The Arithmetic of Tenant Quality

The folk wisdom says "one month of vacancy costs more than €50/month of extra rent." Run it: €50 × 12 = €600. One month of our €600 rent = €600. They are exactly equal. The slogan is wrong as stated – and understanding why the conclusion still mostly holds is the useful part.

1. A turnover is not one month. Realistic Mieterwechsel cost: 1.5 months vacant (€900), redecoration and small repairs (€1,500), letting effort or an agent under the Bestellerprinzip (€500). Call it €2,900 – nearly five months of rent.

2. Amortise it over the tenancy. Tenant A pays €600 and looks likely to stay 4 years: €7,200 − €2,900/4 = €6,475/year effective. Tenant B pays €650 and stays 2.5 years: €7,800 − €2,975/2.5 = €6,610/year. B still wins. Break-even is B moving every 2.2 years. So on vacancy arithmetic alone, taking the €50 is usually correct – the honest version of the slogan is much weaker than the version people repeat.

3. The term that actually decides it is default, not vacancy. A non-paying tenant in Germany runs 6–12 months to a completed Räumungsklage: €4,000–7,000 of lost rent, €2,000–4,000 of legal and enforcement cost, plus restoration. Call the event €10,000 – sixteen years of a €50/month premium. If stretching the rent by €50 lifts your annual probability of that event from 1% to 6%, expected cost rises €500/year and the premium is gone.

So the correct statement is: the €50 is not paid for by vacancy risk, it is paid for by credit risk – and credit risk is the one thing you can actually screen.

Screening that predicts, and that is standard practice: SCHUFA-BonitätsAuskunft, the last three Gehaltsabrechnungen, a Mietschuldenfreiheitsbescheinigung from the previous landlord, and the ratio test – warm rent ≤ 30% of net household income. At €600 cold plus €150 Nebenkosten, that means net household income of at least €2,500/month. A tenant at 45% of net income is not a better tenant because they agreed to €650.

The strategic version: with market rents rising ~3% (vdp had new-letting rents +3.2% year-on-year in Q2 2026), a tenant who stays eight years ends up 10–15% below ortsübliche Vergleichsmiete. That is the price of a reliable counterparty, and it is usually worth paying. You bought a bond, not an auction.