Namespace Considerations in Office Solutions
You cannot change the root namespace (in Visual Basic) or the default namespace (in C#) after you create a Visual Studio Tools for Office project. The namespace will match the name of the project. When you create a project, use the name you want for your project namespace.
Host items in C# projects have their own namespaces. By default, the host item namespace is based on the name of the project.
Use the Namespace for Host Item property to change the namespace of the host item. For more information, see Properties in Visual Studio Tools for Office Projects.