If the building is made of of static meshes you can combine them into one mesh. Do this by going to Edit > Editor Preferences, then under General on the left there is “experimental”, click on that and you’ll see Actor Merging.
All you have to do then is select your meshes, go the Window > Merge Actors.
To move it to a new project just right-click the asset in the content browser, go to asset actions > migrate.
Hope this helps