The Trump administration has an idea to make buying a house more affordable — using the government's control over the mortgage finance system to offer home loans that can be paid off over a much longer time.

The big picture: The arithmetic of loan amortization, however, means that buyers taking out a 50-year mortgage would lose a key advantage of the more traditional loan product, while not saving that much month-to-month. • With a traditional mortgage loan, a homeowner steadily pays down debt and builds equity. A 50-year mortgage would involve very little paydown of debt over the first couple of decades. • The 50-year loan would likely carry a higher interest rate than widely available 30-year mortgages, limiting the savings on the monthly payment.

Driving the news: "Thanks to Presi

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