The indexed price is an adjustment to the original sales price. Statistics Netherlands (CBS) periodically publishes the increase or decrease in house prices in the Netherlands. By correcting the sales price with this increase or decrease, you get the indexed prices. So you can say that an indexed price is the selling price of today.
To calculate the value of a house, look at reference properties. By indexing the selling price of the reference properties, you get a more accurate picture of the value of the property you are looking at. This way you are always up-to-date and you can compare easier houses with each other.
A calculation example:
Suppose that a reference house has been sold for 6 months for € 445,000 but that house prices have already become on average 4% more expensive in that region in the last 6 months.
We then index the purchase price of € 445,000 x 1.04 (4%) and that gives an indexed price of € 462,800.
Since this is an estimate, we round the number up to € 463,000. In those 6 months the house has become about € 18,000 more expensive (€ 463,000 - € 445,000).