||The number of children in the Netherlands is declining and will continue doing so for the coming decade. Consequently, primary schools face vacant floor space. Media reports state an alarming situation, without mentioning exact numbers on the current scale of the problem. Nevertheless, it is very relevant since vacancy costs public funds. This research concentrates on hidden vacancy, which is defined as the situation in which a school uses the entire school building, while not needing all available floor space based on government regulations. __The DAS-framework is used as a conceptual model to approach the problem. Vacancy is a result of a mismatch between demand and supply. To calculate vacancy, demand and supply must be known. Demand can be calculated, based on amounts of students per school. However, the total supply is unknown, since this information is decentralized. Until now only estimates of the current supply are available, which vary between 10 M and 15,8 M m2. This makes it impossible to estimate the current vacancy._ _To solve this knowledge gap, an extract from a Cadastral database (BAG), holding information on Gross Floor Area and building year, was matched with the addresses of schools as registered in a database of the Office of Education. However, the raw version of this database held serious limitations. Often information was clearly incorrect and a substantial number of schools was missing. Therefore 100 municipalities were requested for additional data. As a result the database is enriched with detailed object information of 18% of the Dutch municipalities.__It was found that the current supply of primary schools is 9,6 M m2. This leads to a national vacancy level of 7,9%. However, taking into account a friction vacancy of circa 4% and the rent of floor space to third parties nuances this percentage. Thus it is concluded that vacancy among primary schools is not as alarming on a national scale as reported. However, it is also concluded that the demand will decline in the coming years. The cost of this vacancy, corrected for a friction vacancy of 4%, is between 6,7 and 17,5 M euro annually. If the supply is not adapted to this changing demand, increase of vacancy is expected. __For academics, the findings of this paper nuance existing literature, give insight in the current supply of primary schools and the scale of the current vacancy. Last it provides valuable insights for future scientific use of Cadastral data and its limitations.