An interest-only mortgage lets you pay only the interest owed for an initial period — typically 5 to 10 years — with no principal reduction. After that period ends, the loan converts to a fully amortizing payment covering both principal and interest, which causes monthly payments to jump significantly. These loans are less common after 2008 regulatory changes and typically require stronger credit and larger down payments.
With a standard amortizing mortgage, every monthly payment covers both interest and a slice of the principal balance. An interest-only mortgage delays the principal repayment: during the interest-only period, you pay only the interest that accrues on the outstanding balance. The principal stays flat. When the interest-only period ends, the remaining principal must be repaid over the remaining loan term — and because the amortization clock starts late, those payments are substantially higher.
A borrower takes out a 30-year mortgage with a 10-year interest-only period. For the first 10 years, monthly payments cover interest only on the full balance — keeping payments lower. At year 11, the loan converts: the remaining principal must now be paid off in the remaining 20 years, not 30. That compressed timeline significantly increases the monthly principal-and-interest payment. The borrower has paid no principal for a decade, so the full balance is still owed at the conversion point.
The CFPB flags interest-only loans as one category of higher-risk mortgage structure. Because you're not building equity through payments during the IO period, your equity only grows if the property value increases. If values fall and you need to sell or refinance, you may owe more than the property is worth. At conversion, many borrowers discover the new payment is materially unaffordable — the same dynamic that drove mass defaults in the 2000s housing crisis.
The Dodd-Frank ability-to-repay rule, administered by the CFPB, requires lenders to underwrite interest-only loans based on the fully amortizing payment — the higher payment that will apply after conversion — not the lower interest-only payment. This means lenders must verify that a borrower can afford the mortgage after it converts, not just during the IO period. Interest-only mortgages no longer qualify as 'Qualified Mortgages' under the rule's safe harbor, which limits which lenders offer them and who can qualify. The CFPB's mortgage resources explain how ability-to-repay rules work for non-standard loan structures.
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