1. Electrical
Electrical is the line item most likely to come in over. The upstairs board, the tail-end load on existing circuits and the upgrade scope discovered mid-build all add up. On the Coorparoo project the electrical upgrade scope cost us roughly $5,000 over original.
2. Windows and glazing
Window prices move and lead times can blow your weatherproofing schedule. Lock the spec and the deposit before slab is poured.
3. Engineering variations
Variations to engineering scope mid-build are common. Variable ground conditions, a certifier flagging additional bracing or tie-down, scope additions in the slab. Budget 5-10% of structural category for variation contingency.
4. External works
Driveway, pathways, fencing, landscaping. Often deferred and often forgotten in the original budget. On the Coorparoo project external works were $44,250.
5. The 15-20% contingency rule
Budget contingency above your project envelope. Not 5%. Not 10%. 15% is the minimum on a Queenslander build-under; 20% if you're working with variable ground conditions or significant heritage constraints. This is the single biggest mistake we see in owner-builder planning: contingencies that are too thin.
Tools
Use the Renovation Budget System to model contingency at the line-item level rather than as a single global percentage.
