Decision 1: Defaulting to the 30-Year Without Comparing Terms
Most buyers choose the 30-year because it has the lowest monthly payment. This is rational. What often goes uncalculated is the total cost difference versus a 20-year or 15-year, or the payoff-age difference that has major implications for retirement planning. For a 35-year-old buyer, the payoff ages are 65, 55, and 50 respectively — dramatically different retirement scenarios.
Term comparison for 35-year-old buyer — $400,000 loan, 2025 rates
| Loan | Rate | Monthly P&I | Total Interest | Payoff Age (Buyer 35) |
|---|---|---|---|---|
| $400K, 30yr | 7.00% | $2,661 | $557,960 | 65 |
| $400K, 20yr | 6.625% | $3,059 | $334,160 | 55 |
| $400K, 15yr | 6.375% | $3,451 | $221,180 | 50 |
| $400K, 25yr | 6.75% | $2,868 | $460,400 | 60 |
A 35-year-old who chooses a 20-year over a 30-year is mortgage-free at 55 instead of 65. They save $223,800 in interest and redirect $3,059/month into investments for 10 years — potentially generating $500,000+ in additional wealth by retirement.
Decision 2: Accepting the First Rate Without Shopping
Accepting the first rate quote is the easiest and most expensive path. Studies consistently show that buyers who get 5 quotes receive significantly better terms than those who get 1. On a $400,000 loan, a 0.5% rate improvement saves $125/month and $45,000 over 30 years. Getting 5 quotes takes 3 to 4 hours. That is a $15,000 per hour return — better than almost any other use of your time in the homebuying process.
Decision 3: Buying Points Without Break-Even Math
Paying for discount points without calculating break-even is a common and expensive mistake. At $8,000 for 2 points saving $133/month, break-even is 60 months. If you sell or refinance at Month 48, you spent $8,000 for $6,384 in savings — a $1,616 net loss. The same decision with a 7-year hold would yield $3,156 in net savings. The outcome is determined entirely by how long you stay — and nobody checks this before buying points.
Decision 4: Ignoring the Amortization Schedule
Understanding how slowly the principal falls in early years changes how people make mortgage decisions. Most buyers do not know that after 5 years of perfect payments on a $400,000 30-year loan at 7%, their balance is still $376,000. They have paid $159,660 total — but their balance has only dropped $24,000. This knowledge is valuable for planning refinancing windows, extra payment strategy, and understanding PMI removal timing.
Decision 5: Not Stress-Testing the ARM
An ARM at 6.5% instead of 7.0% fixed saves $119/month initially on a $400,000 loan. This $119/month savings is real and attractive. What is also real: if the ARM adjusts to 9.5% at year 6, the payment jumps from $2,528 to $3,356 — an $828/month increase. For a budget that was already tight at $2,528, this is a crisis. The 30-year fixed buyer at $2,661 never faces this risk.
ARM vs. fixed over 10 years including adjustment risk
| Scenario | Initial Payment | Year 6+ Payment | Risk Factor |
|---|---|---|---|
| 5/1 ARM at 6.5% | $2,528 | $3,356 (at 9.5%) | $828/month increase possible |
| 7/1 ARM at 6.625% | $2,554 | $3,356 (at 9.5%) | $802/month increase possible |
| 30-yr Fixed at 7.0% | $2,661 | $2,661 (forever) | No adjustment risk |
| Break-even (fixed vs. ARM) | — | — | Fixed wins at Year 5.5 if ARM adjusts up |
Decision 6: Down Payment Size Without PMI Math
The most common down payment mistake is choosing a round number (5% or 10%) without calculating the PMI cost and removal timeline. On a $420,000 home, the difference between 5% and 10% down is $21,000 in additional cash — but it reduces PMI by approximately $115/month (from $284 to $169). At $115/month, the extra $21,000 in down payment has a 'return' of 6.6% annually — better than many safe investments.
Run Every Scenario Before You Decide
Compare loan terms, down payments, and rates — find the combination that wins over your specific holding period.