New construction planning - where do I start?

Would you simply get 700,000 euros? I keep seeing that the price per m² is between 1800 and 2000 euros here. There are no golden faucets everywhere and no complicated plots either. I would orient myself on that. If that doesn't help you, you can try it the other way round and see, what kind of loan would I get with my income and equity? Then you choose the house size accordingly.

No, I definitely wouldn't get 700,000 euros. I wouldn't want to do that either.

We have a house that I would then sell - that should bring 100,000 euros in equity after paying off the existing remaining debt, etc.

With KFW funding and the previously mentioned Baukindergeld, some support from the family (own work, guarantees, financial support) I would have to go down to a maximum of 450,000 to 500,000 euros - I could imagine that being affordable without us having to make major compromises anywhere.

At 500,000 euros over 30 years at 2.5% interest (which you would have to secure through a building savings contract or similar), a monthly rate of 1975 euros would be realistic, and I think that would just be manageable. Although banks already activate warning signals at 40% of income... we would be at almost 50%.

We are hoping for KfW subsidies.

We are simply speculating on the announced Baukindergeld

I would try to get the topic off the ground without such constructs. It's a nice extra, but building should not depend on it.
And neither relieves you. KfW funds go toward the measures you need to comply with, but you'll never get that money back. Besides, interest rates on the open market are often better than KfW’s.
How the industry will react to the construction financing is not yet known, but it is quite possible that prices will rise again because of it.

We have just under €5300 available without bonuses and profit-sharing, and I am really scared about the sum I have to borrow.

It's similar for us, and we have over €6,000 per month available.

Calculate your minimum 3 bathrooms, your heating, your double electrical system, etc., then you’ll quickly see where the money is going.

We are just speculating on the announced child construction grant - although it first needs to be clear whether we can receive it with our income. Currently, the limit is €70,000 gross - questionable whether child benefits etc. are included, and whether it is recalculated annually or only at the start of financing... At the start, my wife probably won't be working yet, and will only earn enough after a few years to reach the €70,000. Currently, we are at about €55,000 gross (me) and she earns €5,400 (from a €450 mini-job).

In addition, we are hoping for KfW subsidies.

If we can sell the house we currently own with a "profit" of €100,000, that of course helps enormously.

Then it would theoretically be at least feasible, at least according to my household budget planning.

The house you are talking about is not yet paid off and you would enter the race with €100k equity as a plus?

If yes, then I would really reconsider the whole thing.

a monthly rate of €1975 is realistic, and that would be just about feasible I think.

I consider that highly unrealistic. Or rather, how much would the parents' "rent" be?

I would try to get the topic off the ground without such constructs. It's a nice extra income, but you shouldn't depend on it for building. And neither relieves you. KfW is used up for the measures you need to fulfill, but you never get that money back. Besides, the interest rates on the open market are often better than KfW.

I agree with Robson, at the moment the loans are not particularly attractive, last year it was somewhat different.

The house you are talking about is not yet paid off and you would be entering the race with a surplus of €100,000 as equity?

If yes, then I would seriously reconsider the whole thing.

Yes... The house I own is worth about €280,000 to €300,000 according to various sources, but it can no longer be expanded or remodeled in a way that meets our requirements.

I took out a total of €200,000 for the house in 2012, of which about €170,000 is still outstanding 5 years later.

Assuming I get €300,000 for the house, pay back the €170,000, plus €20,000 closing costs, then €100,000 remains as equity for the new build.

A monthly payment of €2,000 is doable for us. We don’t live extravagantly. I have 13.5 monthly salaries, receive regular raises, bonuses, etc. We earn a few extra euros on the side for luxuries like vacations, and our parents also support us.