Compound Interest: The Power of Investing vs Property Ownership

Why compound interest is central to the buy or rent decision

When people wonder whether to buy or rent, they usually look at price per square meter, mortgage rates around 3.6%, notary fees, and property tax. One key parameter in the simulator is often overlooked: the investment rate (taux_placement) — the potential return on the money you don’t put into real estate if you remain a tenant.

This investment rate activates a powerful mechanism: compound interest. In the buy-or-rent.net / acheter-ou-louer.com simulator, this single parameter can completely flip the result between buying and renting, especially over 20–30 years.

The goal here is not to claim that financial investing is always better than owning a home. The outcome depends on your time horizon, risk profile, rent inflation, general inflation, property tax, mortgage rates, and more. The following is general education, not personalized financial advice.

Quick reminder: what is compound interest?

Compound interest means you earn interest on your initial capital plus the interest already earned. In practice:

• at 0% return, 10,000 € stays 10,000 € (ignoring inflation);
• at 3% per year, 10,000 € becomes about 18,061 € in 20 years;
• at 5% per year, 10,000 € becomes about 26,533 € in 20 years.

The formula is: Final capital = Initial capital × (1 + rate)^years. In the simulator, the taux_placement parameter applies this exact logic to the tenant’s savings (or to any surplus cash of an owner).

Where does the investment rate appear in a buy or rent comparison?

In a typical scenario:

• If you buy: you pay a down payment, notary fees (7–8% on older property, 2–3% on new build), agency fees (3–5%, often built into the price), monthly mortgage payments (loan rate ≈ 3.6%), borrower insurance (around 0.25–0.45% of the principal), property tax (from about €450 to more than €5,000 per year depending on the city, with annual revaluation), and maintenance/renovation.
• If you rent: you pay rent, which rises with the rent index (linked to inflation), but you keep your initial capital and maybe a monthly surplus if rent is cheaper than a mortgage. This money can be invested at a certain investment rate (savings accounts, bonds, ETFs, etc.).

The core question becomes: does the compounded return on this invested capital offset the fact that you’re not repaying a loan and building home equity? That is exactly what the buy or rent simulator helps you visualize.

Example 1: 50,000 € down payment — buying vs investing

Assume a household with:

• Down payment: 50,000 €
• Time horizon: 25 years
• Inflation: 2.5%/year (affects rents, property tax, renovation costs)
• Mortgage rate: 3.6% over 25 years
• Investment rate (taux_placement): 4%/year net of fees and tax (an ambitious but plausible average over the long term for a diversified portfolio, with risk).

Scenario A: buying a home

Suppose you buy a 300,000 € property in the existing stock:

• Purchase price: 300,000 €
• Notary fees (7.5% estimate): 22,500 €
• Agency fees included in price: 4% ≈ 12,000 € (a hidden but real cost)
• Down payment: 50,000 € (used immediately)
• Mortgage amount: 272,500 € (price + notary – down payment)
• Loan rate: 3.6%; term: 25 years

Approximate monthly payment (excluding insurance): 1,375 €. With borrower insurance at 0.30% of principal (about 818 €/year initially, decreasing), the total starts around 1,430 € per month.

Add to that:

• Property tax: 1,200 €/year, with 2.5% yearly increase;
• Maintenance/renovation: roughly 1% of property value per year (3,000 €/year on average), highly dependent on energy performance and needed upgrades.

After 25 years, you fully own the property, whose value will depend on the real estate market and inflation.

Scenario B: renting and investing instead

You choose to rent a similar home for 1,100 €/month:

• Initial rent: 1,100 €/month
• Annual rent increase: 2.5% (aligned with inflation and the rent index)
• No property tax, no structural renovation costs on your side.

You keep your 50,000 € and invest it at 4%/year. On top of that, every month you invest the difference between the owner’s monthly outflow (1,430 €) and your rent (1,100 €), i.e. 330 €, as long as this gap exists.

What compound interest does in this case

1) 50,000 € invested at 4% for 25 years:

• Final capital ≈ 50,000 × (1.04)^25 ≈ 133,000 €

2) Average monthly savings (330 €/month initially, shrinking as rent catches up):

• If we simplify with an average of 250 €/month over 25 years, final capital ≈ 250 × [((1.04)^(25×12) – 1) / (1.04^(1/12) – 1)]
• This gives roughly 145,000 € (order of magnitude).

In total, the renter-investor could end up with around 280,000 € in financial assets (ignoring detailed taxation), thanks to compound interest on the chosen investment rate.

On the other side, the homeowner owns a property whose value depends on:

• price growth or decline (e.g. +1.5%/year or flat markets),
• renovations performed (insulation, heating system, energy rating improvements),
• local tax policy (property tax having sometimes jumped more than 20% over a few years in some cities).

The buy or rent simulator compares the net housing wealth (property value – remaining mortgage – extra costs) with the financial wealth created by compound interest on your taux_placement.

Example 2: small changes in investment rate, big long-term impact

Take the same 50,000 € down payment, ignoring monthly investing. Over 25 years:

• at 2%/year: 50,000 × (1.02)^25 ≈ 82,000 €
• at 4%/year: 50,000 × (1.04)^25 ≈ 133,000 €
• at 6%/year: 50,000 × (1.06)^25 ≈ 214,000 €

Between 2% and 6%, the difference in final capital exceeds 130,000 € for the same initial amount. That’s why the taux_placement parameter is so critical in any investing vs buying analysis.

At the same time, remember:

• a 3.6% mortgage rate is a certain cost;
• a 6% investment return is an uncertain reward (market volatility, possible losses, taxation).

Your buy or rent comparison must therefore factor in risk, not just raw numbers.

Compound interest vs the hidden costs of owning

To understand the real trade-off, you need to pit compound interest against several ownership costs:

• Notary fees: 7–8% in the existing stock, 2–3% in new build. This money is locked in from day one and no longer benefits from compound growth.
• Property tax: 1,200 €, 2,000 €, sometimes more than 5,000 €/year in major cities. With annual revaluation, this burden often grows faster than general inflation.
• Borrower insurance: 0.25–0.45% of the principal per year, totaling several thousand euros over the life of the loan.
• Maintenance and energy renovation: façade, roof, heating systems, compliance with changing regulations, energy rating upgrades to avoid future rental restrictions.
• Prepayment penalties: if you sell before the end of the mortgage, up to 3% of the remaining principal or six months of interest (legal cap).

All of these are amounts that cannot earn compound returns in financial investments. The simulator tracks these cash flows and compares them with a renting/investing scenario.

Compound interest and inflation: a silent tug of war

Another key factor is annual inflation. It affects several levers:

• it pushes rents up via the rent index, sometimes eroding a tenant’s monthly surplus;
• it can push property prices higher over the long run, though not always at the same pace;
• it erodes the real value of your financial assets if your investment rate is below inflation.

For example, if your taux_placement is 2% and inflation is 3%, your capital grows in euros but shrinks in purchasing power. Conversely, a 5% return in a 2.5% inflation environment gives you a real return of roughly 2.5%/year, amplifying the effect of compound interest.

The true power of compound interest is therefore measured in real terms after inflation, not just nominal percentages.

Invest first, buy later: a possible middle way

There is a hybrid strategy in the buy or rent debate:

• stay a tenant for a few years,
• invest aggressively (down payment + monthly surplus) at a potentially higher taux_placement than the mortgage rate,
• then buy later with a larger down payment, reducing the loan amount, the term, and total interest paid.

In this strategy, compound interest works for you before you buy. But there are risks: changing mortgage rates, rising house prices, and uncertain financial market returns.

The buy-or-rent.net / acheter-ou-louer.com simulator lets you test such a plan: for example, rent for 10 years with a given investment rate, then buy, and compare that to buying immediately.

How to use the taux_placement parameter in practice

To make the most of the taux_placement parameter in the simulator:

• Start with a cautious scenario (e.g. 1.5–2%/year, similar to safe savings after inflation);
• Try a moderate scenario (3–4%/year, a balanced diversified portfolio);
• Then a dynamic scenario (5–6%/year, equity/ETF heavy, more volatile).

Watch how the buy or rent outcome changes: the “better” option can flip entirely just by tweaking the investment rate by a few points. This illustrates how powerful compound interest is in the long run.

Keep in mind, though, that higher expected returns generally mean higher risk. The simulator is an educational tool, not a performance guarantee.

Conclusion: property vs the power of compound interest

Compound interest reshapes the buy or rent discussion: money not locked into property can, over the long term, grow into a significant nest egg if your investment rate outpaces both mortgage costs and inflation. At the same time, owning gives you a tangible asset, some protection against rent increases, and control over your home.

There is no one-size-fits-all answer: it depends on your situation, time horizon, risk tolerance, tax environment, future rents, property tax dynamics, and interest rates. Use compound interest as a quantitative lens, not a universal rule.

This article is for information only and does not constitute personalized financial advice. To see exactly how taux_placement and compound interest affect your own case, simulate your situation on buy-or-rent.net.