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Interest-Only Mortgages: A Tool, Not a Trick
An interest-only mortgage lets you pay just the interest for an initial period — usually 10 years — cutting the payment substantially while the balance holds flat. After 2008 these loans got a reputation they only partly earned. The modern version is a regulated non-QM product with a built-in safety: most lenders qualify you at the higher post-IO payment, so approval means you can afford the loan even after the discount ends.
The real payment math
Take an $800,000 loan at 7%:
- Interest-only payment: ~$4,667/month
- 30-year amortized payment: ~$5,322/month
- Difference: ~$655/month for the IO period — roughly $78,000 of payment flexibility over 10 years
The structure is typically a 10-year IO period on a 30- or 40-year total term. When the IO period ends, the loan amortizes over the remaining years and the payment steps up — on the example above, to roughly $6,200 if rates and balance are unchanged. That step is the entire risk of the product, known on day one.
Who it genuinely fits
- Investors: IO maximizes monthly cash flow and, on DSCR loans, many lenders compute the ratio at the IO payment — turning marginal deals into qualifying ones
- Lumpy earners: commission, bonus, and business-exit income — pay the minimum monthly, throw chunks at principal when the money lands (most programs allow principal payments anytime without penalty on primaries)
- Jumbo borrowers: at high balances, the freed-up cash is significant capital that many would rather deploy than amortize
Who it doesn't fit
Anyone using the lower payment to stretch into a house they couldn't otherwise afford. You build no equity from payments during the IO period — if the plan is "hope the value rises," that's the 2008 version, and the qualification-at-the-full-payment rule exists precisely to filter it. The honest test: could you comfortably pay the amortized payment today? If yes, IO is a cash-flow tool. If no, it's a warning light.
What you'll typically need
- Down payment: commonly 15–25% — IO rarely pairs with minimum down
- Credit: ~680–700+ for most programs
- Qualification: expect to be underwritten at the post-IO amortized payment
- Documentation: flexible — IO stacks with bank statement, asset-based, and DSCR programs
Frequently asked questions
Are interest-only mortgages still legal?
Yes — the IO feature makes the loan non-QM, so it lives with non-QM and portfolio lenders. Regulated, with qualification typically at the full amortized payment.
Can I pay principal during the IO period?
Usually yes, whenever you like — and doing so lowers each subsequent interest payment immediately. On investment-property loans, check the prepayment-penalty terms first.
What happens when the IO period ends?
The loan amortizes over the remaining term and the payment steps up. Most borrowers refinance, sell, or simply absorb the step — the right answer is chosen at closing, not discovered in year ten.