You borrow money with a mortgage on the property and put the money in a bank account. The loan must be free of interest. The money in the bank account should be used to record income and to pay interest on the loan.
That means you have to calculate how much you can spend per year
And you have to decide how long you want to extend your extra income. When this period is over, the loan will mature. Either you can choose to extend the period of repayment as long as you presume that you have fallen off when the loan is to be settled.
It can be difficult for us borrowers to find out in the jungle of loan offers. To make it easier to compare different offers, it is statutory that all lenders must state the effective interest rate on a loan. Effective interest rate is what the loan actually costs us. The effective interest rate depends on the nominal interest rate like the interest rate the loan is based on, what fees we have to pay and how we pay the interest rate, down payment or arrears, monthly or annual.
Or you are planning to buy a nine interior and cheaper home
During the period and as the last option you may sell because you got a place in an institution. When you sell your current home and buy a smaller one, the money is released and the loan can be settled. Once you have decided on the time and how much you want to spend, it is easy to calculate how much loan you need to take out.
Tables you will be able to find on the internet give you a clue as to how large a loan you need with different maturities to satisfy the extra income requirement.