You cant copy between projects like that because the propeties for both buildings would conflict.
Some features can be copied from project to project using Ctrl + C , Ctrl + V, but its best not to use this feature for buildings.
If you have created a building that you want to use in another project, why not start the second project with that building loaded as building 1. Any new building could then be created as Building 2. Remember each project can have multiple buildings.
You can also save a building as 3D object, and then load that building into a new project as an object. This is useful for creating an estate for example, but as an object you cant edit it further.
