Write the stay length before you write the rate. Honest number, not the listing-agent “forever home.” Under 3 years, round-trip costs usually beat any reasonable appreciation — rent, then rerun. At 7 years the spreadsheet is often a coin flip. At 12 years buying can win even with boring 3% appreciation if you actually stay. A job that relocates, a relationship that is not settled, a visa, a residency match: those are 2-year files. Do not let a 6.71% average pick this for you. 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 quote. Type the lock you were offered.
Do not skip the match, the 24% card, or the HYSA sleeve to “get in.” A 100% match on the first 6% is a 100% week-of return. A 24% card is a 24% guaranteed hole. Buying with $0 left after closing is how a water heater becomes a credit-card event. The payday file is match → HYSA → Roth/trad → taxable. Size the sleeve to 3 months of the new PITI + food, not 3 months of the old rent. If DTI is why the pre-approval died, that is the DTI tool and the 90-day cut, not a bigger house.
Compare all-in own vs rent, not P&I vs rent. All-in = principal + interest + property tax + homeowners insurance + HOA + maintenance + PMI. Maintenance is not optional; 1% of price per year is a planning number, not a quote — a $420,000 house is $4,200/yr before a roof. Worked defaults in the calculator: $420,000, 20% down, 6.71% 30-year (example, not today’s lock), tax $5,200, insurance $1,400, 1% maintenance, $0 HOA. P&I is $2,170; all-in is $3,070/mo against $2,400 rent. The slogan compared $2,170 to $2,400 and called it a win. It was not.
Cash to close is four lines, not one. Down payment + buyer closing (plan 2–3% of price if you are not rolling seller credits) + moving/immediate repairs + the sleeve from step 2. On $420,000, 20% is $84,000 and 2.5% closing is $10,500 — $94,500 leaves the brokerage before a single payment. FHA vs 5% vs 20% is the down-payment stack, not this file. Emptying the HYSA to hit 20% and skip PMI is how people become house-rich and cash-poor. PMI is a math problem (monthly × 12 × years to 80%); it is not a morality contest. Mechanics: 80% request / 78% automatic.
Price the down payment as capital that still has a job if you rent. The $94,500 in step 4 is not “thrown away” as a renter. At a 4.5% after-tax example (HYSA/T-bill blend — type yours; do not scrape a live APY into this page as if it were frozen) it is $129,413 in 7 years before counting the monthly gap. That is the opportunity cost. A 7% expected brokerage number is allowed only if you will keep contributing through a drawdown; a 4.5% cash number is the conservative pair this file uses. Park-the-cash mechanics: I-bonds vs T-bills vs HYSA.
Count the round trip, both doors. Buy-side 2.5% in. Sell-side typically ~5–6% (commission + title + credits — your market, not a national law). On the worked house, 6% of a 7-year, 3%-appreciation sale is $30,993. Principal you paid is not profit; it is a transfer from checking to equity that selling costs then haircut. A 3-year stay with the same 3% appreciation still leaves the renter ahead because those two friction lines have not been amortized. If you will not stay long enough to eat 8% round-trip, you wanted a lease.
Use conservative pairs for appreciation and rent inflation, then a 0% stress. Both unknown. The 2012–2021 print is not a forecast. Defaults in the calculator: 3% home / 3% rent (example). On the worked file, 7 years at 3%/3% is a coin flip — buy net worth $180,705 vs rent $174,451, buy ahead $6,255. Same 7 years at 0% appreciation: buy $89,951 vs rent $174,451 — rent wins by $84,500. If the 0% case wrecks you, you are underwriting a boom, not a house. Do not paste a Zillow 1-year print in as the forever rate.
The IRC 121 clock is a 24-month file, not a slogan about “tax-free houses.” Exclude up to $250,000 of gain single / $500,000 MFJ if you owned and used it as your main home for 2 of the 5 years before the sale, and you did not use the exclusion on another home in the prior 2 years (IRC 121; IRS Pub. 523 / Topic 701). Gain is sale price − selling costs − basis, not the sale price. Selling at month 18 for a job 50+ miles away can unlock a partial exclusion (prorated by months/24) — not $0, not the full $250k. Depreciation from a rental period is recaptured and is not sheltered. Primer: home-sale exclusion. If the honest stay is under 2 years and it is not a qualifying move, model the gain as taxable.
Read 3 years, 7 years, and 12 years before you pick a team. Same $420,000 / $84,000 down / 6.71% example / $2,400 rent / 3% & 3% / 4.5% opportunity. 3 years: renter $131,205 vs buyer $107,000 — rent by $24,204. 7 years: buyer $180,705 vs renter $174,451 — buy by $6,255 (a rounding error against a 0% year). 12 years: buyer $297,027 vs renter $220,539 — buy by $76,488 if you actually stay and the 3% pair holds. Type your quote and your stay into the All Access calculator (shareable hash, no Chart.js CDN). Defaults are examples.
Lifestyle is allowed to win — write it so you do not pretend the spreadsheet said it. Control of the kitchen, a dog, a school zone, no landlord at month 11: those are values. They are not a 4.5% return. If the 7-year sheet is within ~$10k and you want the house, buy and own the reason. If the sheet says rent by $24k at 3 years and you buy anyway because a podcast said millennials are behind, you wanted a slogan. Nobody is required to buy; nobody is required to rent to prove they can do math.
Do not take the standard deduction out of the payment in your head. 2026 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 that doubling permanent and temporarily lifted the SALT cap to $40,400 in 2026 (MAGI phase-out $505,000). Qualified residence interest is still capped at $750,000 of post-December 15, 2017 acquisition debt. If Schedule A still loses to $32,200, the mortgage did not “save you 22%.” The interest is a cost. Haircut the coupon only for the fraction you actually itemize — same after-tax units as the extra-vs-invest file.
Hard-stop list: buying because rent is “thrown away,” emptying the HYSA to hit 20%, skipping the 401(k) match to afford PITI, treating the Sep 3, 2026 PMMS 6.71% print as your lock, underwriting 7% appreciation because 2013–2021 happened, selling in month 18 and assuming IRC 121 is a free $250k, comparing P&I to rent, stretching DTI past comfortable because the lender said yes, and cashing out a future cheap first lien to “invest the difference” (that file is HELOC vs cash-out — do not rebuild it here). Lock a quote, inspect, rerun at +1% rate. If the +1% version wrecks the leftover, you are one refi-cycle from stress. If the leftover after match + sleeve + this PITI is $0, you wanted the lease.
One-page decision
Stay under 3 years: rent. Stay 7 years, 3%/3% pair, 20% down, all-in $3,070 vs $2,400 rent: a coin flip — lifestyle may break it; 0% appreciation does not. Stay 12 years and you will actually be there: buying can win on the same boring pair. Match, 24% cards, and 3 months of the new PITI first. Cash to close is down + 2.5% + moving + sleeve. IRC 121 needs 24 months. Type the lock; the 6.71% average is not a quote.
Worked example (educational, not advice): $420,000 price, $84,000 down, 6.71% 30-year (PMMS week of Sep 3, 2026, labeled average), $2,400 rent, tax $5,200, insurance $1,400, 1% maintenance, 2.5% buy / 6% sell, 3% appreciation, 3% rent inflation, 4.5% after-tax opportunity. P&I $2,170; all-in $3,070/mo. 3 years: rent $131,205 vs buy $107,000. 7 years: buy $180,705 vs rent $174,451 (0% appreciation: buy $89,951). 12 years: buy $297,027 vs rent $220,539.