How much extra is a seller’s low-rate assumable mortgage actually worth? Compare the equity gap, cash needed, combined payments and remaining debt with new financing over your expected ownership period.
A lower rate does not guarantee a better purchase. Enter both the estimated market value and the seller’s actual asking price. Gray examples are hints only.
Payments include entered mortgage insurance and exclude taxes, homeowners insurance and HOA dues. Initial payments can fall when a loan is paid off. Maximum modeled seller premium is a mathematical estimate, not an appraisal or recommended offer.
Baseline: a hypothetical purchase at your estimated market value, not the seller’s asking price. This does not mean the identical property is available at that price. Both options use the same available cash after reserves, up to their purchase needs. Any unused cash stays yours and is excluded from economic cost.
Each schedule is bounded to the loan’s term (at most 600 months), including months after your ownership horizon. No calendar dates are assumed.
| Month | Beginning balance | Interest | Payment | Principal reduction | Ending balance |
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| Month | Beginning balance | Interest | Payment | Principal reduction | Ending balance |
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| Month | Beginning balance | Interest | Payment | Principal reduction | Ending balance |
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Seller premium = asking price − estimated market value. A negative value is a discount. Seller equity gap = asking price − assumed balance. Usable cash = available cash − protected reserves. Additional funding before secondary fees = max(0, equity gap + assumption closing costs + assumption fee + other costs − usable cash).
All non-secondary transaction costs must be covered by buyer cash. Secondary financing, if selected, covers the remaining equity gap. A secondary fee paid upfront uses cash and increases the equity amount borrowed; a financed fee is added to the second loan. With a fixed cash budget, both can produce the same principal, but the upfront option requires cash for the fee. Secondary fees apply only when a second loan is needed.
Monthly P&I = principal × monthly rate ÷ [1 − (1 + monthly rate)^−months]. At zero interest, payment = principal ÷ months. The assumed mortgage uses its remaining term. Each month adds interest before payment; the final payment is capped to the remaining debt.
Economic cost = upfront buyer cash + P&I paid through the horizon + mortgage insurance paid + remaining debt. Modeled economic benefit = new-financing economic cost − assumption economic cost. This counts principal, premiums and fees once. It is equivalent to purchase price + transaction fees + horizon interest + mortgage insurance. A shared end-property value cancels in the comparison. Equity is estimated market value minus remaining debt.
Monthly cash-flow savings, interest savings and total economic benefit answer different questions. Lower payments alone do not establish savings. This is a nominal-dollar comparison: no discount rate, opportunity cost on cash, appreciation, taxes, sale costs or refinancing is modeled.
The solver changes the asking price, recomputes cash needs, secondary borrowing, fees, payments and remaining debt, and compares against the same market-value baseline. A bounded search locates the highest feasible nonnegative premium with nonnegative economic benefit, including jumps when secondary fees start. If cash availability stops the search first, the result is explicitly a cash limit, not an economic break-even. No positive premium is reported when none qualifies. Price searches stop at $1 billion.
Estimated market value is your assumption, not an appraisal. Selected secondary financing is not lender approval. Actual terms, cash requirements, underwriting, lien limits and transaction costs may differ. No eligibility, minimum down payment or loan-to-value approval is modeled. An infeasible option has no actionable payment or economic-benefit result.
Not every mortgage is assumable. FHA and VA loans may allow assumptions subject to applicable requirements; USDA assumptions depend on the program and may use different terms. Conventional mortgages are often not freely assumable. Obtain loan-servicer and program approval. VA entitlement, substitution of entitlement and release of seller liability need special consideration. This fixed-rate model assumes the existing note’s rate and remaining term continue; confirm that your actual assumption does so.
Mortgage insurance is separate from P&I. Entered monthly insurance is held constant until that first mortgage’s scheduled payoff; no early cancellation, premium refund or automatic disappearance on assumption is assumed. A second loan has no additional insurance modeled. Fees are paid at purchase. Closing costs and the other-cost field must not repeat the same charge. No savings, approval or seller premium is guaranteed.
Sources: CFPB assumption disclosure guidance; CFPB note rate versus APR; VA assumption and release of liability; VA entitlement guidance; USDA program handbooks; HUD housing handbooks.