Flat purchase — box vs. realize

10-year wealth comparison for an owner-occupier with an ETF portfolio. Box-spread rate pulled live from the EUR curve.

€
Tune assumptions
80%
3.85%
2.0%
2.0%
6.5%
40%
Owner-occupied — flat sale tax-free at 10y. With offset on, the box carry nets against Kapitalerträge at 26.375%.
Offset on — Path A gains +€17,495
A · Box spread 2.54%
€1,614,181
Flat (net of mortgage)
€460,337
Portfolio after tax
€1,435,707
Box debt (rolled)
−€299,357
Box interest accrued
€66,332
B · Realize
€1,494,784
Flat (net of mortgage)
€460,337
Portfolio after tax
€1,034,447
Tax paid at t=0
−€27,484
Gross sold at t=0
€260,509
C · All cash
€915,517
Flat (no mortgage)
€914,246
Portfolio after tax
€1,272
Tax paid at t=0
−€98,249
Gross sold at t=0
€931,274
Comfortable portfolio size for Path A: €932,100 — with starting box debt of €233,025 the loan-to-value sits at 25% and survives a Covid-style drawdown without a margin call.
Assumptions & breakdown
Closing costs (Berlin)
€83,025 · 11.07%
Down-payment + closing
€233,025
Mortgage principal
€600,000
Monthly payment
€2,925
Mortgage after 10y
€453,909
Flat value at 10y
€914,246
Assumed portfolio at t=0
€932,100

How the comparison works

Path A — Box spread

Box loan funds the down-payment plus closing costs. The portfolio stays fully invested. The box rolls annually, interest compounds. We treat the box rate as a pure financing cost — any tax-side offset from Termingeschäft losses is deliberately excluded.

Path B — Realize

Sell enough ETF shares so the net proceeds cover down-payment plus closing costs. Pay 26.375% Abgeltungsteuer up front on the realized gain portion. The remaining portfolio compounds; at year 10 the residual gain is taxed again.

Path C — All cash

Sell enough ETF to cover the full purchase price plus closing costs — no mortgage, no box. Big upfront tax hit, but zero financing cost afterwards.