12 Common Rent vs Buy Calculator Mistakes (and How to Avoid Them)

Common Rent vs Buy Calculator Mistakes

Using a calculator is great, but "Garbage In, Garbage Out" applies heavily. A small error in your assumptions (like rent growth or maintenance) can swing the result by €100,000 over a decade.

Here are the most common mistakes users make when modeling their scenario.

Mistake #1: Unrealistic Horizon (#horizon)

"I'll probably stay 10 years."

• Reality: Expats often move within 3-5 years. Young couples move when they have children.
• Impact: Buying has high upfront costs. If you leave early, you may not recover them. Test a shorter horizon to see if you are still safe.

Mistake #2: Ignoring Transaction Costs (#kosten-koper)

"The price is €400k."

• Reality: You pay €400k + ~5% in costs (tax, notary, advice). That's €20k of lost money on day one.
• Fix: Ensure Buyer Costs are set to ~4-6% (unless you have an exemption).

Mistake #3: Mixing Nominal vs Real Returns (#investment-return)

"The stock market does 10%!"

• Reality: That might be a nominal average, but adjusted for inflation it's lower.
• Fix: Be consistent. If you use inflation-adjusted rent growth (e.g., 0% real growth), use inflation-adjusted investment returns (e.g., 5% instead of 7-8%). Our calculator works best with nominal figures for everything, including regular inflation-based rent increases.

Mistake #4: Missing Ownership Costs (#maintenance)

"My mortgage is cheaper than rent."

• Reality: Your mortgage is the minimum you pay. Rent is the maximum you pay.
• Forgot: Maintenance (1% per year), VvE fees (€150+ for apartments), insurance, taxes (OZB). These add up to hundreds of euros per month.

Mistake #5: Tax Double-Counting

"I'll subtract tax from my return input."

• Reality: The calculator has a specific Box 3 Tax input (#box3).
• Fix: Enter the gross return in the return field, and the tax rate in the tax field. Don't do it twice.

A Quick Checklist Before You Trust the Result

1. Is your Time Horizon conservative? (e.g., 5 years instead of 10)
2. Did you include Maintenance costs? (0.5% - 1.0%)
3. Is your Rent Increase realistic? (Standard is 2-3%)
4. Did you account for Box 3 tax on your investments?

FAQ

Should I include inflation?

Yes. Rents are indexed annually (inflation + surcharge). Maintenance costs rise with labor/material prices. It is safest to assume costs rise by 2-3% per year.

What is a realistic investment return?

For a broad index fund (e.g., MSCI World), a long-term average of 7-8% is common. Using 10%+ is risky; using 4% is very conservative.

Related Pages

• Rent vs Buy Framework
• Kosten Koper Explained
• Box 3 for Expats
• Calculator Assumptions

Sources

• Last updated: 2026-07-03
• Nibud: Ownership Costs
• CBS: Inflation Data