The Landlord can according to the rental agreement review the monthly charges if the actual consumption is to exceed the estimated monthly charges.
"This charge is calculated in an arbitrary way but based on a legal Belgian scale. If however the actual consumption is to exceed the estimated monthly charges, the Landlord can, in fact, based on just motives, change the amount of the monthly charges accordingly."
On the anniversary of the rental agreement the Landlord makes an addition of all the yearly expenses (water, electricity, heating, cleaning, insurance,...) to see whether the budgeted amount has been exceeded.
If the budgeted amount has been exceeded, the Landlord can decide to raise the monthly charges per person. This decision, together with the necessary proofs, is communicated to the roomies minimum one month before the actual monthly charges are raised.