changing from full-time to part-time work

It is possible within the framework of credit-temps, either thematic or one without a motive. Both allow you to reduce your hours for limited time and you'll get an allowance from ONEM to cover some of the income loss:
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If you have children under 12 years you can ask for parental leave either for 8 months on 1/2 time, 20 months on 1/5 or 4 months on full time, or a mix of these. If your child is born before 8 March 2012, the last 30 days of the leave is not covered by the allowance. The employer can not refuse parental leave, but can postpone the start of it with max. 6 months on business reasons.
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You can also ask for credit temps 'sans motif'; 24 months on 1/2 time, 60 months on 1/5 or 12 months on full time. To qualify you have to have minimum 5 years working history in Belgium and at least 2 years with your current employer. The eligibility conditions are less strict for 55 and + years old.
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If the company has 10 or less employees, the employer can refuse a request for credit temps without a motive. In bigger companies the employer can limit the amount of employees being on a credit time at a same time, the general rule is that the employer does not have to (but can) allow more than 5 % of his work force on a credit temps at a same time, but there are some variations of the rule in different sectors. The credit temps can not be refused on any reason for 55 and + years old.
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Should you ask to reduce your hours outside of these systems, it is up to your employer to accept or not. In case you would get a go, the change is done by adding an amendment in your contract defining your new work regime. It is important to consider if you want the length of the part-time agreement to be defined in the amendment. If the length is not limited the employer does not have to accept a return to full time even if the employee asks for it. On the other hand if the agreed period comes to the end the employer does not have agree on prolongation.
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For both part time on the above mentioned regimes or part-time based on agreement with the employer the working hours need to be defined and followed in detail in written. Any exceptions of the agreed working hours (for ex. having Tuesday off instead of the regular Wednesday, working extra time etc) need to be asked by the employee and approved by the employer in written, although in practice many companies do not follow this too closely.