The increasing level of device connectivity in today's homes and buildings 
enables numerous opportunities for home owners, building managers, device 
manufacturers and solution providers. Standardized communication protocols 
(e.g., ZigBee, Bluetooth) provide physical connectivity and thus serve as a 
basis for smart home applications. However, beyond physical connectivity, real 
interoperability to effectively develop such applications requires additional 
efforts. It requires harmonizing multiple protocol standards, dealing with a 
heterogeneous device landscape, different data formats, managing resource 
constraints of devices and providing means to react quickly when devices and 
applications leave or join the system. We propose to address these 
application-specific requirements in multiple layers of abstraction. Our 
research has shown that each layer is used by different types of developers 
(e.g., device supplier, service provider, home owner) with specific tool 
support. The paper provides a classification and detailed analysis of 
requirements that have to be addressed in order to enable application 
development in smart homes. It further proposes and analyzes WS4D-PipesBox, a 
multi-layer framework, to illustrate how applications could be developed using 
multiple layers of abstraction.