Second Storey Extension: Is Your Slab Strong Enough to Go Up?

Going up instead of out is the classic Sydney move when the block is tight and the family is growing. But a second storey roughly doubles the load on your existing structure — and not every home was built with that future in mind.

The Three Things That Must Hold

• Footings: the concrete your house sits on was engineered for one storey. A structural engineer needs to confirm it can take two — or design underpinning to upgrade it.
• Walls: ground-floor loadbearing walls become the support structure for everything above. Brick veneer homes usually need new steel or timber posts inside existing walls to carry the load down.
• Soil: remember reactive clay? Doubling the load on a reactive site changes how the whole structure behaves. The soil classification matters as much as the slab.

How You Find Out

A structural engineer's assessment — typically $1,500–$3,500 including footing investigation — answers the question definitively. Test pits beside your footings show their depth and condition. This is non-negotiable due diligence before design work, not after.

What the Answers Cost

• Structure is adequate: a straightforward first-floor addition in Sydney typically runs $3,500–$5,000+ per square metre
• Underpinning needed: add $20,000–$60,000 depending on extent
• Structure can't economically take it: this is the scenario where knock down rebuild suddenly becomes the better investment — and it's more common than people expect on pre-1970 homes

While You're Up There

If the structure checks out, a second storey is also the cheapest moment you'll ever have to fix the ground floor: re-roofing happens anyway, scaffolding is already on site, and trades are already mobilised. Smart extensions bundle the works.