Buy a building plot now for construction start in 23 / 24?

Buy! Budget and income fit!

strongly uncertain and would rather not want to start building this year

I would... - start building as soon as possible despite the incalculable construction cost situation (development probably only from August)

It's not that fast :D Building planning and approval will certainly take until the new year. Everything will be fine! You now have the forum as an advisory support :)

Good conditions. Go for it!
And if it gets a bit more expensive, just take out more credit and repay less.

Who cares whether you’re done in 17 years or not? If you don’t build now, you might still be renting expensively in 17 years. Better to have a small remaining debt as a retiree.

With the salary, a higher repayment adjustment will certainly be possible after 10-15 years. I see absolutely no concern. You basically only have to finance the house. And that is still relatively affordable in SH. Since you don’t have any exaggerated demands… ;)

Buy and start planning. There won't be any groundbreaking before mid-2023 anyway.

Simple and small is okay. However, as a homeowner, you should keep in mind that you need more space due to all the equipment than a tenant in a 90 sqm 3-room apartment.
Please plan enough storage space.

And what does simple driveway mean? Sloping site?

By driveway I meant that we would initially also forgo a garage and just pave two parking spaces (as specified by the development plan). Later, we could possibly then put a wooden carport over it. The plot itself is not on a slope but is quite flat.

By "driveway" I meant that we would initially forego a garage and only pave two parking spaces (as specified by the development plan).

Think about this when planning starts: include it along with 210k equity or liquid assets and the affordable plot of land.
Or if interest rates do actually rise, wait with it. You’ll have to see then.

May I just ask very directly where this uncertainty comes from, whether you can afford it? I mean, the plot is unbeatable cheap and the budget for a 120m² house with 300,000 plus 80,000 ancillary building costs is really generous. This can only blow up in your face if a pile foundation is necessary or if an ancient Viking settlement is discovered underground... maybe it makes sense to have a soil survey carried out soon.

With >5,000 EUR/month net household income, you belong to the top 20%. Who else should be able to take out a 250,000 EUR loan if not these top 20%? ;)

I can also understand the wish to repay as much as possible, but I would rather set the rate a bit lower. Life can sometimes go wrong, and then you sleep poorly if the rate suddenly amounts to 50% (or more) of the net household income. I agreed on a rate of 1,300 EUR for my loan and whatever I can save extra in a given year, I pay off through special repayments. I am allowed to pay in 5% of the total amount free of charge per year. Of course, on average I also want to get to 1,600-1,800 EUR per month... but I don't have to necessarily, that's the difference.