Future-proof prices for water services ought to be ecologically, financially, economically and socially sustainable. Volumetric-only fees exhibit such characteristics just partially, considering the high costs of infrastructure provision. Admittedly, volumetric tariffs are simple, well accepted and offer incentives for wastewater saving. However, other important sustainability dimensions of pricing services are underweighted. Sustainable user fees, in contrast, may induce a reasonable balance between the competing sustainability objectives of pricing. For this purpose, the discrepancy between cost and revenue structure must be overcome by introducing two-part pricing applying a basic user fee. Such a two-part tariff limits the probability of failed cost recovery (financial sustainability), encourages efficient resource use and investment behaviour (economic sustainability) and reduces affordability problems (social sustainability) through increase in (volumetric) price. While this article discusses the meaningfulness of basic user fees conceptually, in Part 2 the design of basic fees for wastewater disposal is dealt with. Part 3 will address sustainable user fees for rainwater drainage.