Combining your job with a side gig is possible but it depends on a few factors which include
Check your employment contract – If your employment contract says you can’t have side gigs, then it’s advisable you don’t. However, most companies are flexible with that so you can at least have a talk with your management. In most instances, they will be willing to let you do it so long as it does not interfere with your work delivery and it is not in conflict of interest with what your company does.
Also if you are getting a new job and you already do something by the side, it is advisable you let your company know ahead of time so they are aware
Also look at your residence permit to be sure it allows you have a job and a self employment
If you’re in doubt of the exact status of things, it might be wise to show your contract and visa details to a lawyer and/or a tax consultant to help you better interpret things legally.
That said, if that coast is cleared, you can decide to register with the KVK as a freelancer but be sure to read through all the details and be certain what the terms are
Also note that additional income rules around tax may still largely apply depending on whether you’re on 30% ruling or not. Usually when you have the 30% ruling, you don’t have a legal obligation to declare any income generated outside of NL. In addition, immovable properties like real estate investments are usually taxed in the country where they are located. but this is not necessarily because the income needs to be declared here but to avoid double taxation rules because you should not be taxed twice.
More information can be found here and here on the KVK website
Wishing you all the best
