1. From the backend, go to the "Listings" tab and click on "Buildings".
2. From there you can see all your buildings. Choose the one you would like to attach the sub-building to and click on "Edit".
3. Once you're on the main building's page, you will find at the bottom a section called "Sub-Buildings". Write in the box the address of the sub-building you would like to add and click on the magnifier button.
4. A list of addresses will appear underneath the box. Click on "Attach" to the address you want to add to this building.
5. Don't forget to click "Update" at the bottom, or you sub-building won't be saved!
There you go!
Now your main building will have a sub-building attached to it.