Your self-employed activity is a secondary activity if you exercise at the same time another professional activity for an employer. 

Before an activity can be seen as a self-employed secondary activity, your principal activity should meet several requirements.

The formalities to be fulfilled in order to be allowed to exercise an independent secondary activity are the same as those for an independent principal activity.

Just like a self-employed person whose independent activity is his principal activity, you must pay social contributions if your reference income exceeds a certain limit.

Suspension of your principal activity
Restrictions with regard to the exercise of your secondary activity
Failed start of your self-employed secondary activity