A perpetual mortgage is a type of mortgage that allows the borrower to increase the principal amount of the mortgage at a later time.
Perpetual mortgage allows the borrower to take a part of the loan amount for which he was approved to cover the cost of his home; by taking only a portion, the borrower can pay a lower interest rate, since he is only required to pay interest on the outstanding balance.
An indefinite mortgage is beneficial to a borrower who is entitled to a higher principal loan amount than may be needed to purchase a home.