Now we have layers showing the flood extent, we can use it to select the affected buildings.
1. In the main menu, go to Vector | Research Tools | Select by Location....
2. In the Select by Location dialog, choose to Select features from buildings with height Where features overlap By comparing to the features from flood1m. Select Modify selection by creating new selection.
3. Click Run. Click Close after processing is completed.
Now you'll see in yellow the selected buildings in the map canvas. These are the buildings affected when the water table rises to 1 m above sea level.