A werfmelding is the declaration of works you file with the RSZ (National Social Security Office) before work starts on a Belgian site. Its official name is Aangifte van werken (Déclaration de travaux). The contractor that has the contract with the client files it online and lists every subcontractor. You must declare contracts of EUR 30,000 or more, EUR 5,000 or more with one subcontractor, and any amount with two or more subcontractors. If you forget, you pay 5% of the undeclared works.
The site number from your declaration is also the key for daily attendance registration. Suivo’s Check in at Work solution handles that daily part for your crews and subcontractors.
What is a werfmelding (declaration of works)?
The declaration of works tells the government which contractors and subcontractors work on which site, for which client. The online service is called Aangifte van werken. The RSZ FAQ uses “werfmelding” for the act of declaring. People also say “unieke werfmelding”, but that is not an official term.
One declaration serves several authorities. Since the Law of 8 December 2013, the declaration to the RSZ, the prior notice for well-being at work and the notice to the construction safety committee all go through the same RSZ application, according to FOD WASO (Federal Public Service Employment). Constructiv and FOD WASO take the data they need from it. You file once.
Who must file it: main contractor, subcontractors, foreign firms?
The declaring contractor. The contractor engaged directly by the client files the declaration. It gives the details of the site, the client and all subcontractors.
Several contractors for the same client. Each contractor with its own contract with the client declares that contract. The first declaration creates a site identification number. The client shares it with the other contractors, who add their contracts to the same site.
Subcontractors. A subcontractor does not file its own declaration. But if it brings in a further subcontractor, it must first inform the declaring contractor in writing. The declaring contractor then adds that company before it starts work. Subcontractors without staff must be declared too.
Other cases:
- Property developers file when they are treated as contractors.
- Private clients: works for private individuals must also be declared, flagged as “private client”.
- Services such as surveyors, safety coordinators and design offices are not declared as subcontractors.
Foreign firms. A foreign company doing works in Belgium follows the same rules and files the declaration. It needs a Belgian company number, which it can request through the declaration form. Foreign subcontractors must be declared with or without a Belgian company number: EU firms with their intra-community VAT number, non-EU firms with their home-country ID number. Works abroad are not declared in Belgium.
Access changes in 2026:
- Since 29 June 2026, only the declaring contractor can create or change a declaration. Other companies in the chain can only view the declarations they appear in.
- The RSZ has announced that during Q4 2026 it will withdraw unsecured access. You will then need secured access to file, change or even view a declaration. Foreign contractors without Belgian eID should request secured access now. Our 2027 compliance calendar for foreign contractors lists the other dates.
Which works and which thresholds?
Works you declare (according to the RSZ):
- Works in immovable property (werken in onroerende staat), except certain agriculture, horticulture and forestry activities. Cleaning of buildings counts and follows the same thresholds.
- Delivery of ready-mixed concrete.
- Asbestos works must be mentioned in the declaration.
- Security (PC 317) and certain meat-sector work: every contract, whatever the amount; in the meat sector, one global declaration per client/declarer.
Thresholds for works in immovable property, per client-contractor contract, excluding VAT:
| Contract value | Subcontractors | Declare? |
|---|---|---|
| EUR 30,000 or more | None or any number | Yes |
| EUR 5,000 or more | Exactly one | Yes |
| Any amount | Two or more | Yes |
| Below EUR 30,000 | None | No |
| Below EUR 5,000 | Exactly one | No |
RSZ examples: EUR 3,000 with two subcontractors, declare. EUR 3,000 with one subcontractor, do not declare. EUR 5,075 with one subcontractor, declare.
Two practical rules:
- Contracts of indefinite duration: use the estimated annual invoiced amount.
- When an exemption stops applying (for example, you add a second subcontractor), declare immediately. The form lets you show that the exemption applied at the start, so the declaration is not treated as late.
When and how do you file it?
When
- Before the works start.
- For temporary or mobile construction sites and for asbestos removal: at least 15 calendar days before the start. Asbestos removal that exposes workers must also be reported to the local Well-being at Work inspection.
How: step by step
1. Log in to the online service Aangifte van werken on the social security portal. Declarations are electronic only.
2. Check whether the site already exists. If another contractor already declared it for the same client, use the site identification number the client gives you.
3. Enter the site, the client and your contract details. Mention asbestos works if relevant.
4. Add every subcontractor, Belgian or foreign.
5. Confirm. You receive a site identification number and a sequence number for your contract. Confirmation documents go to the parties’ e-Box Enterprise and/or by post.
6. Share the site number with the client, so other contractors on the same site can use it.
7. Keep it up to date. You can amend a declaration at any time, even when it is running or closed. Only the RSZ can change the client, the end purpose of the works and the “subject to registration” indicator.
End dates. The end date is the provisional acceptance or the end of the contract. Since 1 January 2025, a continuous-purpose (doorlopend) declaration can run for at most one year from the start date. You can extend it each year, on time. A declaration closes automatically when its end date passes.
Volumes. Batch submission has been possible since 2010. Ready-mix contractors can select the known, potential or all subcontractors from a list per concrete plant.
How the werfmelding connects to Checkinatwork (and CIAO from April 2027)
The declaration of works and daily attendance registration are two different duties that share one number.
Checkinatwork today. Sites with works in immovable property (excluding cleaning) of EUR 500,000 or more excluding VAT (EUR 800,000 for works started between 1 April 2014 and 29 February 2016) must be declared in the Aangifte van werken. Checkinatwork then identifies the workplace by the site number from your declaration. According to the RSZ, all declared parties receive a warning letter with reference “CAWON” when registration is required. From then on, every person on site is registered daily, before they start. Employers and contractors register their own staff, their subcontractors and self-employed subcontractors. Workers can also register themselves.
For which works are exempt, see when Check in at Work is not required.
CIAO from 1 April 2027. From 1 April 2027, according to the RSZ, works in immovable property (excluding cleaning) and ready-mix delivery on sites of EUR 500,000 or more move to Check in and out at work (CIAO). What changes:
- construction attendance can no longer be registered in Check in at Work, according to the RSZ;
- under the Programme Law of 30 May 2026, the time of each arrival and each departure is registered, instead of one daily registration;
- the registration data must be forwarded immediately, and every person must be able to register before entering or leaving the site;
- if the RSZ online app is the registration tool on site, each person needs their own registration device;
- each registration will also include the declaration-of-works number.
The RSZ says more details will follow. The practical lesson is simple: a correct declaration is the foundation. If the site number is wrong or a subcontractor is missing, daily registration fails too.
Cleaning. Cleaning follows the declaration thresholds, not the EUR 500,000 threshold. Every declaration now generates a QR code for Check in and out at work for cleaning.
Where Suivo fits. Suivo does not file the declaration for you. It handles daily registration for your crews and subcontractors in three ways: the Check in at Work app, a mobile site pole that accepts the Belgian ID card and other identifiers, or a vehicle black box. At Cegelec, manual input of 20,000 registrations a month would cost two extra employees (read the Cegelec case).
What happens if you don’t file
According to the RSZ notice to contractors:
- The contractor that fails to declare owes the RSZ 5% of the total amount of the undeclared works, excluding VAT.
- A subcontractor that does not tell its contractor in writing that it uses subcontractors owes 5% of the works it subcontracted.
- If the contractor’s penalty was caused by a subcontractor’s error, it is reduced by what that subcontractor actually paid.
On a EUR 400,000 contract, that is EUR 20,000. There is a hidden cost too. Without a correct declaration, a Checkinatwork site has no valid site number. That makes daily registration of your crews and subcontractors unreliable from day one.
Checklist before the first day on site
- Contract value excluding VAT known, and number of subcontractors counted
- Threshold checked: declare or not?
- Temporary or mobile site or asbestos? Filed at least 15 calendar days before the start
- Site already declared by another contractor for the same client? Use that site identification number
- All subcontractors added, including foreign ones (VAT or home-country ID number)
- Written notice received from every subcontractor that brings in its own subcontractor
- Secured access in place, before the RSZ withdraws unsecured access in Q4 2026
- Site of EUR 500,000 or more? Daily attendance registration set up for every person on site
- Plan ready for Check in and out at work from 1 April 2027
- Subcontractors checked on checkinhoudingsplicht.be before you pay (see our guide to the inhoudingsplicht)
Frequently asked questions
Who must file the werfmelding?
The contractor engaged directly by the client files the declaration of works and lists all subcontractors. If several contractors each have a contract with the same client, each one declares its own contract on the same site.
How do I declare a site?
Log in to the online service Aangifte van werken on the social security portal, enter the site, the client, your contract and your subcontractors, and confirm before the works start. You then receive a site identification number to share with the other contractors.
Which works must you declare?
Works in immovable property (including cleaning) above the thresholds, delivery of ready-mixed concrete, and every contract in security and certain meat-sector activities. Asbestos works must also be mentioned in the declaration.
When is CIAW mandatory?
Check in at Work is mandatory for works in immovable property (excluding cleaning) on sites of EUR 500,000 or more excluding VAT, for ready-mix delivery and for certain meat-sector work. From 1 April 2027, according to the RSZ, construction and ready-mix move to Check in and out at work.
What is the penalty for not filing a werfmelding?
A contractor that fails to declare owes the RSZ 5% of the total amount of the undeclared works, excluding VAT. A subcontractor that does not tell its contractor in writing about its own subcontractors owes 5% of the subcontracted works.
From declaration to daily registration
Your declaration of works sets up the site. Daily registration proves who was there, for your own crews and every subcontractor. Suivo takes care of that daily part, so your site record holds up when an inspector asks.
