Construction financing - Your experiences and tips?

Why should a civil servant in their early 30s necessarily have paid off a new building after 30 years? I don’t really see that necessity. You can just as well aim for closer to 40 years and possibly make special repayments or alternatively save equity in stocks. Better to work with a lower rate that also leaves room to breathe,

Rate with room to breathe? We are talking about just under €4,000 that the OP needs every month for living expenses. Is it really such a high demand to manage with only €3,600 each month but be finished by retirement? With a young couple with less income, I would agree that 1% repayment is enough at first. But not with a couple with €5.5k net.

- Slab: €25,000 - Garage 6x6 m with slab: €25,000

You can only get the slab and double garage in BY for that money if you do all the work yourself or in solid self-construction. I would definitely get some quotes before any nasty surprises come up.

Rate with breathing room? We are talking about nearly €4,000 that the OP needs every month for living expenses. Is it really such a high demand to get by with only €3,600 every month, but then be done by retirement?

With a young couple with less income, I would agree with you that 1% repayment initially is enough. But not with a couple with €5,500 net.

Because in reality, a desire to have children still arises, and then the higher installment simply can't be managed anymore as soon as you have to significantly reduce income? Forced savings with a DINK is all well and good, but if there is no longer a buffer for parental leave (and yes, it's naive to expect only 12 months of parental leave with parental allowance), then you simply run out of liquidity there. I can only repeat myself: liquidity at all times is the be-all and end-all of any financing and far more important than quick repayment.

Because in reality a desire to have children often arises after all, and then the higher installment simply can no longer be managed once you have to significantly give up income?

400€ decide over life and death? You surely don’t believe that yourself. Then please also consider that in reality unemployment can occur and no installment is affordable anymore. The best solution to be prepared for all situations is then to forgo owning a property.

I can only repeat myself: liquidity at any time is the be-all and end-all of any financing and far more important than rapid repayment.

We are not talking about a rapid but about an adequate repayment. And if liquidity is so important to you, then you simply arrange an option to change the repayment rate. Your little problem is then solved. You can’t be 100% prepared for all situations, but approaching the framework conditions with only 1% repayment is simply negligent. According to your logic, there would ALWAYS have to be a child (even in a family with children already or for those in their early 50s) and unemployment considered. That is simply pointless and impossible. Suspenders, parachute and yet another safety net is too much.

You can only get the base slab and double garage in BY for that money if you do it completely or mostly yourself. I would definitely get some offers first before any nasty surprises come up.

So the base slab with 140 sqm floor area including insulation, a clean layer of lean concrete and separation foil as well as reinforcement cost me about €20,000 (gross). Completely without any own work. Although in hindsight, I would have done the insulation myself, you can save some money with little effort there.
However, the garage without own work. That was completely done by myself.
A garage for €25,000 is possible, then probably prefabricated garages, likely made of sheet metal/steel.

Sorry for not replying for so long. Thanks for the tips. Where most seem to agree: With that income, the installment can be higher, which still surprises me. A couple of friends pays a €2,000 installment, but with the big difference that he works in the IG Metall wonderland, so the income there will be worlds higher. First offers should come in soon, possibly I will have to reschedule & reduce the house size in the construction planning. I should have done that earlier, I will pay rescheduling costs for the mistake. But I originally considered my measurements to be very small or a small house. Never mind. I will give feedback on how much the companies offer.