Yes, all bed & breakfasts and hotels are required to install smoke alarms. This is regulated in the new Flemish Tourism Decree of April 1st 2017. You can find the complete text of the law on the website of Tourism Flanders.
The decree divides accommodation into three main categories:
Room-related accommodation with less than 6 rental units and less than 13 sleeping accommodations.
Room-related accommodation with more than 5 rental units or with more than 12 sleeping accommodations.
Terrain-related accommodation (such as campsites, smoke alarms are not required in this case)
The regulation for small accommodations is the simplest. B&Bs and small hotels must provide a smoke detector in every room and one extra smoke detector in every room on the escape route (to the entrance door). You should provide at least 1 smoke alarm on each floor. The smoke alarms should have a built-in non-replaceable battery that lasts for at least 5 years.
For larger hotels, the situation is a lot more complex. In exceptional situations (32 beds and one rental unit max.), autonomous, wireless connectable smoke detectors are still allowed, but in other situations you must provide a fire control panel, so that the entire building can be protected with wire-coupled 230V smoke detectors. Contact your local fire department for a specific audit.