How does it proceed now????

By "as precise as possible" I meant:

Plan the house as completely as possible and also consider things that are usually charged extra, special requests, etc. right from the first planning, so you get a realistic house price and don’t have to dig into your pocket X times afterwards.

I would also say: Put pressure on the BU so that you have something concrete in your hands as quickly as possible! You "have to" have the construction performance description checked by an independent specialist lawyer (for example), discuss possible changes, etc. You have surely already noticed from reading the forum that turnkey does not necessarily mean turnkey. I hope you still have enough buffer for all eventualities! Was there, for example, already a soil survey for the property before purchase? Sometimes the first surprises can be lurking there (i.e. additional foundation costs). I also wonder which bank already lets you sign the loan agreements when it is not even clear how expensive the house will be. Our banks here would not have gone along with that. They made that very clear to us as well: first provide a concrete offer and then see... What happens if you realize, for example, that you have to expect at least €50,000 in additional costs? I hope you still have plenty of cash saved to possibly offset something like that... Phew, for me that would have been too risky...

Well, I think that's ambitious.

Am I right in seeing that you don't have a partner yet with whom you want/will build?

You have really put yourselves under a lot of pressure, both time-wise and financially. I would definitely not let the BU or whoever you want to build with know about this. If he finds out that you actually have no time, I think it will get unpleasant.

"Hurry, hurry" – that’s how things get forgotten here and there.

I hope you find the right one immediately. Then get into planning. Even if there is time pressure, check everything calmly, especially watch out for hidden costs if you don't want to have to finance more later. Your offer should definitely not be exactly on or even above your already signed financing budget.
I honestly don't understand how the bank just gives out a construction loan on that basis. Haven't you really talked to them – that it's actually only a rough estimate and you don't yet know exactly how much it will cost in the end?
Is just one offer enough for the bank? Didn't they want to see contracts or know when it will start or something?
I don't get it.

In terms of timing, nothing must come up here.

- Find a partner
- Planning, best not to forget anything (house + ancillary building costs / garage? / fireplace? must be included)
- Prepare contracts and HAVE THEM CHECKED
(( Up to this point, you should (even if it takes longer) be completely honest with yourselves whether it fits with the partner or not. Otherwise, it'll come back to bite you later ))
- Sign
- Detailed planning
- Soil survey?
- Structural analysis
- Surveying
- Building application (you can really hope that nothing interferes here – between building application and start of construction, it can easily take 3 months)
- hope that construction starts before commitment fees have to be paid

@emer

well, we already have a partner with whom we want to build. as I have already written, the plans for our house are more or less ready (except for minor details that we are still unsure about).

we have been approved the loan agreement from the bank based on a first offer from the bu.

Yes, I have read that. I rather mean being "bound" to a partner, because it may still happen that something goes wrong for you, or the further planning suddenly causes costs to explode.
For example, we planned with a BU for a very long time (almost a year) and were already quite far along. In the end, we decided to go with an architect. Without a loan already signed, such a decision is certainly easy. In a case like yours, we would have thought about it 10 times and would probably have stayed with the BU. Even if that decision would not have made us happy.

Does the loan amount correspond 1:1 to your cost estimate?

The hope is that the first offer was calculated very generously. My experience has shown (I have received several offers / cost estimates from companies) that there is still quite a bit missing or that many things occurred to us afterwards or should be changed, which in turn costs money.

It sounds like you have no financial leeway.

Also, the soil condition and the hillside location play a not insignificant role.
As long as a structural engineer does not have exact values, he makes assumptions. Which can drive up costs.
A lame comparison: If you don’t know the adhesive strength of a glue per cm2, you coat the whole surface with it just to be safe – it uses more, but the chance that it holds is quite high.

All in all, 350,000 does not sound too little at first. The stomach pains only come when it suddenly is estimated at 360 or 360 thousand. Then maybe the leeway you would need is missing. Fingers crossed for you.