Declaration of works to the NSSO: when, how and why
Anyone having immovable works carried out in Belgium must declare them to the NSSO beforehand. Here is who declares, within what deadline, through which channel and what happens if you forget.
The declaration of works is the notification you file with the Belgian NSSO before immovable works start. It sits in the same article 30bis as the withholding obligation but serves a different purpose: the NSSO wants to know who works on which site before the first stone is laid.
Who must declare
The obligation rests with the contractor who contracts with the client, so the main contractor. If you use subcontractors, you list them in the same declaration. A subcontractor who subcontracts further reports this to the contractor above them, who completes the declaration.
| Role | What you declare | When |
|---|---|---|
| Main contractor | the site, the client and all known subcontractors | before works start |
| Subcontractor | any further subcontracting, to the contractor above you | before that subcontractor starts |
| Client | nothing, but you check that the declaration exists | at award |
When the declaration must happen
The declaration is filed before works begin. If something changes during execution, for example a subcontractor joining or leaving, you update the declaration. Sites below the threshold amounts are exempt, but the exemption disappears as soon as a subcontractor appears on site. In practice the safe rule is simple: wherever there is subcontracting, you declare.
Through which channel
The declaration runs through the Declaration of Works online service on the social security portal. You log in with itsme or an eID reader. Larger contractors often use a batch connection from their ERP so sites are transmitted automatically. After filing you receive a site number, which you need later for presence registration.
How this relates to the withholding obligation
Declaration of works and the withholding obligation are two separate duties addressing the same risks. The declaration tells the NSSO who is working. The withholding obligation forces you to pay part of an invoice to the authorities if your contractor has social security or tax debts. You can be fully compliant on your declaration and still pay an invoice incorrectly, or the other way around. So always check both.
What happens without a declaration
Without a declaration you risk an administrative fine and you are jointly liable for the social security debts of the undeclared subcontractor. During a site inspection the declaration is one of the first documents requested, together with presence registration and the identity of the workers present.
Checklist for a new site
- Establish who the main contractor is and who files the declaration.
- File before the start date, with all known subcontractors.
- Keep the site number with the file.
- Update the declaration whenever a subcontractor joins or leaves.
- Check every company involved before every payment.
Frequently asked questions
Do I declare for my own staff? Yes, the site itself is declared, even without subcontractors, once the threshold is reached.
How long does a declaration stay valid? It covers the duration of the site. You keep it current until the works are finished.
Does the declaration replace debt checks? No. The declaration is a notification. Checking social security and tax debts remains a separate, recurring duty at every payment.
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