Employee and self-employed as a side activity: can your employer object?

Do you need your employer's consent to become self-employed as a side activity?

No, not as far as social security status goes. The conditions set by NISSE concern your main occupation, usually a salaried job of at least half-time, not your employer's opinion. No written authorisation is requested when you join a social insurance fund or register the activity.

Half-time is measured against a full-time job in the same company or sector. Civil servants and teachers follow their own rules, including on holding several activities: their HR department is the right contact before starting. The page on self-employment as a side activity sets out every access condition.

That does not mean your employer has no say. Your employment contract, the work rules and the Employment Contracts Act still apply: that is where the real limits lie.

What your employment contract forbids while you combine both

Three obligations follow employees into their self-employed activity. They do not disclose their employer's manufacturing or business secrets, during or after the contract. They refrain from any act of unfair competition. And they perform their work at the time, place and under the conditions agreed: the self-employed activity sits outside working hours, and the work equipment entrusted by the company stays dedicated to it.

Unfair competition is not simply doing the same job. The risk arises when you approach your employer's customers, use their files, prices or methods, or divert a project meant for them. Acts of unfair competition are among the examples of serious cause accepted by case law, which allows dismissal without notice.

Since the Act of 7 October 2022, the FPS Employment states that an employer may not, in principle, forbid a worker from doing parallel work outside their schedule for one or more other employers, nor penalise them for it, unless the law allows it. That text covers work for other employers: for a self-employed activity, reread your contract and the work rules, which may include a duty to inform.

• Reread your employment contract and the work rules before starting the activity.
• Never approach your employer's customers or suppliers.
• Keep a separate computer, email address and schedule for the self-employed activity.
• Inform your employer in writing when the contract requires it, or when the activity touches their sector.

Contributions and tax: what combining both really costs

Combining both costs social security contributions and, above all, tax. Below €1,922.16 of annual net income, a side-activity self-employed person pays no contributions; above it, they pay 20.5% on the whole income. The profit is then added to your salary in the same personal income tax return and follows the progressive scale.

Because your salary already fills the lower bands, each euro of profit is taxed at the rate of the band reached. For 2025 income, the band from €28,800 to €49,840 is taxed at 45%, plus the municipal surcharge, on average 7.5% of the tax. The contribution threshold and its effect are explained in the article on the side-activity income ceiling.

Nothing is usually withheld at source on your invoices: the withholding tax on your payslip only covers the salary. Tax on the self-employed activity therefore arrives in one go with the tax assessment notice, often more than a year after the first profits. Setting money aside each month avoids an unpleasant surprise.

Which steps come before the first invoice?

Three steps come before the first invoice: registering the activity with the Crossroads Bank for Enterprises through a business one-stop shop, joining a social insurance fund before the activity starts, and, depending on the activity, registering for VAT. The one-stop shop can handle the fund membership at the same time, for a fee.

Without membership, you are automatically affiliated to the National Auxiliary Fund, with adjustments to follow. The social security contributions simulator works out what your side income will cost, and the main or side activity comparison shows what changes if your salaried job ever ends.

Further reading

The pages on this site that explore each of the points covered here in more depth.

• Self-employed as a side activityStatus conditions, formalities and tax, the complete guide.
• Self-employed side activity: is there an income ceiling?The two thresholds that change your contributions, worked out for 2026.
• Main or side activity: the comparisonWhat changes if the salaried job ends or drops below half-time.
• Social security contributions simulatorWork out the contribution on your side income.
• Side-activity social statusThe definition of the status in a few lines.

Frequently asked questions