A standard concrete slab-on-ground costs $120 to $220 per square metre on level sites with stable soil, making it the most cost-effective foundation for modern extensions. Suspended timber subfloors (bearers and joists on brick piers) cost $190 to $320 per square metre but are vastly superior on sloping sites, highly reactive clay soils (Class H or E), or properties requiring accessible subfloor plumbing.

Waterproofing failures represent over 75% of all residential building defects. Early warning signs include peeling paint on adjoining room walls, swollen timber skirting boards, musty subfloor odors, and efflorescence along tile grout lines. Rectifying a failed shower membrane requires stripping tiles back to bare framing, replacing rotted joists, and applying certified Class III membrane.

Hairline vertical cracks under 1mm wide in drywall or foundation slabs are typically benign cosmetic settling caused by initial concrete curing and timber drying. In contrast, diagonal stepped cracks in brickwork exceeding 5mm, horizontal foundation fissures, or cracks where one side is visibly displaced outward indicate differential foundation subsidence requiring structural underpinning.

The seven most costly structural defects uncovered during pre-purchase inspections are: foundation footing subsidence, active termite infestation and hollowed subfloor framing, failing subfloor drainage causing rising damp, sagging roof trusses, cracked masonry cavity ties, degraded waterproofing membranes, and unpermitted removal of load-bearing walls.