Land in sight, is house construction financially possible?

The KFW loan is usually hardly paid off at all, so I would arrange everything so that the outstanding balance can be settled at the end of the term.

For example, 7 years ago we took out a normal loan of €180,000 at 1.9% with a term of 20 years and a KFW loan of €50,000 at 0.6% with a term of 10 years. The rate for the KFW loan was only €156 per month, so that now after 7 years there is still an outstanding balance of about €40,000. We now had the €40,000 available, so we paid off the KFW loan directly this year. (So much for no prepayment on KFW). This way we saved ourselves refinancing the remaining balance of about €36,000 in 3 years and can now calmly take care of the rest of the other loan for the next 13 years.

The KFW loan is usually hardly ever repaid,

That is generally wrong. With a term of 35 years, of course the lowest repayment amount applies (2,x%), but the highest interest rate. With a term of, for example, 10 years and two repayment-free years, you are "allowed" to repay a good 12% p.a. But then you only pay about one third of the interest rate of the longer term.

For example, 7 years ago we took out a normal loan of €180,000 at 1.9% with a term of 20 years and a KFW loan of €50,000 at 0.6% with a term of 10 years. The rate for the KFW loan was only €156 per month, so after 7 years there was still an outstanding debt of approx. €40,000.

Then you are confusing fixed interest period and term. Two completely different things like fire and water.

We now had €40,000 available, so we paid off the KFW loan directly this year. (So much for the topic of no special repayments with KFW).

On the topic of special repayments: In your case you are not making a special repayment, but a full loan repayment. That was and still is always possible for us (against a prepayment penalty). But special repayments, e.g. €5,000 last year and €2,500 this year, etc., are excluded by KfW.
Citation from KfW conditions:
"You can repay the entire outstanding loan amount early. For this you pay a prepayment penalty. Partial repayments are not possible."

Ah, ok. Thanks for the corrections. :)
By the way, the prepayment penalty was not due for us.

Ah, ok. Thanks for the corrections. :) By the way, there was no prepayment penalty due in our case.

This is also currently normal. Since you are repaying a loan with a currently low interest rate, the bank does not incur any costs - on the contrary, they even make a profit because the money repaid early can be better invested until the end of the original term. So you don't have to compensate anyone - at most, there may be minor administrative costs.

The problem is that your number has so little significance. I can only roughly estimate a price per square meter from it; there is just too much variability in the other items: land, exterior facilities, garage/carport, kitchen. You should of course always do the total calculation, if only to prevent a provider from shifting costs to other points ("price from slab on grade"). But throwing out 700k upwards as an absolute minimum for a rather cheap plot, I don’t think that’s right. It definitely can be done cheaper, even as a timber frame.

I can break it down again, and as I said, this is a calculation without any personal contribution.

Land including ancillary purchase costs, soil survey, surveying	92,600 EUR
Slab on grade including ring beam, base plaster, batter board framework, backflow preventer, and multi-utility house entry	44,000 EUR	We had an offer made by a local company.
House turnkey including photovoltaic system with battery storage, front entrance platform/stairs, concrete platform for heat pump, carport, small shed, standard equipment	545,500 EUR	Of course, there is potential for savings here. Photovoltaic system removed -25k, offer from another provider was -30k.
Furnishing including kitchen and new light fixtures	30,000 EUR	We are bringing all other furniture with us.
Additional construction costs including fees, construction power/water, insurance, earthworks, utility connections, final measurement, paving work	26,500 EUR
Exterior facilities including terrace and sun protection	15,000 EUR	There will be no more money for more, meaning everything else we will have to do ourselves later on.
Total	753,600 EUR

I only see savings potential in the house itself, which I already mentioned. Photovoltaic system removed, other provider. Another construction method certainly makes it cheaper again, but then of course it is a different house. I have obtained several offers and had to realize for myself that no one will offer this construction method for 100k less. And do me a favor and don’t tell people that, for example, Hanse Haus builds a QNG+ villa for 327,359 EUR. That is completely detached from reality and never what you actually pay in the end. That is exactly the trick all general contractors in model home parks use and try to conceal the actual costs. In the brochure it looks good at first, but in the end the 327k turn into 450k because this and that was missing and here and there you want something changed...

I can only speak from my experience and share it, and then everyone has to draw their own conclusions. And my experience is that you really have to put a lot of effort into properly gathering the actual costs and shouldn’t make the mistake of trusting some brochure price. And if someone says a slab on grade only costs 25k, I would get suspicious. And if someone says a QNG+ house only costs 327k, I would get suspicious. And if someone doesn’t try to include all the other costs, then I would make the effort myself, because with these prices and loan costs 30-50k can quickly break your neck in the end if you suddenly have to refinance.

A little tip on the side: I bought a compilation of all possible costs on the Fertighausexperte homepage, which I used to make my calculation. That helped me get an overview of what you have to pay attention to. Then you can also ask targeted questions about offers and clarify what is included in the offer and what you still have to consider separately in your calculation.

You will never find out all the actual costs in advance. Especially not from the general contractor, as it is very city/municipality/property-specific. But everything related to the building structure very much so. The general contractor can say what this or that additional service costs. Whether you build with A or B is a guideline, but it influences the ancillary building costs less, because nothing changes there. I also wonder if it has to be a 30K kitchen? Show kitchen? Why wouldn’t a 10K kitchen suffice?! I will probably never understand…. Remove carport, small shed, and photovoltaics. Do those separately (have them done). I would be really interested to know what your house would cost at Town & Country or based on yours there, or at Heinz von Heiden, just to throw two affordable builders into the mix…