This has to do with the loan-to-value ratio at the banks, not exclusively with the equity.
The calculation is primarily based on the risk for the bank. I will try to illustrate it with two examples.
- Total amount €500,000
- Equity in cash is €100,000
- Loan required €400,000
Case 1:
The borrower borrows the sum of €400,000 entirely from Bank XYZ.
Bank XYZ naturally wants to be first in the land register. Otherwise, in the worst case, there might be nothing at all.
- Bank calculates: €500,000
- minus €100,000
- leaving €400,000
This results in equity of 20% and a loan-to-value ratio of 80%. Accordingly, the terms of the loan are calculated. Bank XYZ therefore wants to get back the 80% (€400,000) in any case.
Case 2:
The borrower borrows €100,000 from KfW and €300,000 from Bank XYZ.
KfW is content for the time being to have second place in the land register. Bank XYZ wants and is again in first place. This way, Bank XYZ hopefully receives its €300,000 in case of emergency and does not care if KfW gets nothing.
Bank XYZ thus lends only 60%, so the loss is lower in the worst case, which may also affect the offered interest rate; KfW lends 20%.
The equity still remains 20% of the total amount.
...
Hope this makes it transparent.