You found a rental property listing that mentions an assumable mortgage at 3.25%. The seller bought in 2021 when rates were rock bottom, and now you're wondering: is this actually a good deal, or just marketing fluff?
The math on assumable mortgages is different from a standard purchase analysis. Your down payment changes, your monthly payment is locked in, and the returns calculation has some wrinkles that trip up first-time buyers. Here's how to actually run the numbers.
What Makes a Mortgage Assumable
Assumable mortgages allow a buyer to take over the seller's existing loan, keeping the original interest rate and remaining term. Not all loans qualify.
These loan types are assumable:
Conventional loans are almost never assumable. They typically include a "due on sale" clause that requires full payoff when the property transfers.
When you assume a mortgage, you're taking over whatever balance remains. If the seller bought for $250,000 with an FHA loan at 3.25% five years ago, they might owe $220,000 today. You're assuming that $220,000, not the original amount.
The Down Payment Math
This is where assumable mortgages get interesting (and sometimes deal-breaking).
Your "down payment" on an assumption isn't a percentage of the purchase price. It's the difference between the purchase price and the remaining loan balance.
> Down Payment = Purchase Price - Remaining Loan Balance
If that property is now worth $285,000 and the remaining balance is $220,000, you need to bring $65,000 to the table. That's about 23% down, not the 3.5% FHA minimum you might be used to.
This equity requirement kills a lot of assumable mortgage deals for investors. The seller has built equity over years of payments and appreciation. You're buying out that equity position.
When the Numbers Work
Assumable mortgages pencil out best when:
Calculating Monthly Cash Flow
The monthly payment stays exactly the same as the seller's current payment. Principal, interest, and any mortgage insurance that was part of the original loan. Property taxes and insurance reset to current amounts.
Let me walk through a comparison.
Scenario: $285,000 duplex
| Item | Assumable (3.25%) | New Loan (7.0%) |
|---|---|---|
| Loan Amount | $220,000 | $256,500 |
| Down Payment | $65,000 | $28,500 |
| Monthly P&I | $957 | $1,707 |
| Monthly Cash Flow | +$443 | -$307 |
Gross rent is $2,400/month. After taxes, insurance, and a 10% expense factor, you have $1,400 available for debt service. The assumable loan leaves you with $443 positive cash flow. The new loan at 7% puts you $307 in the hole every month.
That $750/month difference in debt service is the whole story. Over a year, you're keeping $8,940 more in your pocket.
Return Calculations Get Complicated
Here's where investors get confused. The assumable mortgage produces better cash flow, but you put more money down. Which deal actually has better returns?
You need to calculate both cash-on-cash return and look at total return over your hold period.
Cash-on-Cash Return
> Cash-on-Cash Return = Annual Cash Flow / Total Cash Invested
For the assumable scenario:
For the new loan scenario:
The assumable mortgage wins decisively on cash-on-cash. But this metric alone doesn't capture everything.
Total Return Over Time
Your total return includes:
The assumable mortgage is further along in amortization, meaning a higher percentage of each payment goes to principal. In year one, maybe $350/month of that $957 payment is principal. On the new loan, only $200/month of the $1,707 payment is principal.
After five years, assuming 3% annual appreciation:
| Return Component | Assumable | New Loan |
|---|---|---|
| Cumulative Cash Flow | $26,580 | -$18,420 |
| Principal Paydown | $24,500 | $13,800 |
| Appreciation | $44,000 | $44,000 |
| Total Return | $95,080 | $39,380 |
| On Investment of | $69,000 | $36,500 |
| **ROI** | **138%** | **108%** |
Both scenarios show positive total returns because appreciation covers the negative cash flow in the new loan scenario. But the assumable mortgage delivers significantly more actual dollars.
The Opportunity Cost Question
Some investors argue differently. "If I only put $36,500 into this deal instead of $69,000, I could buy a second property with the remaining $32,500."
This is a valid point. Leverage cuts both ways.
If you can find two properties that each produce positive cash flow with conventional financing, deploying $69,000 across two deals might beat concentrating it in one assumable deal.
But in the current rate environment, finding properties that cash flow positively with 7% financing is genuinely difficult. The assumable mortgage solves a real problem: making the numbers work at all.
I lean toward the assumable mortgage when the rate spread is above 2.5%. Below that, the extra equity required starts eating into returns enough that I'd rather preserve capital for other opportunities.
A Complete Worked Example
Let me run through a full analysis on a single-family rental.
Property Details:
Your Investment:
Monthly Cash Flow:
| Income/Expense | Amount |
|---|---|
| Gross Rent | $1,650 |
| Vacancy (5%) | -$83 |
| Property Taxes | -$233 |
| Insurance | -$117 |
| Maintenance (8%) | -$132 |
| CapEx Reserve (5%) | -$83 |
| Property Management (0%, self-managed) | $0 |
| Mortgage Payment | -$885 |
| **Net Cash Flow** | **$117** |
Returns:
Comparison to New Financing at 7%:
With new financing, this property doesn't work. The assumable mortgage transforms it from a money-loser into a modest performer.
VA Loan Assumptions Have a Catch
VA loans are assumable, but there's a significant wrinkle. If a non-veteran assumes a VA loan, the original veteran's entitlement stays tied up until the loan is paid off.
Many veteran sellers won't agree to an assumption because it prevents them from using their VA benefit on their next purchase. You might need to offer a higher price or other concessions to make the deal attractive to a veteran seller.
If you're a veteran yourself, you can substitute your entitlement in some cases, releasing the seller's benefit. This makes the transaction much more appealing to the seller.
FHA Assumption Requirements
FHA loans require lender approval for the assumption. You'll need to qualify with the lender, which means:
The lender can deny the assumption if you don't meet their criteria. This isn't automatic. Budget 45-60 days for the assumption process, longer than a typical purchase closing.
FHA loans also carry mortgage insurance for the life of the loan (if originated after June 2013 with less than 10% down). You inherit this cost. Factor the monthly MIP into your cash flow analysis.
Common Mistakes
Forgetting about assumption fees. Lenders charge a fee to process assumptions, typically $500-$1,500. Some charge a percentage of the loan balance. Add this to your closing costs.
Ignoring the seller's equity position. I've seen investors get excited about a 3% assumable rate only to realize the seller has $150,000 in equity on a $300,000 property. That 50% down payment changes everything. Always calculate required equity before getting attached to the rate.
Comparing to the wrong baseline. Don't compare assumable mortgage returns to what returns looked like in 2020. Compare to what you can actually get today with current financing. A 10% cash-on-cash return sounds mediocre historically, but it beats negative cash flow.
Running Your Own Numbers
Assumable mortgages require more upfront analysis than standard deals. You need the exact loan balance, original interest rate, remaining term, and any ongoing mortgage insurance. Ask the listing agent for a copy of the seller's most recent mortgage statement.
Once you have those numbers, run the analysis both ways: assumable versus new financing. The [single-family calculator](/tools/single-family) lets you model both scenarios by adjusting the down payment and interest rate inputs. Compare cash flow, cash-on-cash return, and five-year total return before making an offer.
The math will tell you whether that low rate is worth the extra equity, or whether you're better off preserving capital for a different deal.