A single long-running client, full-time hours and a company laptop can add up to something that reads as employment to a tax inspector, whatever the contract happens to be called. The assessment weighs an overall pattern rather than any one factor, which is exactly what makes it hard to self-diagnose.
Schijnzelfstandigheid, sham or false self-employment, gets assessed on two things together, following the Hoge Raad's Deliveroo ruling: gezag, meaning whether the client directs how, when and where you work the way a boss would, and organisatorische inbedding, whether your work is structurally part of their normal operations rather than a distinct outside service. Signals that point the other way, toward genuine entrepreneurship: multiple clients rather than one, setting your own prices and invoicing, using your own equipment, carrying real financial risk if a job goes wrong, and being free to send a substitute instead of showing up yourself. No single factor decides it on its own — a tax inspector or judge looks at the whole pattern. If you're working full-time hours for one client, under their day-to-day direction, on their laptop, for a year straight, that pattern reads as employment regardless of what your contract is titled.