30 years (fixed interest rate) 90% financing realistic?

There’s too little left of the net income! Or said bluntly: Do you want to impress your friends with your property or do you want children? Trying to do both will turn into a serious burden on your relationship. Because children not only cost time and income but, at the latest with daycare, also real money. And if you then have to significantly (!) reduce your usual standard of living, that causes conflicts.

Is your wish a detached single-family house? A first search shows that Deutsche Reihenhaus builds in the RMG around Hanau or Alzey, starting at 250 or 350 thousand euros. Probably middle houses, corner houses cost a bit more. Even with upgrades, the repayment should only be about half as high. Just as an example of affordable building—whether you like that area or what you are willing to pay for a better location you ultimately have to decide.

“Better to laugh in a hut than cry in a palace” someone here put as a motto. That fits!

The OP writes about a travel time of under 35 minutes to Frankfurt, Wiesbaden, and Mainz. From Alzey, you’ll manage that in rush hour at best with a helicopter. Frankfurt is unlikely to be reached by car in under 90 minutes, Wiesbaden I’d estimate 70 and Mainz 60. Whoever wants to be closer “to the action” pays accordingly—600k is quite realistic for that. Just to put it into perspective:

In Wörrstadt (district Alzey-Worms), there is currently a 700 sqm plot in a new development area for 520k. So much for 250-350k for a semi-detached house. That only gets you the land. Unless, of course, you want to build your property in dying (cow) villages where today there is absolutely nothing left (shops, schools, daycare, doctors, etc.). Then it’s certainly cheaper too.

I would at least wait with this project until your partner is a civil servant. Until then you’ll surely have saved more equity.

There's too little left over from the net! Or frankly said: Do you want to impress friends with your property or do you want children? Both together will develop into a serious strain on your relationship. Because children not only cost time and income but at the latest with daycare also real money. And if then the accustomed standard of living has to be significantly (!) reduced, that causes conflicts.

Is your wish a detached single-family house? A first search shows that the German townhouse in the RMG area builds around Hanau or Alzey, starting from 250 or 350 thousand euros. Probably middle houses, corner houses a bit more. Even with upgrading, the repayment should only be about half as high. Just as an example for low-cost building, whether you like the area or what you have to be willing to pay for the better location you ultimately have to know.

"Better to laugh in the hut than to cry in the palace" someone here used as a motto. That fits!

Great motto. I have to remember that!

The OP writes about a travel time of under 35 minutes to Frankfurt, Wiesbaden, and Mainz. From Alzey, you can only manage that in rush hour with a helicopter at most. Frankfurt is hardly achievable by car in under 90 minutes, I estimate Wiesbaden at 70 and Mainz at 60. For those who want to be closer to the action, they pay accordingly, 600k already fits well there. Just so one can put it into perspective:

I don't know the current travel times (I originally come from the east of the RMG, but it's been more than 20 years since I moved away from there), but the OP wrote

Here we have found a property with which we can both serve Frankfurt as well as Wiesbaden and Mainz within a reasonable travel time (<35 minutes) as employees, in case there in the future - for whatever reason - should be a job change.

I interpret that as both are currently working, for example, in Frankfurt and only in case their work should someday take them to MZ or WI, they want to live exactly in the triangle of these cities so that commuting does not get out of hand. This of course limits the choice of location very much and raises the costs since then only something in the heart of the RMG comes into question. If Frankfurt as a commuting destination would be sufficient for the time being, HU as a home base would not be so bad (train line HU HBF to F Bankenviertel 20 minutes). And in the worst case, you can sell the house in HU and buy something near WI (in a western suburb) for commuting. You just pay well 100 to 200k more to bet on flexibility in the middle of the RMG.

Flexibility costs!

These 100 to 200k are exactly the crunch between relaxed financing and the nightly drama or mental worry.

The incidental purchase costs of a one-time change certainly do not compensate for this. Hence the attempt at a change of perspective.

The monthly burden would be too high for me, especially if children are still to come. I would probably give up the dogma of 30 years full repayment. You are still young, and you won't retire before 70 anyway.

Key data:
joint monthly net income: 5000 EUR
of which
- 2500 from my wife, 31 years old: employed in the public sector ((not yet) a civil servant);
- 2500 from me; financial service provider, 29 years old.
Fixed special payments are included in these figures.

How much is it without special payments? Because this value counts at the banks.

Money for the move (double rent etc.), kitchen, necessary furniture and small items (lamps, curtains, etc.) is already secured. However, we cannot finance special requests (we do not want more than the standard from the building description: tiles, painter's fleece, parquet, described room layout, fittings, ...). Here I am unclear whether it is even realistic to plan that no further (large, i.e. > 5000) costs will arise.

Does that mean you have no buffer at all? A construction ALWAYS (!!) costs more than you think.
May I ask how much the house costs and how much you have budgeted for incidental construction costs?

Thanks for all the feedback!

How much is it without special payments? Because this value counts with the banks.

Actually, I gave wrong numbers above. It is a net 5000 without and 5400 with special payments calculated per month in order to be able to service the 1950 installment. That is 39% under the 40% that I have often read as the maximum recommendation. However, I am unclear whether this rough recommendation also applies to a 30-year financing.

Does that mean you have no buffer at all? A build ALWAYS (!!) costs more than you think.
May I ask what the house costs, and how much you have estimated for incidental construction costs?

It is a turnkey purchase from the developer (i.e. including wallpaper, floors, etc.) "without further/hidden costs". Does the statement "it ALWAYS gets more expensive" also apply here?

600 commission-free + 6% property transfer tax + 1.5% notary costs + 0.5% land registry costs = 648 TEUR