How is the Belgian foreign KI calculated?

You take the purchase price excluding expenses and divide it by an adjustment factor to calculate the (notional) value of the property in 1975. From this, you take 5.3% to arrive at the non-indexed foreign KI.

By 2024: purchase price excluding costs / 15.726 * 5.30% = the non-indexed foreign KI

If you buy a €400,000 property in Spain in 2024, the indexed Belgian foreign KI is €1,348 (400,000 / 15.726 * 5.30%).

Find more information on the foreign KI declaration here.

Here you can read how to declare this amount in the Belgian personal income tax return.