How much house is in it?

Because of the large sums, one quickly tends to stop paying attention to 1000 euros. That is actually true in the end, but if you allow it at every opportunity and selection, it becomes a bottomless pit.

Therefore, take your land, your ruin, and the development plan and see what the best solution is for you. What can still be gained from the existing structure? Always try to plan as small as possible. 160 sqm sounds good – 180 too much, etc.

Yes, that’s how we want to do it.
Maybe it would actually be worth taking a closer look at the basement. I have to discuss that with my husband and later also with the provider we decide on.
It also depends on whether the house contains asbestos. The 60s were particularly prone to that. Then everything would have to go anyway.

Yes, the thing about the large sums is true as well. Although I really believe that the selection process is the safest day for us during the house construction because neither of us tends to be flashy. We also always build many things ourselves, e.g., desks or shelves. We really enjoy that, and it is a great balance to desk work.

Difficult to give an estimate here. The standards diverge here. On the one hand, the bathroom should be functional, on the other hand, you want KNX. Unfortunately, there is no clear line here, with this you could be below or above 2000€/sqm. Leaving out things I also find difficult depending on the position; things like a terrace are out of the question, photovoltaic systems are more likely but then lead to additional costs that actually wouldn't have to be.

If we now calculate with 2000€/sqm, it would be for 160sqm,

320k€ Kfw55 house without basement + photovoltaic + electric blinds + little patterning
25k€ KNX
25k€ demolition
155k€ land
30k€ purchase/construction incidental costs
25k€ landscaping
20k€ finished garage
15k€ kitchen
10k€ small stuff
= 625k€

That would be a calculation with which I personally wouldn't have any concerns about plunging into an additional financing.

Who thinks it will be less or more?

All the prefab house providers state that the corresponding insulation values can only be maintained with their own brand basements, but those are ridiculously expensive with prefab construction companies and I’m actually not keen on paying 40,000€ just for a basement...

[...]

Therefore, I’m calculating with the standard equipment of a corresponding house at 289,000€ plus 5,000€ for special equipment.

I don’t find the 40k basement excessively expensive. 5,000 for upgrading completely? You can easily do that with two vanity cabinets. Also, you’re very limited with the floors and nicer tiles cost a pretty penny at 180sqm. If you don’t take the standard 30x30 tiles, the installation costs also go up.

Yes, but as far as I understand, with an existing basement the insulation values are bad again, because we would only reach max. KFW 55. And whether you use the funding or not (due to worsening conditions), I would still like to build as energy-efficiently as possible. That might still be a dream for now that will burst when we get an offer, but I’m still holding on to the hope that we can realize a KFW 40 little house.

I repeat myself: "And plus-energy... read up a bit on energy trickery, then you might quickly change your target." KfW values have nothing to do with your real consumption, even if it’s often presented that way. The measures to achieve the standard can easily cost more than you’ll ever save on heating costs.

And the prefab garage is made of precast concrete parts. It doesn’t even look bad and an acquaintance retrofitted the electrical system. All in all, he paid just over 15k. Of course not from Zapf or another well-known manufacturer, but with a normal garage that doesn’t matter much to me. It just shouldn’t fall on my car someday, but it looks stable enough

[...]

Regarding smart home, at first we would just like the basic requirements so that we can gradually "upgrade" later.

I don’t consider a double garage for 15k feasible nowadays. Even if the prefab element itself is just about in that range, you still need a foundation underneath, drainage has to be arranged, and electricity installed. In 10 years you might need to charge your e-car there.

No fence that is "cat-proof" at all. The cats at our place always find a loophole.

Thank you for your assessment
Only, I think we would rather leave something out or possibly go for the other property (1000sqm; 138k; completely "forested" with small trees that we could remove ourselves including the stumps over several weekends - I would have to check with the building authority to what extent this is feasible and whether it really does not count as forest as the realtor said).
That way we would save the demolition (and possibly the asbestos) plus the property is cheaper and the fencing would have to cover 500sqm less.
I just really don’t want to break the 600k mark because that is my personal pain threshold for financing. Above that, I feel uncomfortable.

Thanks for your assessment
Only, I think we’d rather leave something out or possibly go for the other plot (1000sqm; 138k; completely "forested" with small trees, which we could remove ourselves over a few weekends including the stumps - I’d have to check with the building authority to what extent that is feasible and whether it really doesn’t count as forest like the realtor said).
That way we’d save the demolition (and possibly the asbestos) plus the plot is cheaper and the fencing would need to cover 500sqm less.
I just really don’t want to go over 600k because that’s my personal pain threshold for financing. Above that I feel uncomfortable.

Then only KNX and demolition are left out + plot is cheaper. That’s 75k€ less, then 180sqm also works. If something else like blinds or large-format tiles are added, you’ll be over immediately.

It won’t be better than KFW 55 either.