Dimona Declaration (Déclaration Immédiate / Onmiddellijke Aangifte)

What Is Dimona?

Dimona (Déclaration Immédiate in French, Onmiddellijke Aangifte in Dutch) is a mandatory electronic declaration system administered by Belgium's National Social Security Office (ONSS - Office National de Sécurité Sociale, also known as RSZ - Rijksinstituut voor Sociale Zekerheid in Dutch). It requires employers to immediately register all workers and work starters with the social security system.

Legal Basis & Purpose

Dimona was established under Royal Decree 27/3/1998 (Belgian construction safety law) and is operationalised under ONSS regulations. The system serves multiple purposes:

1. Social Security Registration: Ensures workers are registered in the Belgian social security system, entitling them to unemployment benefits, disability insurance, and pensions.
2. Employment Transparency: Creates a real-time record of who is working, where, and for whom. This transparency deters informal/undeclared work and tax evasion.
3. Chain Liability (Ketenaansprakelijkheid) Compliance: Dimona declarations create a traceable chain showing which companies employed which workers on-site, enabling regulators to hold principals accountable for subcontractor compliance.
4. Health & Safety Oversight: ONSS uses Dimona data to verify that workers are covered by health and safety protections; missing Dimona indicates workers operating outside legal protections.
5. Posted Worker Control: Dimona declarations include a "posted worker" designation, enabling Belgian authorities to track cross-border workers and ensure Posted Workers Directive compliance.

Who Must File Dimona?

Dimona declarations are mandatory for:

• Every worker employed in Belgium (local or foreign, full-time or part-time)
• Self-employed persons working on construction sites (declaration to register business)
• Posted workers from other EU countries
• Temporary agency workers
• Apprentices and trainees on construction sites

Dimona is mandatory for construction, dredging, maritime, and energy projects (all classified under Royal Decree 27/3/1998).

Filing Deadline & Consequences

Deadline: Dimona must be filed within 72 hours of the worker's first day of work (or, if work start date is known in advance, before work starts).

Consequences of Non-Compliance:

• Fines: €150-€5,000 per worker per violation
• Criminal prosecution: Employer and responsible individuals can face criminal charges
• Chain liability exposure: Principal contractor may be liable for subcontractor non-compliance
• Payroll recovery: ONSS may claim back unpaid social contributions
• Project stoppage: Regulators may halt work until Dimona compliance is demonstrated
• Insurance implications: Policies may be voided for workers without Dimona coverage

Dimona Declaration Contents

A Dimona declaration includes:

• Worker name, date of birth, nationality
• Employer name and ONSS registration number
• Job title and occupational classification (NACE code)
• Work location (site address)
• Work start date
• Expected work end date (if temporary/project-based)
• Gross salary/wage
• Work status (employed, self-employed, apprentice, posted worker, temporary agency)
• For posted workers: home country, sending company, posting designation

How Dimona Filing Works in Practice

Electronic Filing Process

Step 1: Registration with ONSS

• Company must be registered with ONSS (employer registration number)
• Employer or designated HR/payroll person registers for Dimona access

(online portal or via certified intermediaries)

Step 2: Information Gathering

• Collect worker information:
• Full name, date of birth, nationality, passport/ID number
• Salary/wage and payroll classification
• Job title and occupational code (NACE classification)
• Work location details (site address)
• Start date (must be filed before start date if known)
• Expected end date (if temporary assignment)
• For posted workers: home country, sending company, reference number

Step 3: Declaration Filing

• File Dimona electronically via ONSS portal or certified intermediary
• Declaration is submitted to:
• ONSS (social security registration)
• CIAW (Comité Interprofessionnel pour l'Application des Salaires / construction industry wage authority)
• RSZ (in Dutch regions)

Step 4: Confirmation & Status

• ONSS provides electronic confirmation with Dimona number
• Confirmation confirms worker is now registered and covered by social security
• Employer maintains confirmation as proof of compliance

Step 5: On-Site Verification

• Principal contractor (or inspecting authority) can request Dimona confirmation
• Employer must be able to produce Dimona number and confirmation within 24 hours
• Missing confirmation indicates non-compliance

Step 6: End of Work Declaration

• When worker leaves (end of project), employer files end declaration
• Ends social security coverage for that position
• Provides final payroll and contribution settlement

Timeline Example: Construction Project

Day 1: Project manager confirms 10 subcontractor workers will start on Day 10 Days 2-9: Employer collects worker information (names, dates of birth, nationalities) Day 8: Employer prepares and submits Dimona declarations for all 10 workers Day 9: ONSS sends confirmation with Dimona numbers Day 10: Workers arrive; Dimona confirmations are available for verification Day 11: Regulatory inspection occurs; principal contractor shows Dimona confirmations to Compliance demonstrated; no fines

CONTRAST: Day 10: Workers arrive; no Dimona filed Day 11: Regulatory inspection; Dimona not available to Non-compliance; fines of €150-€5,000 per worker (€1,500-€50,000 total) to Project may be halted pending compliance

Posted Worker Dimona Example

For a posted worker from Poland working on a Belgian construction site:

Dimona Declaration (Posted Worker):

• Worker: Jan Kowalski (Polish national, passport PL123456789)
• Employer: BuildCorp Poland Sp. z o.o. (based in Warsaw)
• Work location: Brussels, Belgium (construction site address)
• Work dates: 15 Jan 2025 - 28 Feb 2025 (6 weeks)
• Job title: Structural Carpenter
• Status: POSTED WORKER (designated)
• Gross salary: €2,500 per month
• Home country: Poland
• Sending company registration: Polish Tax ID 12345678

Declaration filed in Belgium 72 hours before work start (13 Jan 2025) to Posted worker is now registered in Belgian social security to Entitled to Belgian health insurance, unemployment benefits (if work ends) to Belgian authorities have visibility of posting to Posted Workers Directive compliance demonstrated

Why Dimona Matters: Operational impact

For HSSE Teams

Dimona compliance is foundational to legal operations in Belgium. Missing or late Dimona declarations expose the company (and sometimes the HSSE team members responsible) to criminal liability and fines. HSSE teams must ensure all workers arriving on-site-including subcontractor workers-are registered via Dimona. Pre-mobilisation checklists must include Dimona verification. When incidents occur, the first regulatory question is "Was this worker properly registered?" Missing Dimona in an incident context can escalate liability dramatically.

For IT & CIOs

Dimona data must be centrally tracked and verified. Systems should flag when workers are scheduled to arrive, automatically check Dimona status, and alert HSSE/payroll if Dimona is missing or will expire. For projects with multiple subcontractors and high worker turnover, automated Dimona tracking is essential to prevent compliance gaps. Integration with payroll systems ensures Dimona is filed before first pay run.

Industry context

According to ONSS enforcement records (2021-2023), Dimona non-compliance is cited in approximately 40% of construction site regulatory inspections. Missing or late Dimona declarations account for approximately €50-100 million in fines annually across Belgium's construction sector. For individual construction companies, a single Dimona violation can cost €150-€5,000 per worker; typical construction projects involving 30-50 workers face potential exposure of €4,500-€250,000 for Dimona non-compliance. Conversely, projects demonstrating robust Dimona compliance (filed on-time with documented confirmations) face no Dimona-related penalties and significantly lower overall regulatory violation rates.

Implementing & Monitoring Dimona: From Manual to Digital

Manual approach: A construction principal contractor receives subcontractor worker lists but does not systematically verify Dimona status. Some subcontractors file Dimona late; some forget entirely. The principal assumes Dimona is the subcontractor's responsibility. When an ONSS inspector arrives, Dimona confirmations are not available; the principal is unable to demonstrate compliance. Fines are issued; multiple subcontractors face prosecution; the principal is held liable for subcontractor non-compliance under chain liability law.

Digital approach: Before subcontractors arrive on-site, the system automatically checks Dimona status. If Dimona is not yet filed, alerts are sent to subcontractor HR with the 72-hour deadline highlighted. On-site, workers cannot be assigned to tasks until Dimona confirmation is received. The principal contractor maintains a real-time Dimona register showing all on-site workers and confirmation status. When regulatory inspectors arrive, Dimona compliance is immediately demonstrable. For projects with posted workers, the system flags posting designation and Posted Workers Directive requirements alongside Dimona.

Dockt's platform integrates Dimona tracking with broader credential and workforce compliance management. When projects are planned, the system automatically generates worker lists and Dimona filing reminders. For posted workers, it flags both Dimona and Posted Workers Directive requirements. On-site mobile access allows supervisors to verify Dimona status instantly. Regulatory compliance dashboards show Dimona filing status in real-time; audit trails document that files were submitted on-time. For principal contractors managing multiple subcontractors, Dockt ensures visibility of Dimona compliance across the supply chain.

Best Practices for Dimona Compliance

• Plan Ahead & File Early: Do not wait until 72 hours before work starts. File Dimona as soon as worker assignment is confirmed (ideally 1-2 weeks before work). Early filing prevents last-minute delays and shows good faith compliance.
• Verify Worker Information Before Filing: Ensure all worker information (name spelling, date of birth, nationality, passport number for foreign workers) is accurate before filing. Incorrect Dimona may be rejected or create compliance issues.
• Maintain Dimona Confirmations: Store all Dimona confirmations (confirmation numbers, filing dates, PDF confirmations from ONSS). If a Dimona is not confirmed, follow up with ONSS immediately; do not allow workers on-site until confirmed.
• For Subcontractors: Principal Must Verify: If using subcontractors, do not assume Dimona has been filed. Actively verify Dimona status for all subcontractor workers before they arrive on-site. Include Dimona verification in pre-mobilisation checklists and site access procedures.
• For Posted Workers: Coordinate with Sending Company: If receiving posted workers from another EU country, confirm that the sending company understands Dimona requirement and will file within 72 hours. Provide clear deadline in the contract. If Dimona is not filed on time, the principal contractor is liable.
• Document Dimona Status in Incident Context: If an incident occurs, immediately verify that the injured/affected worker was properly registered via Dimona. If missing Dimona is discovered during incident investigation, it escalates liability significantly. Prevention is critical.