STRATEGY PAPER

The True Cost of a Down Payment

$100K down isn’t $100K. It’s $100K plus everything it could have earned.
Last reviewed March 2026 · DwellQ Research6 SOURCES

Key Findings

01$100K at 7% annual return grows to ~$140K in 5 years, ~$197K in 10 years
02$200K down (coastal markets) forgoes $280K–$394K in potential 5–10yr growth
0320% down avoids PMI ($167–$333/mo on $400K loan) but locks up more capital
04If expected return > mortgage rate, less down payment may yield higher net worth
05PMI elimination often occurs sooner than expected due to home appreciation
06DwellQ quantifies this tradeoff by running both paths simultaneously

Capital Allocation, Not Sunk Cost

A down payment is a capital allocation decision, not a simple expense. Every dollar deployed as a down payment is a dollar removed from your investment portfolio. The true cost of a $100K down payment isn’t $100K—it’s $100K plus the compound returns that money would have generated if invested elsewhere.

The Compounding Math

At 7% nominal annual return: $100K grows to ~$140K in 5 years, ~$197K in 10 years, and ~$276K in 15 years. A $200K down payment (common in coastal markets) would grow to $280K in 5 years and $394K in 10 years. This forgone growth is the opportunity cost that most calculators ignore entirely.

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PMI vs Capital Allocation

Putting 20% down avoids Private Mortgage Insurance (PMI), typically 0.5–1.0% of the loan annually. On a $400K loan, that’s $2,000–$4,000/yr ($167–$333/mo). But keeping more capital invested may generate returns that exceed the PMI cost. At 7% returns, $50K in additional invested capital generates ~$3,500/yr in expected growth. PMI also often falls off automatically when the loan-to-value ratio reaches 78%.

When Less Down Is More

If your expected investment return exceeds your mortgage rate net of tax benefits, a smaller down payment (with PMI) may produce higher total net worth. At 6.1% mortgage and 7% expected return, the math is close. At 8%+ expected returns, the case for a smaller down payment strengthens. DwellQ models this tradeoff directly.

THE BOTTOM LINE
The down payment is a capital allocation decision. Every dollar in your down payment is a dollar not invested. DwellQ quantifies this tradeoff by running both paths simultaneously.

Frequently Asked Questions

Should I put 20% down to avoid PMI?+
Not necessarily. If your expected investment return exceeds your mortgage rate, the math may favor a smaller down payment even with PMI. Run both scenarios in DwellQ to compare 10-year net worth outcomes.
How much does PMI actually cost?+
Typically 0.5–1.0% of the loan balance annually. On a $400K loan: $2,000–$4,000/yr ($167–$333/mo). It falls off automatically at 78% LTV.
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RELATED ANALYSIS
The Opportunity Cost of a Down PaymentWhy Most Rent vs Buy Calculators Get It WrongHow Much Home Can I Actually Afford?
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METHODOLOGY
DwellQ research uses a net worth comparison framework. Both paths—buying (building equity minus all ownership costs) and renting (investing the down payment plus monthly surplus)—are modeled month-by-month over the full holding period. Assumptions are documented, sensitivity-tested, and sourced from publicly available data. This is scenario analysis, not financial advice.
SOURCES & REFERENCES
  1. Federal Reserve Bank of St. Louis. FRED: S&P 500 Total Return Index.[fred.stlouisfed.org]
  2. Urban Institute. Housing Finance Policy Center: Down Payment Assistance Programs.[urban.org]
  3. Freddie Mac. PMI Fact Sheet and Rate Schedules.[freddiemac.com]
  4. National Association of Realtors. Profile of Home Buyers and Sellers, 2024.[nar.realtor]
  5. Beracha, E. and Johnson, K.H. ‘Lessons from Over 30 Years of Buy vs Rent Decisions.’ Real Estate Economics, 40(2), 2012.
  6. Goodman, L.S. and Mayer, C. ‘Homeownership and the American Dream.’ J. of Economic Perspectives, 32(1), 2018.