Interest-only is the most misunderstood structure in DSCR lending. Borrowers treat it as a cash-flow gimmick; lenders treat it as a distinct credit decision with its own pricing grid, LTV caps, and qualification rules. Used correctly, a 10-year IO period turns a deal that fails at 1.08 into one that prices at 1.25, adds $250 to $400 a month of holding-period cash flow on a typical $300,000 loan, and buys a value-add investor a decade of runway before amortization starts. Used carelessly, it sets up a payment jump of 20 to 30 percent in year eleven on a property that never grew into the debt. The difference comes down to three things: which payment the lender qualifies on, what the IO option costs in rate, and whether the exit plan survives the recast.
How the structure actually works
The standard DSCR interest-only product is a 30-year loan with a 10-year IO period followed by a 20-year amortization, all at a fixed note rate. A growing number of lenders also offer a 40-year term with a 10-year IO period and 30-year amortizing tail, which softens the post-IO payment jump considerably. During the IO window the payment is simply rate times balance divided by twelve: on a $300,000 loan at 7.75%, that is $1,937.50 a month, versus $2,149 on a fully amortizing 30-year — a difference of about $212 a month, every month, for ten years.
Nothing about the rate adjusts at the end of the IO period on these fixed products; only the payment does. The 30/10 IO recasts to a 20-year amortization of the full original balance, which on the example above pushes the payment to roughly $2,463 — a 27 percent jump. The 40/10 structure recasts to a 30-year tail and lands near $2,149, the same payment the borrower would have had on a standard 30-year from day one. That single difference is why the 40-year IO has become the default recommendation for buy-and-hold investors who want the IO cash flow without the year-eleven cliff.
The qualification question that decides everything
Here is where lender selection matters more than rate. Some lenders qualify the DSCR ratio using the interest-only payment; others qualify on the fully amortizing payment even when the borrower takes the IO option. The gap is enormous. Take a property renting at $2,400 with taxes and insurance of $350 a month, against the $300,000 loan above. Qualified on the IO payment, the ratio is $2,400 / ($1,937.50 + $350) = 1.05 against the IO number — but run the same deal through a lender that uses the amortizing payment and the denominator rises to $2,499, dropping the ratio below 0.96 and out of most programs entirely. On stronger deals the effect works in your favor: a deal that computes to 1.10 amortizing often clears 1.20 to 1.25 on the IO payment, which at many shops moves it into a better pricing tier worth 25 to 50 basis points.
The market splits roughly down the middle. Most top-tier non-QM lenders qualify on the IO payment, which is precisely why they offer the product — it expands the box for thin-ratio deals in expensive markets. A conservative minority underwrites to the amortizing payment as a stress test, treating the IO option purely as a borrower cash-flow election. The lender directory flags which programs qualify on the IO payment, and it is the first question to ask when a deal is anywhere under 1.20, because the answer determines whether the IO option helps you qualify or just helps you hold.
What IO costs in rate and LTV
The IO option is not free. Typical pricing adds 12.5 to 37.5 basis points to the note rate, with 25 bps the most common adder at the big national shops. On current pricing, where standard DSCR money runs roughly 7.25% to 9.25% depending on ratio, LTV, and credit, an IO election usually lands the final rate a quarter point above the amortizing equivalent. Some lenders price it instead as a half-point cost at closing rather than a rate adder — worth taking when you expect to refinance inside five years, since the upfront point amortizes out cheaper than 25 bps paid forever.
LTV caps also tighten. Where a standard purchase allows 75–80% LTV, most IO programs cap at 75%, and several cut cash-out refinances with IO to 70%. Minimum DSCR floors often rise a notch as well — programs with a 1.00 floor on amortizing loans commonly require 1.10 computed on the IO payment, and a few require 700+ FICO for any IO election versus 660 for the standard product. None of this is prohibitive; it just means IO is a prime-borrower product, not a marginal-deal rescue at maximum leverage.
Where the math favors IO
The structure earns its premium in three situations. First, low-cap-rate appreciation markets, where the rent-to-price ratio cannot produce a qualifying DSCR on an amortizing payment. A condo in Dallas renting at 0.55 percent of purchase price may compute to 0.98 amortizing and 1.12 on the IO payment — the difference between no loan and a loan. High-tax states compound this: the same rent miss hurts twice as much in Texas, where effective property taxes of 1.6 to 2.2 percent already burden the denominator, so IO qualification is doing real work in exactly the markets where ratios are structurally thin.
Second, value-add holds. An investor repositioning a tired duplex over 24 months wants minimum required payments while rents climb from $1,650 to $2,100. The IO period carries the property through stabilization, after which a cash-out DSCR refinance at the higher rents and value resets the whole capital stack anyway — the borrower never meets the recast.
Third, portfolio builders optimizing for acquisition velocity. The $212 a month saved on one property is trivial; across ten properties it is $25,000 a year of retained cash flow that funds the next down payment. For investors running that playbook, the IO election is a deliberate choice to rent the amortization schedule rather than own it, and the math is covered in more depth in our guide to the DSCR ratio.
Where it goes wrong
The failure mode is holding a 30/10 IO past the recast on a property whose rents did not grow. A borrower who took the IO payment at $1,937 and faces $2,463 in year eleven needs rents up roughly 25 percent over the decade just to keep the ratio flat. In strong Sun Belt metros that is a reasonable base case; in slow-growth markets it is a coin flip. The structure also interacts badly with long prepayment penalties: a 5-year step-down prepay on a 10-year IO sounds harmless, but borrowers who plan to refinance at stabilization in month 30 are paying a 3 percent penalty to exit — on top of the IO rate premium they paid to get there. Match the prepay term to the actual hold plan, not the IO term.
Cash-out refinances deserve particular caution. Pulling maximum equity at 70% LTV on an IO payment produces the lowest possible payment on the highest possible balance — seductive, and the closest thing DSCR lending has to the pre-2008 structures that ended badly. If the deal only works IO at max leverage, it does not work.
Running your own numbers
Before electing IO, price the deal three ways: amortizing 30-year, 30/10 IO, and 40/10 IO, each at the lender's actual rate adder. Then compute the recast payment and the rent growth required to hold the ratio at the lender's floor. The DSCR calculators handle the payment math directly. In cash-flow metros like Detroit, where ratios commonly start above 1.30, the IO election is pure cash-flow strategy and the recast risk is minimal; in coastal and high-growth markets it is a qualification tool that demands a real exit plan. Know which game you are playing before you pay the 25 basis points.