Dimona (Declaration Immediate/Onmiddellijke Aangifte) is the mandatory real-time declaration system for ALL Belgian employers to report the start and end of every employment relationship to the NSSO. In the construction sector Belgium, additional requirements apply that go beyond standard rules. What Is a Dimona Declaration? Dimona is the electronic declaration that an employer must submit: - At the start of every employment relationship (Dimona IN) — mandatory BEFORE the first working day - At the end of every employment relationship (Dimona OUT) — by the next working day at the latest - When the contract type or status changes (Dimona UPDATE) Dimona sits alongside Checkinatwork attendance registration: both are mandatory in the construction sector, but they register different things. Construction Sector Belgium: Specific Dimona Requirements 1. Dimona IN for Construction Workers Required data: - Worker's NISS/INSZ number - Expected start date - Joint committee number: PC 124 (construction) or PC 200 (support functions) - Type of employment contract (permanent, fixed-term, student, flexi-job) 2. Dimona OUT Must be submitted by the next working day after the last day of work. This applies even for single-day contracts. 3. Dimona for Temporary Workers in Construction Temporary employment agencies are responsible for the Dimona declaration for each temporary worker. The user company (construction firm) must additionally ensure correct Checkinatwork registration. 4. Foreign Workers and Subcontractors Additional obligations beyond Dimona: - Limosa declaration: Mandatory for every foreign worker before starting work in Belgium - A1 certificate: Proof of social security affiliation in the home country - Belgian work permit if applicable (outside EU) Common Mistakes in Construction Sector Dimona - Late Dimona IN declarations: The declaration must be submitted BEFORE the first day — not on the first day itself - Forgetting Dimona OUT for short-term projects or day contracts - Not updating Dimona when the contract type changes - Missing Limosa declarations for foreign subcontractors - Invalid NISS numbers leading to processing errors at the NSSO Penalties and Sanctions The NSSO and Social Inspection actively monitor Dimona compliance in the construction sector: - Administrative fines: €50 to €2,500 per violation - Social corrections: up to €6,000 per undeclared worker - Criminal prosecution for systematic fraud - Exclusion from public tendering Dimona and Checkinatwork: The Combined Obligation In the construction sector Belgium, Dimona and Checkinatwork are complementary obligations. Checkin OK integrates both: when uploading your attendance data, the platform also checks the Dimona status of workers and flags missing or expired declarations. Read also our post on Checkinatwork compliance or our guide on NSSO attendance declarations. Frequently Asked Questions about Dimona Declaration Q: Can I cancel a Dimona declaration? A: Yes. A Dimona IN can be cancelled if the employment relationship has not started. This must be done as soon as possible after the planned start date via the NSSO portal or through an automated system. Q: Does Dimona also apply to interns and students in the construction sector? A: Yes. Students and interns must be declared via Dimona, but with a specific student status (Dimona type S). Threshold hours apply for holiday work. Q: What if I make a Dimona error? A: Errors in Dimona declarations must be corrected as quickly as possible via Dimona UPDATE or via the NSSO helpdesk. Checkin OK flags potential Dimona conflicts before you submit your attendance data. Try Checkin OK free today — automate your Dimona declaration, Checkinatwork, and CIAO compliance in one platform.

Construction Sector Dimona Rules in Belgium: What Employers Must Know in 2026