The equity is not too low, the monthly input is currently too low.
If the equity were higher, the monthly input would be sufficient.
If I take the bank's perspective, it looks like this:
... as a bank, I give a relatively secure loan with the very high probability that shortly after completion the foreclosure threat already arises because the installments are unaffordable. The problem is not the equity but the missing income.
Because the bank gets interest for it. That’s what banks do. Loans against interest.
Loan from credere = Latin for trust. The bank has no trust that the loan will be repaid. No bank wants to deal with houses it has to auction.
Why the rental income from the granny flat should not be considered is also completely unclear to me.
They are considered. But only 80% of it is taken into account – the rest are operating costs. However, a higher LH flat rate is also taken into account for that.