Joining a social secretariat in Belgium takes two signatures: a written agreement and a mandate in Mahis, the application of the NSSO (RSZ), which you and the secretariat both confirm within 60 days. If you are a new employer, the secretariat then registers you with the NSSO and files the Dimona before the first working day, using the provisional number that starts with 51. A first affiliation can start at any time; a switch always takes effect on the first day of a quarter.
Joining a social secretariat at a glance
| Question | Answer | Source |
|---|---|---|
| Is joining compulsory? | No, you may file your returns yourself | NSSO, instructions on mandataries |
| How many employers join? | 83% in 2025 | NSSO annual report |
| What do you sign? | A written agreement and a mandate in Mahis | NSSO, Mahis guidelines |
| Deadline to confirm the mandate | 60 days, then it is cancelled | Mahis guidelines, point 2 |
| When does the mandate start? | First day of the quarter of affiliation | NSSO power of attorney |
| Switching from another secretariat | Start of a quarter, up to 1 after it begins still for that quarter | Mahis guidelines, point 4.3 |
| First DmfA | Within 1 after the end of the quarter | NSSO, DmfA instructions |
| NSSO contribution to the costs | € 36.45 per quarter with the first-employee reduction | NSSO, first hires |
Joining is not compulsory. Why, you can read briefly in is a social secretariat compulsory?. This guide covers what happens when you do choose to join.
What social secretariat affiliation means in law
A social secretariat in the legal sense is a non-profit association recognised by the Minister of Social Affairs. Article 27 of the Act of 27 June 1969 gives it the exclusive right to collect the contributions owed by its affiliated employers, only by bank transfer, and to pass them on to the National Social Security Office. On the official list of 20 June 2025 we count 25 recognised secretariats with an active head office.
When you join, the secretariat becomes your mandatary. It completes in your name the formalities you have as an employer: the Dimona, the quarterly DmfA return, and the collection and transfer of contributions. Responsibility stays with you. The NSSO writes in its guidelines that a company always remains responsible itself for meeting its obligations correctly. If something goes wrong, the institutions turn to you.
Besides the recognised secretariats there are social service providers without recognition. They may file returns under a mandate, but they may not use the title social secretariat or collect contributions. The difference, and where a social bureau fits in, is explained in our guide recognised secretariats and social bureaus and in what is a social bureau.
Employer joining a social secretariat: the steps
This is the route for a company hiring staff for the first time. An employer who only switches secretariat skips steps 1 and 4.
| Step | Who does it | Deadline |
|---|---|---|
| 1. Company number (KBO/BCE) | you, through a business counter | before anything else |
| 2. Agreement with the secretariat | you and the secretariat | before the first working day |
| 3. Mandate in Mahis for Dimona, DmfA and Flexi At Work | one party enters, the other confirms | 60 days after creation |
| 4. Registration as an employer through WIDE | the secretariat with a mandate, or you | before or on the day the employee starts |
| 5. Determine the joint committee | the secretariat with you | before the contract and pay |
| 6. Employee details and contract | you provide, the secretariat processes | before the start |
| 7. Dimona IN | the secretariat or you | at the latest when the employee starts |
| 8. First pay and payslip | the secretariat calculates, you pay | on the pay date |
| 9. First DmfA | the secretariat | 1 after the quarter |
1. The company number. Without a KBO number there is no registration as an employer with the NSSO. A company or a sole proprietorship already has one. Are you self-employed and hiring your first employee? Also read social secretariat for a sole proprietorship.
2. The agreement. Before a secretariat may act for you, you sign a written agreement that expressly sets out which tasks the mandatary will carry out for the employer, as the NSSO instructions put it. Check the term and the notice rules: that is where your freedom to switch later lies. The questions to ask beforehand are in which social secretariat to choose.
3. The mandate in Mahis. You or the secretariat enter the mandate in Mahis; the other party confirms it in the same application, with an eID or itsme. Those two confirmations are enough, no handwritten signature is needed. Without confirmation within 60 days, Mahis cancels the mandate and you start again. There can only be one mandatary for the DmfA, and a DmfA mandate always comes with a mandate for Dimona and Flexi At Work.
4. Registration as an employer. A secretariat can only send your WIDE registration once a validated mandate is in Mahis, at least for Dimona. WIDE immediately issues a provisional number starting with 51, and the Dimona can go out with it. The final number, your employer category and your NACE code follow later on form ID101. More in the NSSO number.
5. The joint committee. Your activity decides which joint committee (paritair comité) you fall under, and that committee sets the minimum wage, bonuses and indexation. See how to find your joint committee.
6 and 7. Contract and Dimona. According to the NSSO, the Dimona IN must go out at the latest at the moment the employee starts working. Never let anyone start without a Dimona: a missing Dimona is a level 4 offence, with an administrative fine of € 3,000 to € 35,000 per employee. Everything about the declaration is in the Dimona declaration.
8 and 9. First pay and first DmfA. The secretariat calculates the pay and produces the payslip. After the quarter it files the DmfA, within 1 after the end of that quarter. What that return contains, you can read in what is the DmfA.
Some obligations are handled outside the secretariat: the occupational accident insurance, membership of an external prevention service and the work rules. The full list for the first working day is on becoming an employer.
The details and documents the secretariat asks for
Every secretariat has its own forms. What it asks for follows from what the returns and the payroll calculation need:
| Detail or document | What it is needed for |
|---|---|
| Company number and details of the manager or director | agreement, Mahis mandate, WIDE |
| Description of your activity | joint committee, NACE code, employer category |
| NSSO number, if you are already an employer | taking over the existing file |
| Occupational accident insurance policy | returns after a workplace accident |
| Name of your external prevention service | the annual fee and health surveillance |
| National register number or BIS number of the employee | Dimona and DmfA |
| Start date, contract type, working schedule and job | Dimona, contract, payroll |
| Gross pay, benefits and commuting | payslip, contributions, withholding tax |
| Family situation and bank account of the employee | withholding tax and payment |
| When switching: the individual accounts and leave balances | continuous payroll and holiday pay |
Under Article 48 of the Royal Decree of 28 November 1969, the secretariat must keep a complete file for each affiliated employer, at a location in Belgium, that allows the inspectorate to check the accuracy of the returns. What an individual account contains is explained in our guide on the subject.
First affiliation or switch: what is different
| First affiliation | Switch to another secretariat | |
|---|---|---|
| Start date | any time; without a previous provider even retroactively | always the first day of a quarter |
| Last moment | none | up to 1 after the quarter begins, still for that quarter |
| Notice | not needed | under the contract with your current secretariat |
| NSSO number | new, through WIDE | stays the same |
| History | none | the old mandatary must give the new one all the information it needs |
| Last return | none | to agree: who files the DmfA for the last quarter with the old secretariat? |
The rule for switching comes from the NSSO instructions: a mandate can only pass to a new mandatary at the transition to a new quarter. When entering the mandate, you confirm in Mahis that you terminated any previous contract correctly. How to do that and which dates apply for 2027 and 2028 is in social secretariat notice period; the full playbook is in changing social secretariat.
What the secretariat collects after you join, and why
Once you have joined, you no longer pay your NSSO contributions directly to the NSSO, but to your secretariat, which passes them on. That is the law: recognition gives the secretariat the exclusive right to collect contributions, and a provider without recognition is not allowed to. If you owe monthly advance payments to the NSSO, those go through the secretariat as well.
On top of that, the secretariat charges its own fee for the work. It is set out in your agreement. Is the money you pay for the NSSO safe? Article 48 of the Royal Decree of 28 November 1969 requires every recognised secretariat to lodge a guarantee with the National Bank or the Deposit and Consignment Office, from € 37,184.03 to € 185,920.14 depending on the number of employees of its affiliates. That guarantee covers employers' losses if the secretariat fails to pass the amounts on.
Proof of affiliation and affiliation number at a social secretariat
If you need proof of affiliation, you have three sources:
- Mahis. The NSSO describes Mahis as the application in which an employer can view its mandates. If there is doubt about a mandate, the data in Mahis prevail. Your company's access manager gives you the role to view them.
- The signed agreement with the secretariat, with the tasks and the start date.
- The NSSO confirmations. For a new employer, the secretariat receives a copy of form ID101 with the final number, and an electronic confirmation ID102 that the power of attorney has been registered.
Three numbers come up around an affiliation and are often confused:
| Number | Who issues it | What it tells you |
|---|---|---|
| NSSO number | the NSSO, through WIDE | that you are an employer; needed for Dimona and DmfA |
| Recognition number | the minister, at recognition | which secretariat it is; Acerta has number 250 |
| Affiliation number | the secretariat | the number under which the secretariat manages your file |
The affiliation number is a separate field on the NSSO power of attorney, next to the recognition number and the branch office of the secretariat. No law sets its format: each secretariat picks its own numbering. Under Article 48 of the same decree, the secretariat must show its recognition number on all its deeds, invoices and documents. So every invoice tells you which secretariat you belong to.
What joining costs, in broad terms
There is no statutory fee. Secretariats usually charge a one-off cost to set up the file and then a fee per employee, per payslip or per month, with extras for special tasks. Some ask for a guarantee or an advance. How these items work is set out in the cost of a social secretariat and in social secretariat rates.
Two government contributions lower the bill:
- For your first employee. If you had nobody employed in the previous 12 months, you get up to € 2,000 per quarter off your employer contributions. If you are affiliated with a social secretariat, there is also a contribution of € 36.45 towards the affiliation costs for each quarter with that reduction, not prorated to the hours worked. Explained in the reduction for the first employee.
- In hospitality. Employers in the hospitality joint committee have received since 1 April 2007 a contribution of € 10 per full-time equivalent per quarter. It is settled directly with the secretariat. More on payroll in hospitality.
Example: a first employee on 1 February 2027
A fictional baker in Ghent, a sole proprietor, hires his first shop assistant on Monday 1 February 2027.
- In mid-January he signs the agreement with the secretariat. The secretariat enters the mandate in Mahis, and the baker confirms it the same day with itsme.
- The secretariat registers him as an employer through WIDE and receives a provisional number starting with 51.
- Together they determine the joint committee and the pay. The baker provides the national register number, the schedule and the contract.
- By 1 February at the latest, before the assistant starts, the Dimona IN goes out.
- The first pay follows at the end of February. After the first quarter, by 30 April 2027 at the latest, the DmfA is with the NSSO, with the first-employee reduction and the contribution of € 36.45 for that quarter.
Checklist before you sign
Go through these questions before you sign:
- Do you know which tasks the secretariat does and which you keep doing yourself?
- Are the term, the notice period and the form of notice in writing?
- Who enters the mandate in Mahis, and who confirms it within 60 days?
- Who handles the WIDE registration if you do not have an NSSO number yet?
- Who determines the joint committee, and based on which activity?
- Who files the Dimona, and how far ahead must you report a new employee?
- What does the set-up cost, what does each month cost, and what is not included?
- Is the first-employee or hospitality contribution reflected in the invoicing?
- Who is your fixed contact, and how do you reach them?
If you cannot answer a question, ask the secretariat before you sign. The answers belong in the agreement, not only in an email.
Joining through Business Centrum
Business Centrum is a social bureau in Ghent and a recognised partner of Acerta. You sign an agreement with Business Centrum, and we arrange the affiliation with Acerta together with you; Acerta is the secretariat that files the returns and collects the contributions. You keep one fixed contact who knows your file. How that cooperation works is described on social bureau and Acerta.
Business Centrum then takes over your social secretariat through Acerta, your payroll administration and your Dimona management. Want to know what joining means for your business? Request a call back.