Build or save? What do you think?

Doc. Schnaggels has already clearly shown you the costs, and it seems that your wishes cannot be reconciled with the current income. However, if you are not yet willing to give up the idea of building immediately, the question is whether you want/can make compromises, such as foregoing the basement (which can cost more than 40,000 with special sealing measures), or building smaller than 150 sqm (with the right layout, even 120 sqm can be very spacious), and a double garage does not have to be there from the start either. Despite everything, I think it will be very tight for you, and in your position, I would currently not want to spend more than 800 or 900 EUR on a loan installment.

No one here surely wants to talk the OP out of their dream of owning a house, but this is a young family with soon two small children, so the financing should be on solid footing and not just optimistically calculated. Unexpected costs can arise very quickly when building a house, and in this case, one should still have a buffer or the possibility of additional financing so that one does not suddenly face existential fears. One should also not use all of their available free capital for the financing (as the OP has calculated here) because after all, one does not live only for building the house!

We do know that the salary will still increase well over the next 5 years, but unfortunately not exactly how fast. And I will also start working again or take a mini-job as soon as the children are in kindergarten, however, a third child is probably planned (that's why the 150m2), so that could still take a while. If we keep saving, we would have at least €30,000 more equity in 3 years, but by then everything will be more expensive again, interest rates will rise again, and the children will get older... I also don't think we can manage €350,000, so the house would have to be cheaper, but where to save?

Buy a used large house and renovate it step by step - it may not be cheaper, but the costs are flexible. Alternatively, leave out the basement. Forego the double garage. 150 m² can also be enough for 3 children without a basement.

Buying second-hand would of course also be an option, but there is practically nothing to buy in the village / area where we want to stay, and we have been observing that for 3 years.

I have the impression that the basic idea, namely to build in the new development area of the village, has already been established.

Why does a plot of land in a village cost so much? How large is this plot supposed to be?

150 sqm are sufficient for four bedrooms even without a basement! It depends on the house type and the floor plan.

A double garage costs between 10,000 and 18,000 €, this amount is by far not enough in my experience. I recommend a minimum estimate of 22,000 €.

Alternatively, there are extra-large carports with space for just under 10,000 €.
Alternatively: log cabins garages... they are cheaper, but I have not yet seen them cheaper than a double garage. Costs for a single garage: just under 4,000 € without installation.

I find that under the given circumstances, study and small family, the equity capital is quite decent, but I would not include this amount in a possible financing. You will always need money from time to time, even if only for another vehicle or a washing machine.

I would point out that the child/children are still small and that over the course of their/your life, some costs that cannot yet be estimated will have to be considered.

I would seek a conversation with a bank. They will give you a loan amount. Then you have your estimate and can continue to think it over.