Pay the house or buy the market — with the coupon in front of you
Free primer: refi vs extra principal. PMI clock: 80% request / 78% automatic. Cheap first lien you might tap: HELOC vs cash-out. Paid steps assume you already know “the market beats 3%” is a slogan, and you want a file that treats a 3% COVID lien and a 7% 2023–26 lien as different animals.
Do this in order
Write the coupon on the payoff letter, not a national average. Remaining balance, remaining term, current P&I, whether PMI is still drafting. Freddie Mac’s Primary Mortgage Market Survey printed 6.71% on the 30-year the week of September 3, 2026 — that is an average of applications, not your rate. A 2020–21 first lien at 2.75–3.5% and a 2023–26 first lien at ~7% do not get the same extra-principal answer. If you cannot name the coupon to two decimals, you are about to automate a podcast.
Do not skip the match, the 24% card, or the HYSA sleeve to “feel aggressive” on the house. A 100% match on the first 6% is a 100% week-of return. A 24% card is a 24% guaranteed hole. Extra principal on a 3% loan is a 3% guaranteed return with no Tuesday liquidity. The payday file is match → HYSA → Roth/trad → taxable. This playbook starts after those three exist. If DTI is why a refinance died, that is the DTI tool, not extra principal.
Translate both options into the same after-tax units. Mortgage prepayment return ≈ coupon × (1 − federal marginal × the fraction of that interest you actually itemize). For many households after the 2026 standard deduction, that fraction is ~0, so 3% is ~3% and 7% is ~7%. Investing return is expected, volatile, and wrapper-dependent: Roth is the full expected number; taxable long-term gains sit in the 0/15/20% schedule (2026 15% band tops out at $545,500 single / $613,700 MFJ — Rev. Proc. 2025-32). Use a conservative expected number you would still accept in a flat decade, not last year’s S&P. Do not scrape a live quote into this page as if it were frozen.
Run the 2026 itemize test before you shave the coupon for “the deduction.” Standard deduction is $16,100 single / MFS, $32,200 MFJ, $24,150 HOH (Rev. Proc. 2025-32 / IR-2025-103). OBBBA (P.L. 119-21) made the doubled standard deduction permanent and temporarily lifted the SALT cap to $40,400 in 2026, phasing out from MAGI $505,000. Qualified residence interest stays capped at $750,000 of post-December 15, 2017 acquisition debt ($375,000 MFS); the old $1 million cap only still applies to debt incurred on or before that date. Home-equity interest is deductible only if the cash bought, built, or substantially improved this house — consumer-debt HELOCs do not get a 2026 write-off. If Schedule A still loses to $32,200, extra principal does not “lose a deduction.” It never had one.
If PMI is still on the stub, that is the first extra dollar — not the market, not principal beyond 80%. Conventional: request cancellation at 80% of original value; automatic drop is typically 78% if you are current (Homeowners Protection Act). A 0.8% PMI on a $340,000 loan is $2,720 a year — a guaranteed ~0.8% on top of the coupon until it dies. Extra principal that gets you to 80%, then stop and rerun this file. FHA annual MIP is a different animal; removing it is usually a conventional refinance, priced in the break-even guide. Do not cash-out back over 80% and re-attach PMI you just killed. Mechanics: how to remove PMI.
The 3% first lien is a keep-the-loan file. Extra $500/month on $300,000 remaining, 25 years, 3%, taking the standard deduction: paid off in 199 months instead of 300, interest saved $46,061. Same $500 invested 10 years at a 7% expected return (example, not a forecast) is $86,542 vs extra equity of $69,871 — the brokerage is ahead on expected net worth and you can sell it on a Tuesday. Do not refinance this loan into a 6.7% cash-out to “invest the difference.” That is lighting the cheap first lien. If you need a slice of equity, the file is tap equity without lighting the 3% loan — HELOC or recast, not a new first lien. Recast is for putting a lump in (same 3%, lower payment, small fee).
The 7% first lien is an extra-principal file until the after-tax expected return clearly beats 7% with money you can lose. Same $300,000, 25 years, extra $500: paid off in 190 months, interest saved $139,558. After 10 years the extra equity is $86,542 — the same dollars as investing $500/month at 7%, because avoided interest compounds at the coupon. Extra wins on guarantee; the market only wins if your after-tax expected return is higher and you keep contributing through a drawdown. Taxable at 15% long-term on a 7% expected is ~5.95% — extra still wins if you are not itemizing. Itemizing at 22% haircuts the 7% coupon to ~5.46%; a Roth at 7% expected is then slightly ahead on the mean and behind on the worst year. Type your coupon and your expected number into the extra-vs-invest calculator (All Access). Defaults are examples.
Price liquidity as a number, not a vibe. Principal you send the servicer comes back only by selling the house, cashing out (new first lien), or drawing a HELOC. All three get awkward in a recession — underwriting tightens the week your job looks shaky. Brokerage equity can be sold on a Tuesday; a Roth has a 59½ clock unless an exception applies. If the emergency sleeve is under 3 months of PITI + food, extra principal is how people become house-rich and cash-poor. Job-risk premium of 50–100 basis points on the “invest” side is allowed. If you are shopping the tap anyway, start at HELOC vs cash-out so you do not rewrite a 3% first lien by accident.
Size the “free and clear” feeling so it can lose. A paid-off house is a housing-cost floor of tax + insurance + maintenance, not $0. It is also a 3% or 7% return you can no longer earn on that principal because the loan is gone. If the reason you want extra principal is “I sleep,” write the years of PITI you are buying: $500/month extra on the 7% loan cuts ~9 years. If the reason is “the internet said debt is bad,” you wanted the 3% vs 7% split in steps 6–7, not a slogan. Nobody is required to carry a mortgage; nobody is required to prepay a 3% loan to prove adulthood.
Refi vs extra vs recast are three products. Extra monthly principal is a refinance you already own — same coupon, shorter term, $0 closing. Recast is a lump-sum principal dump that lowers the payment and keeps the rate. A rate-and-term refinance only wins if the new coupon, after costs, beats the remaining coupon on money you will still be in the house to enjoy — break-even months, match or shorten the remaining term unless cash-flow is the actual goal. A 30-year reset that shrinks the payment and raises lifetime interest is a cash-flow product, not a win. Do not let a loan officer pick extra vs refi before you see the remaining term on the payoff letter.
Automate one draft, labeled, with a kill rule. Extra principal: a second ACH the servicer codes as principal only — not “put it in escrow,” not a 13th payment that the website applies to next month’s P&I. Investing: payday+2 draft to the Roth or brokerage after the HYSA sleeve is full. Screenshot the election. Kill extra principal the month a 24% card reappears, the HYSA sleeve is raided, or you refinance. Revisit the 3% vs 7% split if you recast, if PMI drops, or if you leave the job that funds the extra. Future-you will not remember a verbal “I’ll throw the bonus at the house.”
Hard-stop list: skipping the 401(k) match to prepay a 3% loan, cashing out a 3% first lien to buy index funds, extra principal while a 24% card is open, emptying the HYSA to feel house-rich, treating the Sep 3, 2026 PMMS 6.71% print as your coupon, signing a 30-year reset because the payment shrank, extra principal past 80% LTV while PMI is still drafting, and a HELOC as a checking account. If the leftover after match + sleeve + minimums is $0, you wanted the stub and the card, not a target-date and not a “debt-free” sticker. If the leftover exists and the coupon is 3%, the brokerage is the default; if the coupon is 7% and you take the standard deduction, extra principal is the default.
One-page decision
Match, 24% cards, HYSA sleeve of PITI first. PMI to 80% next. Then the coupon: 3% first lien, standard deduction, leftover dollars → Roth/brokerage, keep the loan, HELOC only if you must tap. 7% first lien, standard deduction, leftover dollars → extra principal, or a Roth only if you will accept a down year. Itemizing at 22% haircuts a 7% coupon to ~5.46% — close enough that liquidity and job risk break the tie. Do not rewrite a cheap first lien to “invest the spread.” Type your numbers; the 6.71% average is not a quote.
Worked example (educational, not advice): $300,000 remaining, 25 years, extra $500/month, 22% federal, standard deduction. 3% P&I $1,423; extra pays off in 199 months; 10-year extra equity $69,871 vs $500/month at 7% expected $86,542. 7% P&I $2,120; extra pays off in 190 months, interest saved $139,558; 10-year extra equity $86,542 — a tie on expected dollars, extra wins on guarantee. Freddie Mac PMMS 6.71% (week of Sep 3, 2026) is the labeled average in step 1, not an input.