re way off.

We will definitely make appointments with financial advisors and get advice anyway. Otherwise, I’ll just look at catalogs for 2 years, I really have no problem with that. It’s also interesting to watch what the federal government is doing at their housing summit. Maybe there’s something coming with subsidies (but probably only if you build to rent out).
Thanks in any case for your assessments. This way we are definitely much better prepared already. I will definitely keep browsing the forum and learn from your experiences
Great forum here

Don't get me wrong: the bank bases the child planning on the current situation, but you need to think about it mentally. And yes, a car is mathematically expensive. I find your salary very good, but not for 600,000 without equity.

Alright, about the equity: that’s just our savings (also because of some larger purchases recently so that we have a few years of peace with not spending a lot of money). We haven’t told our parents yet that we would like to build a house. Since we both come from fairly wealthy families, they would definitely "chip in" (possibly part of the inheritance in advance). But we actually want to avoid that. Psychological independence. But you can probably register something like the car as a debt, right? After all, it’s a 1 1/2-year-old "new car" with a new value of €45,000. It will probably still be worth about €30,000 considering the depreciation in the first year.