For visualisation, it is much nicer to give the neighbourhood polygons a gradient for the percentage of building cover. Therefore, we need to join the building percentage field to the neighbourhoods layer. That's not a spatial join, but a join based on a key field in both attribute tables. In this case, the neighbourhood name.
1. In the Layers panel, right-click on the neighbourhoods layer and choose Properties... from the context menu.
2. In the Layer Properties dialog go to the Joins 
tab.
3. Click 
to add a new join.
4. In the Add Vector Join dialog, make sure you choose Buildings dissolved as Join layer and buurtnaam for both the Join field and Target field.
5. Expand Joined fields and choose only building percentage, otherwise all fields will be joined.
6. Under Custom field name prefix, delete everything. In that way there will be no prefix for the joined field, which is already clear by itself.
7. Click OK to close this dialog and click again OK to close the Layer Properties dialog.
8. Check the attribute table of the neighbourhoods layer.
In the next section, we'll style the results.