Complete invoice: how much land can we afford?

I would have already found the amount critical at 1% interest, now it is definitely too much.

For the calculation itself, I would set the costs for the basement higher and the ancillary construction costs lower. And while a 15% buffer is great, you do not have the leeway for such large additional expenses.
Ultimately, however, all of this is irrelevant since you do not yet have a plot of land and do not know what expenses the plot will entail. Keywords here would be hillside location and infiltration capacity.

We currently have a net household income of €13,000. This will temporarily reduce to up to €4,700 for 3-5 years. During this phase, we will not start any financing.

Even if you don’t start it then, but make a commitment/a contract with €13k net, you still have the problem that you want to manage a €3k financing for several years with under €5k net. Either you have corresponding cash reserves that you don’t use in the financing, or the construct is harakiri.

Another problem I see is with the 15% buffer. Planning for it is certainly sensible, but you have to present it differently to the bank. They won’t finance you for a “buffer.” You have to adjust the other positions accordingly and have this cost estimate stamped by the architect. At least that’s what our bank required.
By the way, the effort involved in the preliminary loan check for my two house financings bore no reasonable relation to the effort later required to check how the money was used. I don’t know why they are so obstructive when you have a safety buffer included in the calculation. Later on, nobody cares how the money was distributed across the trades, as long as it went into the house and not into a car.

Are you both reducing your working hours or is the person with the significantly higher income taking parental leave for that long? Where does the 8300 euro difference come from? Or is it just assumed that a contract with that income amount will be signed after the end of parental leave?

Is it even certain that your wife can re-enter under the conditions in 5 years? Civil servant? As far as I can see, she currently earns 8,000 EUR net? I don’t want to be pessimistic, but honestly, that doesn’t sound like a position you can return to part-time in 6 or 7 years? MdB or MdL? Professor? Besides, it has already been said. No bank will agree to "in 5 years we will definitely have 8,000 EUR net again." And that’s not possible anyway. You will be missing about 2,000 EUR per month over the 5 years to service the loan. That’s 120,000 EUR. Where do you want to get that from?

The plan is a separate financing, right? And house construction when the woman's money is back. 200,000 with 4,200 net should be affordable. The question is more about how to manage it, so variable is better?

It's really confusing what information is provided here regarding the income situation. I certainly only understand "train station" from it.