Missing the four-month window is easy to do in a chaotic first year, and the letters that follow often get mistaken for junk mail. Because CAK escalates in fixed steps rather than all at once, where someone currently sits in that sequence makes a large difference to what it ends up costing.
Worse than people expect once it actually lands. Miss the four-month window and CAK — the government body that checks who's insured against tax and municipal records — first sends a warning giving you another three months to sort it out yourself, no fine yet, just a deadline. Ignore that too and you get hit with a first boete (fine) of €529.74 (2026 figure), followed by a second fine of the same amount three months after that if you're still uninsured. So the two-warning, two-fine cycle really can cost you over €1,000 before anything else happens. If you're still uninsured after both fines, CAK enrols you automatically in a basic policy on your behalf, and from 1 January 2026 the premium for that arrangement is €172.70 a month, deducted straight from wages or benefits where possible — you don't get a say in the insurer or the policy at that point. None of this cancels the backdated premium you owed anyway for the period you should have been insured. The fastest way out at any stage is to register a normal Dutch basisverzekering yourself before the next deadline hits, which stops the clock even mid-process. If a fine already arrived and you think it's wrong, you can dispute it with CAK directly rather than just paying and hoping it resolves itself.