Guide for employers
Changing your
social secretariat
You change social secretariat by terminating your affiliation as your contract allows and having a new mandate registered with the NSSO (RSZ). The switch always takes effect on the first day of a quarter, and most contracts require 6 months' notice, often to the end of the calendar year.
By Business Centrum editorial teamPublished
Updated and figures checked on
- When
Start of a quarter
In January, April, July or October. Never in the middle of a quarter.
- Notice
6 months, typically
The law sets no notice period. Your affiliation contract decides.
- Who registers
Your new partner
The new mandate goes to the NSSO through Mahis. The old one then ends automatically.
The rules
What the law says, what your contract decides
The rules for a switch sit in two places. The law decides when a mandate can change hands and who must pass on what. Your affiliation contract decides how long you are bound and how you give notice.
The law
A written agreement
You and your social secretariat have a written contract that sets out the content of the mandate (article 31quater of the law of 29 June 1981). The mandate itself is registered with the NSSO in the Mahis application.
Only at a new quarter
A mandate only passes to a new representative at the transition to a new quarter. Once the new mandate is validated in Mahis, the NSSO automatically closes the old one on the last day of the previous quarter. Your new partner can still do this for the current quarter up to 1 month after it starts. Going back further is not possible without an extra procedure.
The past moves with you
The new representative takes over the management of past, present and future. Your former secretariat must, on request, keep providing all information on the quarters it handled. The contract with your new partner must state what the former one still does for its quarters.
You remain responsible
Whoever files your declarations, you remain responsible as the employer for getting them right. If your representative makes a mistake, the NSSO turns to you.
Your contract
Minimum term
Many contracts bind you for the first 3 years. Within that period you can only leave with the secretariat's agreement or against the fee the contract provides for.
Notice period and timing
6 months' notice is common. Some contracts only allow you to leave at the end of the calendar year, others at the end of every quarter. That one difference sets your earliest switch date.
Form of notice
The contract usually requires a registered letter. An email or a phone call then does not count as proof.
Exit costs
Check whether the contract charges for transferring your file or for leaving before the end of the term. How pricing works is explained in our guide on the cost of a social secretariat.
Art. 31quater, § 3, law of 29 June 1981
A mandate can only be transferred to a new representative at the moment of transition to a new quarter.
Tacitly renewed without a notice period?
For contracts concluded, renewed or amended since 1 December 2020, the Belgian rules on unfair terms between businesses apply. A clause that tacitly renews a fixed-term contract without a reasonable notice period is presumed to be unfair. The secretariat can prove otherwise, so have such a clause reviewed before you rely on it.
Timing
Switching social secretariat: when is the best time?
Legally, any new quarter will do. In practice the turn of the year is almost always best, because three things run per calendar year.
January
Best moment
One year, one partner. Your former secretariat closes the year, the new one starts a full year.
April
Possible
Shortly after the year-end closing. The tax forms are filed, but the current year is split over two partners.
July
Possible, more work
Mid-year. Holiday pay has just been processed and half of the year's data sits with the former partner.
October
Possible, more work
Just before the end-of-year bonus and the year-end closing. Your new partner calculates that bonus on data it did not process itself.
What runs per calendar year
The individual account
For each employee you keep an individual account that is closed every year. If you switch mid-year, that account has to be merged from two sources.
The tax forms
The 281.10 forms for an income year must reach the FPS Finance by 28 February at the latest. If you switch at the turn of the year, your former partner usually prepares the forms for the closed year. If you switch during the year, agree in advance who prepares the form and with which data.
Holidays and end-of-year bonus
This year's number of holiday days depends on last year's work, and the end-of-year bonus is calculated over a full year. The less that data is split between two partners, the smaller the risk of a calculation error.
The last DmfA: usually still your former secretariat
A representative remains competent for the declarations of its own mandate period. As a rule, your former secretariat therefore still files the DmfA for its last quarter, within 1 month after that quarter. Put that arrangement in the contract with your new partner.
Worked example
One day late costs a year
Say your contract requires 6 months' notice and only allows you to leave at the end of the calendar year. You want to switch at the turn of the year.
On time
- Your registered letter arrives by 30 June at the latest.
- The notice runs until the end of the year.
- Your new partner starts in January, with a full new year.
One day late
- The letter only arrives after 30 June.
- The notice now applies to the end of the following year.
- You pay one more year for a partner you wanted to leave.
So read your own contract first and work out the notice date as soon as you decide. If the clause is unclear, have it reviewed: the earliest date sometimes hangs on a single sentence.
The handover
What moves to your new secretariat
Your NSSO employer number stays the same, and whatever was declared through Dimona and DmfA is held by the NSSO. What does need to move is everything your secretariat keeps itself to calculate correct pay.
| What | What it contains | Why it matters |
|---|---|---|
| Personnel files | Employment contracts, job titles, schedules, pay agreements and annexes | Without contract data, correct payroll is impossible. |
| Payroll history | Payslips, fixed and variable pay components, benefits, deductions | Basis for guaranteed pay, severance and later corrections. |
| Holidays and leave | Holiday balances, leave taken, time credit, parental leave | This year's holidays are based on last year's work. |
| Seniority and pay scale | Start date, joint committee, job class, sector benefits | Sets minimum wage, end-of-year bonus and your employees' notice periods. |
| Ongoing absences | Illness, workplace accidents, temporary unemployment | A social risk declaration is tied to the quarter of the risk date. |
| Certificates and annual documents | Individual accounts, 281.10 forms, certificates already issued | You keep the individual account for 5 years after its annual closing. |
| History at the NSSO | DmfA declarations and corrections of past quarters | Your former secretariat keeps providing information as long as those quarters are not time-barred: normally 3 years, 10 years in case of fraud. |
Timeline
Switching social secretariat step by step
From the first decision to the last declaration of your former secretariat. Order matters more than speed.
Over 6 months ahead
01Read your affiliation contract
Find the minimum term, the notice period, the date you can give notice for, the required form and any costs. From that, work out your earliest switch date.
Before you give notice
02Choose your new partner
Compare on what matters to you: knowledge of your joint committee, a fixed contact person, speed and price. Our guide on which social secretariat to choose lists the criteria. To compare the big names, see the largest social secretariats in Belgium and the differences between Acerta, SD Worx, Securex, Liantis and Partena.
By the notice date
03Give notice by registered letter
Send the termination letter as your contract requires and ask for written confirmation of the end date of your affiliation.
In the last quarter
04Transfer the data
Your new partner requests the files and history from the former secretariat. Meanwhile your parameters are set up: joint committee, pay scales, benefits, leave balances and the account for your NSSO payments.
Before the first day of the quarter
05Confirm the new mandate
The new mandate is entered in Mahis and you confirm it. The NSSO then closes the old mandate. Too late? Up to 1 month after the start of the quarter it can still be done for that quarter.
Day one
06First Dimona and first payroll
From now on every Dimona declaration goes through your new secretariat. Compare the first payslips line by line with the last ones from the former secretariat.
Within 1 month after the quarter
07Last DmfA from the former secretariat
As a rule, the former secretariat files the declaration for its last quarter. Check that it has gone out and that the contributions are paid.
By 28 February
08Tax forms for the closed year
When you switch at the turn of the year: check that the 281.10 forms are filed and that your employees received their copy.
Termination letter
What goes in your termination letter
A termination letter is short. Above all it must prove that you gave notice on time and correctly, and it should start the transfer of your data.
Termination letter
Send the letter by registered post and keep the proof of posting. If the date is ever disputed, that proof is what counts.
Re: termination of our affiliation
- Your company number and your NSSO employer number
- Your affiliation or client number with the secretariat
- A reference to the contract and the notice clause
- The end date of the affiliation: the last day of a quarter
- A request for written confirmation of receipt and end date
- A request to transfer your data to your new secretariat
- Date and signature of the person authorised to represent your company
Pitfalls
Where a switch goes wrong
Most problems arise in the weeks around the switch. Here is how to avoid them.
Notice too late or in the wrong form
You stay bound for another year, or the secretariat charges a fee for leaving early.
How to avoid it
Work out your notice date as soon as you have doubts, and send the letter in the form the contract requires.
A new mandate without proper notice
By confirming a new mandate in Mahis, you declare that the previous contract was ended correctly. The NSSO therefore does not wait for your former secretariat to resign. A mistake in your notice remains a contractual problem between you and that secretariat.
How to avoid it
First the confirmation of your notice, then the new mandate.
Payments to the wrong account
A recognised social secretariat collects your social security contributions and passes them on. After the switch you pay your new secretariat. A payment to the old account does not find its way to the right place by itself.
How to avoid it
Update your recurring payments before the first due date after the switch.
Leave balances that are wrong
Nobody spots a wrong balance until an employee asks for holiday. How we track leave and absences is set out on our service page.
How to avoid it
Check every balance at the handover against the former secretariat's data.
Illness or an accident from before the switch
A social risk declaration follows the quarter of the risk date. An accident from the previous quarter therefore normally still belongs to the former secretariat, even after you have switched. You or your new partner may still always file the declaration yourselves.
How to avoid it
Hand over a list of all ongoing files at the transfer.
A hire on the first day
Anyone starting on the first day of the new quarter needs a Dimona before they begin. In the rush of the switch it easily falls through the cracks.
How to avoid it
Tell your new partner about hires around the switch well in advance.
A split year without agreements
When you switch during the year, duplicate or incomplete tax forms appear if nobody agrees who closes the year.
How to avoid it
Put in writing what the former secretariat still does. The law in fact requires this in the new contract.
Business Centrum
How Business Centrum handles your switch
Business Centrum works as a partner of Acerta, a recognised social secretariat. Your affiliation runs through Acerta, and we are your fixed contact for all your payroll administration. During a switch we do the work and you make the decisions. How we work day to day is described on our social secretariat page.
What you do
What we do
What you do:You send us your current affiliation contract.
We work out your earliest switch date and tell you which clauses matter.
What you do:You sign the termination letter.
We draft it with the right references and follow up on the confirmation.
What you do:You confirm the new mandate.
We make sure the mandate is in Mahis before the quarter starts.
What you do:You do not have to request anything yourself.
We request the files and history from your former secretariat and check balances, seniority and pay scales.
What you do:You receive your first payslips.
We compare them with the last ones from your former secretariat and explain every difference.
What you do:You keep one contact person.
After the switch we also follow up on the former secretariat's last declarations.
Frequently asked questions
Questions about switching social secretariat
The questions employers ask us most often before they make the move.
Free consultationSources
Every amount and deadline on this page was checked against these official sources, last consulted on .
- FOD Financiën, Belcotax-on-web, Indieningstermijnen fiches
- FOD WASO, De sociale documenten
- RSZ, Administratieve instructies 2026/3, De verplichtingen, DmfA algemeen
- RSZ, Guidelines voor dienstverrichters (Mahis), punt 4.3, versie 2026-03-26
- Wet van 27 juni 1969 (RSZ-wet), art. 42, eerste lid
- Wet van 4 april 2019 (art. 18 voegt art. VI.91/5 WER in; art. 39 inwerkingtreding)
More at Business Centrum
Changing social secretariat
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