I have a vector layer showing the premises of hospitals in a city (purple layer) and a layer of all buildings (not just hospitals). From this I used Clip to find the hospital buildings (pink layer). Some hospital premises are present but the buildings within them aren't (within the red boxes). How can I select the premises where there are no buildings and add them to the pink layer? Essentially finding the polygons where the all buildings layer does not overlap with the purple layer.
You could use the 'Select by Location' tool which is found in vector > Research Tools > Select by Location. Use the disjoint predicate to select features from your original
Hospital layer which do not intersect with features in your
Hospital Buildings layer.
copy and paste the selected features between the two attribute tables,
save the selected features as a new layer, and then use the vector > Data Management Tools > Merge Vector Layers tool to add them to your