Convert WCF Library Project to WCF Web Service (WCF Application) Project Type
How do I convert WCF Library Project to WCF Web Service (WCF Application) Project Type in Visual Studio?
I don't agree with Geert's answer. Changing that guid would end up having project convert mid way.
Also having your service code in host project is not considered as best practices. As host project should bother only about hosting, means what end point you want expose, What type of security you want... things like that. Not about service's internal implementation.
I would recommend having library/class project as it is and have separate project in same solution to host it. Host project can be anything you want Console, Web Service or Web/WCF Application project.
By changing the projecttypeguid in the project file (csproj file -> unload project and open file).