Driven by servitization, an increasing number of manufacturing companies are transforming from being a pure product provider to a provider of Product–Service Systems (PSS). PSS offer unique benefits to both providers and customers by combining physical products and intangible services. However, this results in an increase in variety for the provider, which causes an increase in variety-induced complexity and, finally, also in costs. This correlation is proven based on the literature; possible approaches to manage the complexity are presented and the current research gap is addressed. To address the gap, a method for the development of modular architectures for PSS is presented in this paper. The method comprises five phases and supports companies from identifying PSS-based business models up to modularizing product components and service components. To validate this method, it is first applied to the example of a product family of pressure-reducing valves and then its results are discussed afterwards. It is proven that this method is suitable for the development of modular PSS architectures, but there is also a need for further research, which is described in the Conclusions section.