It's pretty rare for me to end up with a single csproj, It's just that my umbraco projects tends to only be concerned with CMS stuff, I'd end up with csproj's for domain concerns that have no references to Umbraco.Core, as far as the business logic is concerned it has no idea what the CMS is. My controllers will get a bit of CMS content from umbraco, some dto's from some application services, and map them over to view models.