[mamon]

No, it is not illegal - I've been practicing this for the last 10 years, never had any trouble with tax office.

It all depends on how your contract is written - if you have clauses that put some of the business risk on you (e.g. you give 3 month warranty on the code you write, or have some bogus penalties for contract breach, or other things, which I've never seen enforced) then it cannot be treated as full time employment, even if you have just one client for which you issue invoices.

The only thing that is an actual red flag for tax office is when you start as a full time employee, and then switch to running sole proprietorship and your previous employer remains your only client - then it is too obvious that this is just employment in disguise.

EDIT: Also, those "independent contractor" contracts come in different shapes. I, for example have just an hourly rate set, and then I get paid for the hours worked. Some people prefer to have flat monthly rate, and even some paid vacations included in their "B2B contracts", which makes it more suspicious.