For most FHA and VA loans, the seller must obtain the lender's approval for an assumable mortgage. In most cases, conventional mortgages are not affordable.An assumable mortgage is one that allows a new borrower to take over an existing loan from the current borrower. Typically, this involves a home buyer taking over the mortgage from the home seller. The new borrower, the person who will 'assume' the loan, is in exactly the same position as the person transferring it.