Depends on your necessity. If you need to provide service to the world outside your app domain go for SOA. If you need only project organization, just fallow Microsoft Application Architecture, or some Business Architecture Guide for .NET. You should see some of those guides using legacy Web Services
or WCF
, but Web API
come to replace those in some way.
Web API today already can go outside IIS, and It's easy to do that using OWIN middleware. This should be a good addition to your project given that you want modularity.