In this section, we're going to calculate areas of neighbourhoods.
1. In the Layers panel, right-click on the neighbourhoods layer and choose Open Attribute Table from the context menu.
2. In the attribute table, click 
to toggle on the editing mode.
3. Click 
to open the Field Calculator.
4. Create a new field with the Output field name Area (m2) and Output field type Decimal number (real).
5. Under expression add $area.
6. Click OK.
Now you'll see that the area of the neighbourhood polygons is added to the attribute table.
7. Click to toggle off editing. Confirm that you want to save the results.
In the next section, we're going to add this information to the building polygons.