5 Mortgage Myths I Keep Hearing from First-Time Buyers

I got my CFP in 2009, which, if you're doing the math, means my entire professional career was shaped by a housing crisis. The first fifteen clients I ever worked with were underwater on their mortgages. Two of them lost their homes. I have some feelings about housing finance, is what I'm saying.

Here's the thing: of all the financial topics I've sat across a kitchen table and explained to people — retirement accounts, estate planning, variable annuities (ugh) — mortgages are the one where the folk wisdom is most aggressively wrong. Not kind-of wrong. Wrong in ways that cost real families tens of thousands of dollars.

So here are the five myths I heard most often in my years of practice, in no particular order. A couple of these will make you mad if you've been telling yourself them for years. I'm sorry in advance. I'm trying to save you money.

Myth 1 "You need 20% down or you shouldn't buy"

This one is repeated so often it's almost a religious tenet. And I get why — it's defensible, it's conservative, and it sounds like responsible adult financial advice. The problem is it's wrong for a huge number of buyers, and the people repeating it don't usually do the math.

Here's the calculation people skip. In 2018, I worked with a couple in Seattle — let's call them the K's. Two public school teachers, combined income around $135K, they had $60K saved. They wanted to buy a $550K house (this was 2018; that house is $900K now, and that's the whole point of the story).

Standard advice would have said: you have $60K, that's 10.9% down, come back when you have $110K. They listened to that advice for about a year. By the time they had $85K saved, the same house was $640K. They now needed $128K. By the time they had $110K, the house was $720K and they needed $144K. You see where this is going.

They finally bought at $640K with 8% down and PMI of about $180/month. Was PMI annoying? Yes. Did the PMI cost them less than the $90K of home appreciation they would have missed waiting? Obviously yes. By a factor of about 40.

Now — to be fair to the 20% crowd — there are situations where waiting makes sense. If home prices in your area are flat or declining (parts of the Midwest, some older suburbs), the math changes completely. If you're not sure you'll stay in the home for 5+ years, buying at all might be a mistake, regardless of down payment. If you can't afford the monthly payment comfortably at 10% down, the solution isn't more time to save — it's a cheaper house.

But the blanket "20% or nothing" rule has cost more first-time buyers more wealth than almost any other piece of financial advice I can think of. Run the actual numbers for your house in your market, ideally with a mortgage calculator that shows PMI separately, and make a decision based on that. Don't make it based on something your uncle told you at Thanksgiving.

Myth 2 "A 15-year mortgage is always better than a 30-year"

I'm going to get hate mail for this one. I can feel it already.

The argument for 15-year is real: lower interest rate (usually 0.5% to 0.75% lower), enormous interest savings over the life of the loan, you own your house free and clear in half the time. All true. All math I agree with.

The problem is the argument stops there. It ignores what the higher payment does to the rest of your financial life.

Quick numbers, using roughly 2024 rates. $400K loan. 15-year at 6.25% is about $3,430/month. 30-year at 6.875% is about $2,630/month. The 15-year "saves" you roughly $170K in total interest. Huge number, right?

Except: the $800/month difference has to come from somewhere. For almost every client I worked with, it came out of retirement contributions. They'd shrug and say "I'll catch up later." Reader, they did not catch up later.

Run the opportunity cost honestly. That $800/month, invested in a 401(k) with a typical 4% employer match, growing at even a conservative 7% real return over 15 years, becomes roughly $315,000. That's almost double what you "saved" in mortgage interest. And — this matters — it's liquid, it's tax-advantaged, and it's diversified, instead of being locked up in the walls of your specific house in your specific neighborhood.

I worked with a young doctor (I'll call her Priya) in 2017 who was about to sign a 15-year because "that's what her parents did." We walked through the numbers on her household's retirement trajectory with a 15 vs 30. She took the 30, redirected the difference into her employer's 403(b), maxed her backdoor Roth, and — fast forward — her retirement accounts are now worth more than her house.

The only buyers I think should actively pick a 15-year are: people who have already maxed every tax-advantaged account they have access to, people close to retirement who want the psychological weight of the mortgage gone, and people who genuinely cannot trust themselves to invest the difference. That last one is legitimate. If you know yourself, and you know the extra cash flow from a 30-year will just evaporate into lifestyle creep, then sure, a forced-savings 15-year is better than nothing. I just think it's the third-best option, not the default.

Myth 3 "Pay off your mortgage as fast as possible"

This is the same myth as #2, dressed up in different clothes, and it shows up constantly in FIRE subreddits and Dave Ramsey's books. With enormous respect to people who've paid off their houses — I understand the emotional weight of it, and I'm not going to argue with anyone about how they feel. But from a pure financial standpoint, for most buyers with sub-7% mortgages, it's a mistake.

Here's the cleanest way I know to explain it. Every extra dollar you put toward mortgage principal is a dollar you're "investing" at your mortgage rate, after tax. If your mortgage is at 6.5% and you itemize (or live somewhere that the standard deduction already covers you), your effective rate might be more like 5%. Meanwhile, the historical S&P 500 real return is something like 6.8% over long periods. Long-term treasuries, even in 2026, are paying north of 4%. A boring total-bond-market fund is clipping 4.5% with near-zero correlation to your home equity.

If you prepay your mortgage with money that could have gone into a diversified investment portfolio, you're voluntarily accepting a lower expected return and concentrating more of your net worth in a single illiquid asset. That's not prudent. That's the opposite of prudent.

(The one real counter-argument — and I'll grant it — is psychological. Debt stresses people out. If paying off your mortgage lets you sleep, that's worth real money. I've had retired clients in their late 70s who wouldn't touch a stock market for any reason, and for them, a paid-off house was the right answer. But most 35-year-olds in the prepayment camp are optimizing for a kind of safety they don't actually need yet.)

If you want to see the cost of prepayment in dollars instead of percentages, plug your numbers into a debt payoff calculator alongside an investment growth calculator at your expected portfolio return. The gap between the two, compounded over 25 years, is usually somewhere between "a new car" and "a second down payment."

Myth 4 "Fixed-rate is always safer than ARM"

This one is more defensible than the others, I'll be honest. After 2008, the industry — and my entire profession — got religion about fixed-rate mortgages, and I understand why. Adjustable-rate mortgages got a lot of people into houses they couldn't afford, especially the exotic teaser-rate products that don't really exist anymore. I'm not going to defend Option ARMs. They deserved to die.

But the modern ARM is a different animal. A 7/6 ARM (fixed for 7 years, then adjusts every 6 months) with a 2% annual cap and a 5% lifetime cap is not the same product as a 2006 negative-amortization nightmare. And for a specific kind of buyer, it's actually the smarter choice.

Who is that buyer? Someone who knows they won't be in the house for 10+ years. Military families who move on a known rotation. Medical residents with a predictable training timeline. Tech workers in a phase of their career where they expect to relocate. People buying a starter home they plan to upgrade from.

If you're confidently moving in year 5 or year 7, paying the 30-year fixed premium (typically 0.5% to 1.0% higher than a 7/6 ARM) is just lighting money on fire. You're paying for 23 years of rate protection you will never, ever use.

A client I had in 2015, an Army major, was about to take a 30-year fixed on base housing near JBLM because "fixed is safer." She had orders that meant she'd be PCSing within 4 years. We ran the numbers on a 5/1 ARM. She saved about $180/month for the 4 years she owned the house and sold it before the rate even adjusted once. The "safety" of the fixed-rate would have cost her roughly $8,600. Safety isn't free, and sometimes you don't need as much of it as you think.

To compare scenarios cleanly, use a loan comparison tool with both options side by side, including the realistic sale year. Don't let "ARMs are scary" do your math for you. (That said: if you're financially stretched, if you'd be in trouble at the worst-case capped rate, if your time horizon is fuzzy — take the fixed. I don't want anyone taking my endorsement of ARMs and running off a cliff with it.)

Myth 5 "My monthly payment is what the house actually costs me"

This one is less of a myth and more of a blind spot, but it's the one that bites first-time buyers the hardest. I saved it for last because it's the most universal.

Your mortgage P&I is the part everyone sees. The actual cost of owning a home includes, in no particular order: property taxes (which rise), homeowners insurance (which is going up fast in 2026, especially if you're anywhere near a coast or a wildfire zone), HOA fees (if you have them), PMI (if you put less than 20% down), maintenance (budget 1% of home value per year, honestly more in older homes), utilities (often double what you paid as a renter), and opportunity cost on your down payment.

I had a young couple in Austin in 2021 who bought a $480K house. Their quoted payment was $2,400. They budgeted $2,400. By year two, their actual monthly cost — P&I, taxes (which had gone up with reassessment), insurance (which went up 34% in one renewal because of the Texas insurance market), maintenance (a $7,000 HVAC replacement, spread out), and utility delta from their old apartment — was about $3,650. They weren't in financial trouble, but they were stressed, and they felt lied to, and they were kind of right to feel that way.

When a buyer comes to me now, I make them calculate a true monthly cost of ownership number before they ever talk to a realtor. That number is the mortgage P&I, plus 1/12 of annual property taxes, plus 1/12 of annual insurance, plus HOA, plus PMI, plus 1/12 of 1% of home value for maintenance, plus the delta in utilities from their current housing. For most buyers, it's 30-50% higher than the number their loan officer quoted them.

And — this is what most buyers miss — they should also be thinking about the APR of the mortgage, not just the note rate. Two loans at "6.5%" can have wildly different APRs once you fold in origination fees, points, and lender credits. An APR calculator makes this apples-to-apples. Get quotes from at least three lenders. Compare APRs, not rates. The difference between the best and worst offer for the same borrower is routinely 0.25% to 0.5%, which on a $400K loan over 30 years is — you can do this math yourself now — a real amount of money.

The meta-advice

If I had to compress fourteen years of financial planning into a single paragraph about mortgages, it'd be this: most of the traditional "rules" were written for a different macro environment (flat home prices, 4% CDs, predictable careers) and they haven't been updated for the world you're actually living in. Don't outsource the thinking to a rule of thumb. Actually run the numbers for your actual situation, with your actual income trajectory, at your actual house price, in your actual market. Use real calculators, not rough guesses. And be honest with yourself about what you're optimizing for — total wealth, peace of mind, optionality, legacy — because those optimize to different loan products.

I'll admit the things I still don't fully know. I don't know where rates are going (nobody does, don't trust anyone who says they do). I don't know how the insurance market will settle out after the last few years of climate-driven losses, and I think that's the single most underpriced risk in homeownership right now. I don't know whether rent-vs-buy math in places like Austin or Phoenix will look reasonable again this decade.

But I do know this: the couples who made the worst mortgage decisions I ever watched up close were not the ones who didn't know the rules. They were the ones who only knew the rules, and followed them past the point where the rules stopped applying. Don't be them. Think one level deeper than the advice. That's the whole job.