Can I start a small advisory side business in Belgium as an employee in 2026?

Yes, in many cases a full-time employee in Belgium can start a small advisory activity as a sole proprietor in secondary occupation, but only if the employment contract, immigration status, VAT position and tax treatment are checked first. For income year 2026, this structure is often lighter than creating an SRL/BV for a few thousand euros of annual revenue, but the correct answer depends on the individual situation.

The figures below are illustrative and relate to income year 2026 / assessment year 2027 unless stated otherwise.

The usual starting point: secondary self-employed activity, not a company

For a small advisory activity generating only limited annual revenue, the most proportionate Belgian structure is often a sole proprietorship in secondary occupation.

This means that the person remains employed, while also carrying out a small independent activity on the side. In practice, this usually involves:

• obtaining or activating a Belgian enterprise number;

• registering the activity through an approved business counter;

• affiliating with a Belgian social insurance fund before starting;

• choosing the correct VAT regime;

• keeping basic accounts and issuing compliant invoices.

An SRL/BV may become relevant later if the business grows, if contractual risks increase, if subcontractors or employees are hired, or if liability separation becomes important. For a small test activity, however, a company can be unnecessarily heavy because of incorporation costs, double-entry accounting, annual accounts, company tax compliance and director remuneration planning.

The employment contract is often the first real risk

Before registering the side activity, the employee should review the employment contract carefully. Many contracts do not prohibit all outside work, but they may restrict ancillary activities that are incompatible with the employer’s interests.

Typical red flags include:

• carrying out the side activity during working hours;

• advising clients who compete with the employer;

• approaching the employer’s clients, suppliers or business partners;

• using confidential information obtained through employment;

• using the employer’s tools, documents, templates or know-how;

• creating advisory material too close to the employer’s commercial environment.

In a typical case involving startup advisory, the safest approach is to ask the employer for written confirmation before starting. The request should define the activity narrowly: outside working hours, with personal equipment, without use of confidential information, without contact with the employer’s clients or competitors, and without impact on the employee’s duties.

This is especially important where the employee works in a regulated, financial, technology or commercially sensitive environment. Even if the activity is tax-compliant, it can still create an employment-law or confidentiality issue.

Non-EU nationals: check the professional card exemption

A non-EU national living in Belgium must also check whether a professional card is required before starting an independent activity.

In Brussels, non-EU nationals generally need a professional card for self-employed work unless an exemption applies. One important exemption may apply to people who hold a residence status allowing them to live in Belgium without limitation, such as certain permanent residence statuses.

For a person with Belgian permanent residence, the practical step is to ask an approved business counter, or the competent regional administration, to confirm whether registration as self-employed in secondary occupation can be done without a professional card.

This should be verified before issuing the first invoice. A limited residence card could lead to a different conclusion.

Main registration steps before the first invoice

A cautious registration sequence would normally look like this:

1. Confirm whether a professional card is required or whether an exemption applies.

2. Check the employment contract and obtain written employer approval where needed.

3. Register the sole proprietorship through an approved business counter.

4. Activate the enterprise number in the Crossroads Bank for Enterprises.

5. Affiliate with a social insurance fund before starting.

6. Choose the VAT regime, often the small business exemption regime at the beginning.

7. Open a separate bank account for the activity, even if not always mandatory.

8. Set up basic invoicing and bookkeeping.

9. Consider professional liability insurance, especially for strategic, commercial or fundraising-related advice.

The key point is that affiliation with a social insurance fund must be handled before the activity starts. Delayed registration can create administrative issues and possible penalties.

Social contributions in secondary occupation

A full-time employee who starts a small independent activity will often qualify as self-employed in secondary occupation.

This status is more favourable than a main self-employed activity because the main social security coverage remains linked to employment. Social contributions are calculated on the net taxable self-employed income, not on gross turnover.

For example, if a person invoices around €5,000 and has around €1,000 of deductible professional expenses, the first tax basis is not €5,000 but roughly €4,000 before social contributions and income tax.

For a very small side activity, final social contributions may be low or even nil if the income remains below the applicable threshold. However, provisional quarterly contributions may still be requested unless the social insurance fund accepts a reduction based on expected low income.

The classification must be reviewed if the person later reduces employment, becomes unemployed, takes a career break or changes status.

VAT: the small business exemption can be useful, but cross-border clients complicate the analysis

Advisory services are normally VAT-relevant services. For a small activity, the small business VAT exemption regime is often the simplest starting point.

This regime is available where annual turnover does not exceed €25,000 excluding VAT. If the activity starts during the year, the threshold is prorated. Under this regime, the business does not charge Belgian VAT to clients, but it also cannot deduct VAT on professional expenses.

For Belgian clients, the invoice will generally be issued without VAT, with the appropriate exemption wording.

For international clients, the VAT analysis becomes more technical:

• for EU business clients with a valid VAT number, B2B services are often subject to the reverse-charge mechanism in the client’s country;

• for non-EU business clients, advisory services may be outside the scope of Belgian VAT under the place-of-supply rules;

• for private individuals, the result can differ and may create more complexity.

For that reason, a small advisory business may want to limit its first phase to B2B clients only, with clear evidence that each client is a business customer.

Belgian income tax: the side profit is added to salary

As a sole proprietor, the independent profit is taxed in the Belgian personal income tax return.

The basic calculation is:

• gross advisory fees invoiced;

• minus deductible professional expenses;

• minus social contributions;

• equals net taxable self-employed income.

That net income is then added to the person’s other taxable income, usually salary. Because Belgium uses progressive tax rates, the marginal tax rate on the additional side profit can be high when the employee already has a substantial salary.

For income year 2026, the top federal tax bracket starts above €51,070 of taxable income, and the 45% bracket applies before that level. Depending on salary, deductions and municipal surcharge, a small independent profit may effectively be taxed at a high marginal rate.

As a cautious cash-flow rule, someone starting a small side activity may reserve around 55% to 65% of net profit before final tax to cover personal income tax and social contributions.

For example, if the activity produces around €4,000 of net profit before contributions and tax, it would be prudent not to treat the full amount as disposable income. A reserve of around €2,200 to €2,600 may be more realistic until the final tax calculation is known.

Advance tax payments may be relevant

Salary is usually subject to withholding tax, but independent income is not. This can create a tax balance to pay later.

Belgium has an advance payment system for self-employed income. For income year 2026, the tax increase rate for insufficient advance payments is 4.50%.

For a very small side activity, the financial impact may be limited. But if the activity becomes regular, voluntary advance payments during the year can help avoid surprises, especially once the profit becomes clearer after the second or third quarter.

The starter exemption from tax increases should not be assumed to fully protect a person who is self-employed in secondary occupation.

Deductible expenses: keep separation and evidence

Common deductible expenses for a small advisory activity may include:

• accountant or tax adviser fees;

• registration costs;

• professional liability insurance;

• business bank account fees;

• invoicing or accounting software;

• website, domain name and professional email address;

• marketing and branding costs;

• research tools, CRM tools and productivity software;

• professional training, books, conferences or courses;

• coworking space or meeting room rental;

• documented business use of phone and internet;

• conservative home office expenses, where justified;

• travel costs to meet clients;

• laptop or equipment bought specifically for the independent activity.

The two main principles are evidence and separation. The person should avoid using the employer’s laptop, software, documents, client lists, templates or confidential information.

Home office expenses should be treated carefully, especially if the employee already receives a telework allowance from the employer. The same costs should not be counted twice.

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