At which level does high level Risk Assessment and solution scope comes in business case ?
According to the Business Analysis Practice Guide, the Business Case is defined as:
Business Case. A documented economic feasibility study used to establish the validity of the benefits of a selected component lacking sufficient definition and used as a basis for the authorization of further project management activities.
As Business Case set the basis of further project management activities, the risk assessment and documenting solution scope activities are done only at the high level. To initiate a project, the risk assessment and discovering of solution scope is done which is enough to understand the viability of the project.
As soon as project is initiated, during Analysis, a detailed solution scope is defined.