Self-employment as a secondary activity in Belgium: conditions, contributions and formalities

Key takeaways

• Your principal employment must represent at least half-time to carry on a secondary activity.
• In 2026, no social contributions are due if your annual net income remains below €1,922.16.
• Administrative formalities are identical to those of a principal self-employed person: CBE and social insurance fund.
• Your supplementary income adds to your salary in the personal income tax return and is taxed at your marginal rate.

Carrying out a self-employed activity alongside a salaried employment is a reality for a growing number of Belgian workers. Self-employment as a secondary activity in Belgium benefits from a reduced social contribution scheme below a certain income threshold, but the administrative framework remains identical to that of a principal self-employed person. This guide details the access conditions, social and tax obligations, and the concrete steps to start on a compliant basis.

Secondary self-employment: definition and scope

We speak of a secondary self-employed activity when a person carries out both a self-employed activity and a separate principal occupation: a salaried job, a civil service function or a tenured teaching position. The self-employed activity supplements a principal career, it does not replace it.

The status also applies to certain persons receiving a replacement income derived from a previous salaried or civil servant activity, such as certain unemployment benefits, provided that this income meets the conditions set by the National Social Security Institute for the Self-Employed (NSSI).

Conditions for carrying out a secondary activity

The central condition is that your principal occupation must be of a minimum duration. For employees in the private or public sector, the principal employment must represent at least half-time, i.e. a monthly working time equivalent to 50% of a full-time position in the sector concerned. For permanently tenured teachers, the threshold is 6/10 of a full timetable. For civil servants, the conditions also relate to a minimum duration of activity in their function, at least half-time.

These conditions are assessed based on the actual situation at the time the self-employed activity is carried out. If you reduce your salaried working time below half-time, you automatically lose secondary status and switch to principal status, with the minimum contributions that come with it.

Points to check before starting

• Salaried or civil servant employment of at least half-time

  Monthly working time equivalent to at least 50% of a full-time position in your sector.

• No exclusivity clause in your employment contract

  Some employers prohibit or restrict parallel professional activities.

• Legal and authorised self-employed activity

  Some regulated professions (architect, auditor, doctor...) require specific prior authorisations.

• Affiliation with a social insurance fund before the start of the activity

  Choose a fund accredited by the NSSI: Securex, Partena, UCM, Acerta, Xerius or other.

A frequently overlooked point: read your employment contract before getting started. Some employers include an exclusivity or non-competition clause that may prohibit or restrict any parallel professional activity.

Social contributions: what you actually pay

The main difference between secondary status and principal status lies in the social contributions. Below a threshold of annual net income, you pay no contributions in respect of your secondary activity. Above that, contributions are proportional to your income, without automatic minimum contributions on modest income.

Contributions are calculated on the net professional income from your self-employed activity, i.e. your supplementary turnover after deduction of business expenses. Your social insurance fund collects these quarterly contributions and pays them to the NSSI. Payments are provisional in the first years, then regularised once the definitive income is communicated by the tax administration.

An important point regarding social rights: below the contribution threshold, the secondary activity opens no additional rights in the self-employed scheme. Neither for the pension nor for health and disability insurance. Your rights remain those accrued through your principal salaried occupation.

Taxation: how your supplementary income is taxed

Income from your secondary activity is professional income subject to personal income tax (PIT). It adds to your salary income in the annual tax return and is taxed according to the progressive scale that applies to all your professional income.

This accumulation mechanism has a concrete consequence: if your salary already places you in a high tax bracket, your first euros of supplementary income will also be subject to it. This often surprises workers who start a secondary activity imagining separate or reduced taxation.

The counterpart: you can deduct all your real business expenses related to the activity (equipment, software, professional subscriptions, home office expenses if you carry out the activity there, travel, training). These deductions reduce the taxable base. The flat-rate deduction is generally not recommended for self-employed persons: it is designed for employees and does not reflect the reality of the expenses of an active self-employed activity.

Formalities for starting your activity

The administrative formalities for a secondary self-employed person are identical to those of a principal self-employed person. There is no simplified regime for secondary activities.

1. 1

   Check the employment contract

   Before anything

   Ensure there is no exclusivity or non-competition clause applicable to the planned activity.

2. 2

   Affiliate with a social insurance fund

   Before the start

   Choose a fund accredited by the NSSI and affiliate before or at the latest at the start of the activity.

3. 3

   Register with the Crossroads Bank for Enterprises (CBE)

   Before the start

   Go to an accredited enterprise counter to register the activity and obtain a company number.

4. 4

   Declare the activity for VAT if applicable

   Before the start if subject to VAT

   If annual turnover exceeds €25,000 excluding VAT, a VAT registration declaration is required with the FPS Finance.

5. 5

   Keep accounts from the first day

   From the start

   As a natural person carrying out a secondary activity, simplified accounting is sufficient for most activities: a record of professional receipts and expenses.

Creating your secondary activity through an accredited enterprise counter can be done quickly, provided you have previously chosen your social insurance fund and have a precise description of the activity to be carried out.

Launching a secondary activity?

Monsiegesocial supports you in the formalities for business creation and CBE registration in Belgium.

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Advantages and limitations of secondary status

Further reading

• Incorporating an SRL in Belgium: the essential steps: when the secondary activity grows, switching to a company may prove more fiscally advantageous.
• Company domiciliation: a professional registered office address for your secondary activity, without renting offices.
• The NSSI website centralises the conditions and official scales for secondary self-employed persons.