The Zinsbindung Decision Is an Option Trade

August 2026 benchmark quotes (Dr. Klein, 11 Aug, €350k reference loan): 10 years 3.70% Sollzins, 15 years 3.91%, 20 years 4.04%. Add roughly 0.5pp for a 95% Beleihungsauslauf like ours. Context: the ECB deposit rate is 2.25% after a +25bp move on 11 June 2026 – the first rise since 2023. Nobody, including the ECB, knows the path from here.

The naive framing is "how long do I want certainty?" The correct framing is: you are buying a one-sided option, and §489 BGB hands you the exit for free.

§489 Abs. 1 Nr. 2 BGB: a borrower on a fixed-rate loan may terminate in full ten years after full disbursement, with six months' notice and no Vorfälligkeitsentschädigung. It cannot be contracted away.

So a 15-year Zinsbindung is not fifteen years of commitment. It is 10.5 years of commitment plus 4.5 years of protection you may abandon at will. Rates fall → you leave at month 126 and refinance. Rates rise → you keep 3.91% to 2041. Heads you win, tails you don't lose.

Price the option. On €170,000 the 10y→15y premium of 0.21pp is €357/year, roughly €3,570 over the committed decade. What it protects: 4.5 years on a balance of about €129,000. If 2036 rates are 2pp higher, that is 2% × €129,000 × 4.5 ≈ €11,600. Paying €3,600 for €11,600 in a plausible bad state is cheap insurance. But recompute rather than inheriting the rule: when the 10y→15y spread was 60–80bp, as in 2022, the same arithmetic said no. One caveat – the ten-year clock restarts on any Nachtrag that alters the loan. Don't reset it by accident.

Stress-test at +2pp. At year 11 the balance is €128,800, rent has grown to about €8,780 and costs to €2,070, so NOI is €6,710. The break-even refinancing rate – where the flat stops covering its own interest – is €6,710 / €128,800 = 5.2%. At 6.2% you carry €1,280/year of negative spread pre-tax, while the annuity at 2% Tilgung is €880/month, essentially unchanged from today because the balance shrank.

That is the real lesson. A rate shock arrives as a cash-flow problem long before it arrives as a return problem. Test whether you can pay, not whether the IRR still looks respectable.