To calculate the building density per neighbourhood, we need to first calculate the total area of the buildings per neighbourhood.
In Section 2.1 of this tutorial, you have already calculated the area of neighbourhoods.
1. In the same way, use the Field Calculator to calculate the area of the dissolved building features. Call the new field Building area (m2).
Now we can calculate the percentage of buildings per neighbourhood.
2. Go to the Field Calculator of the buildings dissolved attribute table.
3. Create a new field with the name building percentage with the Output field type Decimal number (real).
4. Create the following expression: (use the middle panel of the dialog, under Fields and Values double click on the fields you want to add to the expression)
"Building area (m2)" / "Area (m2)" * 100
5. Click OK.
6. Toggle off editing mode by clicking 
and confirm that you want to save the changes.
7. Check the results.
8. Close the attribute table.
In the next section we'll add the percentages to the neighbourhood layer.