Financing through building savings? I don't see the catch.

According to the consultant, the bank's response is now available. The property is being completely revalued according to "Basel III," which throws a wrench in our plans. Now the interest rate drops from 2.6% to 3.25% because a different loan-to-value ratio applies. Calculated over the term, that amounts to 42,000 euros if we plan without special repayments. We are at our wit's end, constant setbacks...

That is quite intense. I would get quotes from somewhere else again.

Well, in the end it’s even 3.3%, of course we will get a different consultation again. Especially since our "consultation" "can" not forward us the justification to read through. On Monday we’re going back to the house bank, let’s see if they can’t undercut the new offer that our "consultation" wants to send us on Monday. In parallel, we’re looking ourselves and requesting the written justification from the bank. Slowly I really feel f***ed. Already wasted so much time that we have to extend the financing contingency with the provider....

But I'm surprised, since Basel III has been in effect since early 2013, what happened before that? The equity capital is not that low either, so how do they now arrive at an interest rate jump of 0.65%?

Furthermore: Basel III does not change/regulate the interest rate / the loan-to-value ratio. It only requires banks to build up more equity. Of course, the banks pass this on to the customer (if little or no equity is available). But such a "miscalculation" is strange.

What percentage loan-to-value was actually applied in your case?

According to our advisor and our calculation, we had a loan-to-value ratio of just under 72%.
Breakdown
1. Land is valued at 37,313 euros (rounded to full euros)
2. Construction costs according to the contract then add 300,865 euros
3. Outdoor facilities flat rate of 15,000 euros added
4. Other incidental construction costs, but without kitchen, probably 33,000 euros
5. Additional electricity flat rate of 5,000 euros
6. Stove and sauna at full price 13,500 euros

Total sum thus 404,678 euros

For a loan amount of 290,000 euros.

However, the bank apparently assumes such a depreciation that it arrives at a loan portion of just under 88%.
The basic condition at the bank is 2.6% and for loans over 100k€ a further reduction of 0.1%. This is how the 2.6% came about. For loan-to-value ratios between 80 and 90%, 0.7% is added (for 72-80% it would be 0.4%).

So my sparse information.