Calculating the WOZ value

Municipality of Rijswijk

The algorithm calculates the WOZ value of real estate using statistical calculation models and market data. It supports the implementation of the WOZ Act and forms the basis for various local taxes.

Last change on 20th of November 2025, at 7:09 (CET) | Publication Standard 1.0

• Publication category: Impactful algorithms
• Impact assessment: Field not filled in.
• Status: In use

General information

Theme

Field not filled in.

Begin date

2009-08

Contact information

[e-mail]

Responsible use

Goal and impact

The purpose of the algorithm is to objectively and uniformly determine the value of homes and other real estate.

The outcome forms the basis for property tax (WOZB), water board tax and income tax, among others.

The impact on residents is great, as the WOZ value affects various taxes. Therefore, the calculations are carefully checked and objections are possible.

Considerations

The use of a statistical calculation model ensures consistent valuation and reduces the likelihood of human error.

Models are tested and calibrated annually against current sales figures and property characteristics.

The advantage is that values can be determined efficiently and evenly. A point of attention is that market developments or incomplete data can affect the outcome. Therefore, human assessment remains necessary.

Human intervention

The model outcome is always checked by a valuer from the municipality or an external WOZ specialist.

In case of discrepancies or inexplicable differences between model values and market prices, the value is adjusted manually.

Residents can object to the established WOZ value; an individual reassessment always follows.

Risk management

The municipality secures the quality of the calculations by:

• annual updating of the models based on new market data;
• testing by certified WOZ valuers;
• validation of model outcomes through spot checks;
• compliance with the Valuation Instruction of the Valuation Chamber.

The models are not used for automated decision-making without human review.

There is a low privacy risk because the data used is largely object- and market-related.

Legal basis

• Property Valuation Act (WOZ)
• Municipal Act
• General Act on national taxes (AWR)

Links to legal bases

• Property Act: https://wetten.overheid.nl/BWBR0007119
• Municipal Act: https://wetten.overheid.nl/BWBR0005416
• AWR: https://wetten.overheid.nl/BWBR0002320

Operations

Data

The algorithm uses property and market data, including:

• sales prices of comparable properties;
• year of construction, type, area and location of the property;
• data from the Basic Registration of Addresses and Buildings (BAG) and the Land Registry;
• market figures from national data sources such as the NVM or Valuation Chamber.

No special personal data are processed.

All data are used exclusively for determining the WOZ value under the WOZ Act.

Technical design

The algorithm is a statistical computational model working with multiple regression methods and feature comparison.

Its operation is basically as follows:

1. Property and market data are collected and cleaned.
2. Similar properties in the surrounding area are selected for each object.
3. Based on characteristics (such as residential area, year of construction and location), a model value is calculated.
4. The model is calibrated based on realised sales prices.
5. An appraiser checks the results and makes any corrections.

The models are recalibrated and validated annually in accordance with Valuation Chamber guidelines.

External provider

Ortec