As far as we know it is permitted to search the beach of Katwijk with a metal detector.
Generally it is allowed to search the Dutch beaches using a metal detector at amateur level. Metal detection is a search method for finding metal objects in the ground. Beach searchers use a metal detector to find lost items (jewelry and coins) of bathers.
Of course, Dutch laws and regulations also apply on the beach. This means that you, also when searching on beach, need to take the following into account: Law on Archaeological Heritage, the Monuments law, the weapons and ammunition law, the local General Local Regulation (APV), Lost and Found items according to the civil law etc. Aside from that you should always leave behind your search location the way you found it. In other words, neatly close any dug holes.
A local municipality may decide to prohibit the use of a metal detector (detection ban) via the General Local Regulation. Therefore always check the General Local Regulation and/or contact the municipality where you wish to search. Katwijk is subject to the municipality of Katwijk. Here you will find contact information about this municipality: http://www.katwijk.nl/
Note: Searching in the Dutch dunes is generally strictly prohibited. This is due to the protection of nature.