In Wallonia, the regulations of the Code de Développement Territorial of the Walloon region stipulate that you have to apply for a permit, unless your swimming pool meets the following conditions.
If any of these conditions are not met, you need to apply for a permit with limited effect (without the need for an architect) from your municipal council, which is usually issued within 30 days. But again, it's best to check with the municipality where you live to see exactly what is allowed.