House construction, how much can we finance?

With your wishes, you would need at least 650-700 sqm, preferably a bit more. In Marburg, you won’t get by with 100k for the plot. Why exactly does it have to be Marburg itself? I know the area quite well, so if you tell me the reasons, I might be able to suggest places nearby that would also fit and be cheaper.

So a train connection would be the absolute minimum. You want to go to the cinema sometimes, have a relaxed drink, and if I had to rely on the car for that, I might as well stay and build in the little village where I grew up.

For example, I know building plots in the city forest. There the square meter is 210€.

Building a castle in the air won’t help you.
Therefore, I would talk to your girlfriend, just calculate everything together, and see a financial advisor without obligation. A lot is possible together.

Imagine an apartment for two children near Marburg.
You pay at least 1000€ without a garden.

She doesn’t want to think about it right now, but if I build, then only together.
I at least want to inform myself and take further steps like consultation talks, etc.

It doesn’t matter to wait another 2-3 years; by then, you’ll have accumulated a large amount of equity.

Good plan - use the time and your self-employment to continue building up solid equity. Currently, you are not yet forced to build - waiting & planning can be a proper strategy. Age-wise definitely. A lot has been said and published in the press about current real estate prices. With an income of 4500€ and no equity, you won't move much today, maybe enough for a 400,000 - 450,000€ loan. You can get something for that, but ultimately not yet the "typical" 160m² city villa with 800m² plot. The question is what the situation will look like in 3-5 years. Interest rates can't fall significantly anymore, will construction costs still increase by 5-10%? So 20,000 - 40,000€ per year? Who has the net income to keep up here? It's similar with plots for single-family houses - sure, there's always someone, but at some point it becomes obviously too hot when in the middle of nowhere prices are called for that require being a manager in a big city.

I don't think it makes sense in the current situation to discuss how much the OP could spend on a house. I think you and your girlfriend should first decide whether to purchase a property or not. Then you can consider where you want to go and what you would like. After that, you can look at the prices in those areas (for plots, developer projects, and existing buildings). Then you can calculate with the help of the forum whether the project is feasible for you. It doesn't always have to be a house "built" by yourself. Already finished or pre-planned houses can be an alternative. Or initially an apartment, which in my opinion can be sold again without losses in any larger city if you then want to switch to a house.

The question is what the situation will look like in 3-5 years. Interest rates can no longer fall significantly, will construction costs still increase by 5-10%? So 20,000 - 40,000€ per year? Who has the net income to keep up with this? It’s similar with plots for single-family houses – sure, there will always be someone, but at some point it becomes obviously too hot when prices are asked for in the middle of nowhere that require you to be a manager in a big city.

Oh, the same thing was said 4 years ago. Back then, there were still areas considered "too far out" that today are seen as "close." And then things really took off again.

And do you know why I believe this will continue? The inheritance generation is only just starting to really get moving! Many home builders/buyers here earn significantly less than I do. But they approach house hunting very differently because their wallet is well filled with grandmother-grandfather/mother-father money. If you bring 300k into the financing, you don’t have to be a manager in a big city. That will keep prices high.

Income and Asset Situation: Household income (net)? 4500€ monthly + secondary income between 20,000€-50,000€ annually.

Is that the profit after taxes? Let's just assume so, then the matter is super simple: save for 3 years and comfortably build with well over 100k of equity. Then the bank will also take this secondary income seriously. Until then, convince your girlfriend.

I mean, as of today, what good is a rolled number of 20-50,000€ annually (why such a wide range??), if nothing has actually remained in the private account – in other words, you can't or don't want to contribute anything from it?

And where is all the money of the heirs supposed to come from if not from real estate (Germans are not a people of stocks & funds)? And who else should buy the overpriced heirlooms except other heirs?

What you also forget - the demography. With the Baby Boomers, the best-provided generation is slowly retiring and can relocate their center of life, besides, they live their lives very differently from the thrifty (pre-) & war generation. Example: big motorhome with sticker "We are wasting our children's inheritance"..
With appropriate care in old age, many big dreamed inheritances can turn out very differently.

Everything that comes after the Baby Boomers is numerically smaller, and significantly so. This can hardly be sensibly compensated by immigration, and certainly not in the segment of high-priced detached or semi-detached houses. Surely there will still be large inheritances, but many more houses will also become "available" - and the generations mainly eligible as buyers (20-45 years) no longer have the prerequisites as before. At least broadly speaking. So, there are definitely hard limits in this game - and no cycle runs forever. A perpetual motion machine does not exist for fundamental reasons.