Some mortgages in the past when interest rates were very low have been transferable. But consider: A new purchaser will have a different credit history than the original purchaser. If the new purchaser is a better credit risk, chances are good that they can get better terms. If they're a worse credit risk, the bank is going to want different terms.

In a relatively new mortgage, the savings will be slight, since you'd still have a lot of the same costs involved in transferring the mortgage as in originating it. And in an older mortgage, there'd be too much of the principal already paid- why would you assume an existing mortgage that required you to put down 50%? Even it you wanted to put down 50% or more- say, from the sale of an existing house- you'd do better to originate a new mortgage, since you'd be paying proportionally more interest- whcih is tax deductible- up front.