The IP Multimedia Subsystem (IMS) provides a framework that accommodates current and future services in wired and wireless networks. However, IMS does not handle non- 3G elements such as Wireless Local Area Networks (WLANs). In order to provide interconnection at the service layer between 3G and WLANs, interworking between IMS and WLAN is necessary. Extending IMS beyond 3G to WLANs is a crucial step towards the evolution of a seamless universal next generation wireless network, commonly known as 4G. In this paper, a novel architecture for service layer interworking between WLAN and 3G is presented. The architecture takes into consideration the interaction of WLAN Application SIP servers with the IMS Call Session Control Functions (CSCFs) and the extensibility of Application Servers (ASs) beyond the core IMS network. A WLAN AS is introduced into the IMS network and an SIP server into the WLAN. These act as interworking arbitrators and communicate with each other to provide service and session continuity. The main advantage of this architecture is its feasibility within the standard. It is also non-intrusive to the IMS core or the WLAN.