Social Service Number / BSN
It is compulsory for every person that works in the Netherlands to have a Social Services Number (burgerservicenummer, BSN, sofinummer). Every Dutch citizen has his or her own unique BSN number which is used for contact with government services such as the tax administration. You will receive a BSN number after registering with your municipality, the BSN number is stated on the summary of personal details which you will receive by post a number of weeks after your registration.
If your stay in the Netherlands will not exceed a period of four months, you will not be able to register with your municipality. Consequently, you will not receive a BSN. In this case you can apply for a "sofi number" ("social fiscal number", social security number). Foreign employees who live in Leiden can apply for a sofi number at the Tax Office at the Willem de Zwijgerlaan. This is one of the 16 locations where sofi numbers are issued.
During your appointment at the Tax Office you will be required to show valid proof of identity (not a driver's licence) and you will be asked to state your address. If you are not registered with the local council, then you may be asked for proof of your address. Depending on your nationality, you may also be required to show a residence and/or work permit.
Non-EU citizens are required to have a passport containing a residence permit and a work permit as well as an employment contract. A new regulation which was recently introduced enables the Tax Office to issue a sofi number on the basis of a document which proves that a residence permit has been applied for. When you apply for a residence permit, the local council authority will place a sticker in your passport. If the sticker states that you are permitted to work in the Netherlands, it may serve as proof in order to issue a sofi number.
Please note: the fact that you have a BSN or sofi-number does not automatically mean that you are permitted to work in the Netherlands!