SQL Server management studio allows you to have Database Diagrams to graphically show the structure of the database. In addition, you can create and modify tables, columns, relationships, and keys. Plus, you can modify indexes and constraints.
While trying to Create a New Database Diagram you might come across to the following error:
To address this problem you might consider granting access to [sa] database account:
ALTER AUTHORIZATION ON DATABASE::DatabaseName TO [sa];
Or if you want to change the owner to that local Administrator account, then it should be:
ALTER AUTHORIZATION ON DATABASE::DatabaseName TO [PCName\Administrator];