Can you insist on working from home?
There is no legal right to work from home. A bill to give employees that right was rejected by the Senate in 2023. Under the Flexible Working Act you can ask to change your place of work; your employer must consider the request seriously but may refuse.
Allowances
For a single day your employer can pay one or the other, not both. Employers may also provide a desk, chair and screen tax-free under the work-related costs scheme.
Living in Belgium or Germany?
An EU framework agreement lets cross-border workers work from home up to just under 50% while staying insured in the employer's country, if both countries signed and you apply.
Days worked at home abroad can move part of your income tax to your country of residence. Keep a record of your home-working days.
See DTAA in depth for treaty rules, and the job offer guide.
Sources
This article is based on these official pages. If anything here differs from them, the official page is right.