The introduction of a basic fee for the management of wastewateris beneficial from a sustainability standpoint. A variety of bases for assessment are feasible. This article looks at three models based on sustainability criteria and using empirical data from three sample municipalities in the German state of North Rhine-Westphalia. The number of residential units turns out to be a suitable yardstick that is robust in the face of fluctuations in the quantity of wastewater and fair in allocating costs. The fairness of allocating costs is based on the high correlation with the quantity of wastewater, meaning that capacity-related contingency costs can be better mapped. In making the conversion, it is also important to minimise friction with the old fee model. In this regard, the size of the water meter is less effective as a potential basis for assessment than the residential unit model. It also proves to be less fair in allocating costs so the residential unit model, meaning that the latter appears advisable on the whole. The degression in contingency costs with regards to a building’s number of residential units requires that a basic fee tariff proportionate to efficiency must reflect this degression. What is more, an equivalency rule is needed for commercial users.