Build or wait a little longer?

If interest rates do rise... I can't imagine that interest rates in Europe will reach levels in the next few years that would be dangerous for the homebuilder. Many countries would certainly go bankrupt faster than the German homebuilder.

Due to the upcoming new [Energieeinsparverordnung] regulations, prices will certainly not become cheaper. And the order books are still full for the next ~2 years.

It's six of one, half a dozen of the other. Speculating now about if and when prices will rise or fall is completely irrelevant in your situation: You have a building obligation in 3 years, so start planning now and then get going. The time will be up faster than you can say "save more equity." Because whether you save another €30,000 now is hardly decisive given the total investment. The deal will be close to 100% financing whether it's now or in 2 years. Which is okay considering the income and the possible condominium to be sold. Therefore: If you want to build, start planning already.

With the construction obligation, however, you have put the gun to your own head, which leads to the project becoming unnecessarily expensive. High loan-to-value means higher interest. With the required amount, that is many thousands of euros in interest that you will have to pay due to lack of equity.

That is true, but I wonder whether it would have been wiser to let such an opportunity pass!? We already got the plot below market value, and the only risk we have seen is the loss of the additional costs we paid for the plot. In addition, the location of the plot also has an ideal value for us, and the likelihood of getting another plot in this location in the coming years is very low.

Don’t be upset. You already belong to the "fools" who financed 100% for an apartment they don’t want two years later That’s life.

Here too, we thought we were doing something good for ourselves. What speaks against trying to rent out the apartment first? If that doesn’t work well, you can still sell and use the money for special repayment on the mortgage, right?

The spiral breaks as soon as interest rates rise. Then the majority of people can no longer afford current prices because you get less loan amount for the same monthly rate.

That is exactly what I am afraid of. That interest rates will rise slightly within the next 2 years and construction costs as well. And despite having more equity, you will not be better off or even worse than now.

Time passes faster than you can say "save more equity." Because whether you save another €30,000 now is hardly decisive given the total investment. The deal will be close to a 100% financing either now or in 2 years.

Well, it is actually a bit more than €30,000. Calculated over 24 months we would have:
24 x €1200 savings (plus x) = €28,800
24 x €600 apartment repayment (856 - interest = ~600) = €14,400
24 x €700 plot repayment = €16,800

makes ~€60k more equity

But opposed to that are the ~4% construction cost increase = €18,000, the 2 years of life passing by, and the family planning which would probably conflict much more severely.

Do you have experience with what interest rate I would currently have to expect with a 100%+ loan-to-value?

Due to the speculation tax, I couldn’t really sell it for a higher price anyway at first.

You don’t have to pay speculation tax because you haven’t rented it out. Have you already spoken with your bank? I would also approach them regarding selling or renting out the apartment.

On the other hand, there are the ~4% construction cost increase = €18,000, the 2 years of life that will then have passed, and the family planning that would probably collide much harder.

Price increase is speculation, your years of life are not wasted in the apartment either, and as I wrote before, I wouldn't build a house for a speculative family. Are you sure how many children you want = number of children's rooms? Are you sure you can have children at all? I think this is the wrong order, and as with the apartment (getting it cheaply), building the house now sounds like an impulse decision because there was an opportunity. Hmm.

Interest rate for 10 years approx. 1.6% at 100%. Anything above that as a consumer loan. Someone with 80% financing would get it for 1%. Assuming a monthly rate of €1,500 on a loan amount of €400,000, the homeowner with 100% financing pays €54,000 interest in 10 years, plus interest on the consumer loan if >100%, the one with 80% financing €32,000. And that is only the current comparison, we are in an absolute low interest rate environment.

Everything is a matter of perspective. I would always prefer to build before starting a family. Especially when building a house and having a baby/baby period coincide, it is already a lot of stress. Of course, it can be managed, but I wouldn't voluntarily put myself through that. Without children, parental allowance is still possible. With toddlers, it's only very limited or connected with much more stress. Of course, it may be that the children do not come as planned or desired. For me, that is a general life risk. Just as it can be that you have two children, think that that's how it will stay, and build accordingly, and then a surprise egg arrives. There are many scenarios you can imagine, so I would first plan in the way that is most desirable for me, and the rest life will bring along.

And I personally consider it certain that construction costs will continue to rise at least for the next 2 years. Only the extent of the price increase is questionable to me.