If landowners have to pay ground rent then they will sell any land that is not making money. This means the market will be flooded with new land. So the price of land will go down. So mortgages on land will go down.
Ground rent means work is not taxed, so it's cheaper to hire builders. So building becomes cheaper, and mortgages on buildings go down.
Ground rent means no unearned profit from land, so nobody speculates on land. So land prices will be less volatile. So once the system settles in mortgage deposits will then be smaller (because deposits exist to ensure the lender against price volatility).