Adelaide is built on reactive black/grey clay across most of the metro. This affects pergola footing design dramatically.
The problem
Reactive clay swells when wet and shrinks when dry. Seasonal movement of 30-50mm is normal in Adelaide's east, west and north. Standard 300mm-diameter, 600mm-deep concrete pads can heave, tilt or crack within 3-5 years.
The fix
Engineered piers (typically 450-600mm diameter, 1.2-1.5m deep, reinforced with steel cages) or screw piles. Sized per AS 2870 for the specific site reactivity. Adds $1,500-$4,000 to a typical pergola/verandah build.
Where it matters most
Eastern Adelaide (Norwood, Burnside, Magill, Beaumont), inner-western Adelaide (Mile End, Henley, Fulham), northern post-war estates (Salisbury, Elizabeth, Tea Tree Gully). Hills properties on rock typically don't need engineered footings.