Centennial Park 2021 · Woollahra Council
House Extensions in Centennial Park: Joining New to Old
The hardest part of your extension is a joint you'll never see.
Most houses around the Parklands went up between the 1880s and the 1920s, on footings nobody drew. Your new wing will sit on engineered concrete. Where the two meet, they'll move differently for the rest of the building's life, and that junction decides whether the plaster cracks in year two.
Short answer
What matters most when extending an old Centennial Park house?
Where the old house meets the new wing. Victorian footings are often shallow brick or sandstone, so test pits and an engineer's design come first, then a movement joint so each part settles independently, and underpinning where new work loads or undermines the old wall. Woollahra DAs take 14 to 20 weeks. Ground-level additions run about $2,465 to $2,655 per m² here.
Written and reviewed by Oliver Alameri
Licensed builder, NSW Contractor Licence 487805C · Founder, Buildana
Fact-checked
What's actually under a Victorian wall
Sometimes it's dressed sandstone blocks sitting straight on rock. Great. Sometimes it's a few courses of stepped brick on a bed of rubble, 400mm down, on clay that swells every wet winter. You don't know which until someone digs.
So before the design is locked, I get test pits dug beside the wall the new work will attach to. The structural engineer looks at depth, width and what the footing bears on. Twenty minutes with a shovel can save a redesign.
It matters more here than most places. Centennial Park runs across sandstone ridges, but the fall toward the harbour brings softer, more reactive ground. Neighbours three doors apart can be standing on completely different ground.
- Footing depth and width at the junction wall
- What it bears on: rock, clay or old fill
- Condition of the brick or stone and the mortar
- Any past cracking or earlier patch-up repairs
Dig first. Draw second. Doing it the other way round is how budgets blow out.
Let them move, just not against each other
A new concrete footing and a century-old brick one won't settle the same way. Try to lock them rigidly together and the weaker one loses. Usually that's the old wall.
The better answer is articulation. A deliberate vertical joint where old meets new, filled with a flexible sealant, with flashings and roof junctions detailed to slide rather than tear. Inside, a shadow line or a change of material hides it. Nobody notices it's there. That's the point.
Old lime mortar is soft on purpose, too. Repointing it with hard cement mortar near the join traps moisture and shifts the stress into the brick itself, so any patching gets matched to what's there.
Want to know what's under your house first?
Send me your Centennial Park address and a sketch, even a napkin one, of what you'd like to add. I'll tell you where I'd dig, what the engineer will look for, and how the junction should be handled.
0476 300 300When underpinning is on the table
Two situations usually trigger it. The new work loads the old wall, say a first floor bearing on it. Or the new footings go deeper than the old ones, close enough to undermine them.
Underpinning means extending the old footing down to better ground in short sections, never all at once. Dig a bay, pour it, let it cure, move to the next. On a semi-detached house the shared wall needs particular care, and the neighbour is told in writing before work starts, with their house photographed first.
It's slow. It's also predictable once the test pits are in, and that's why they come first.
Cost and the Woollahra timeline
Woollahra's heritage conservation areas cover nearly every street here, which puts the complying-development route out of reach for most additions. Plan on a development application: 14 to 20 weeks in council's own figures, longer if a state-listed item is involved, with base fees of $2,500 to $4,200.
For budgeting, Rawlinsons puts a 50 to 75 square metre brick veneer ground-level addition at $1,825 to $1,965 a square metre in Sydney. With Centennial Park's 1.35 loading, that's roughly $2,465 to $2,655. A new first floor starts nearer $3,700. Kitchens, bathrooms and any underpinning are extra, each costed off the engineer's design rather than guessed.
Before the drawings are finished
What I'd want answered on any house extension in Centennial Park.
- 01
Have test pits been dug at the wall the extension joins?
- 02
Has an engineer designed the junction, not just the new footings?
- 03
Where do movement joints go, and how are they hidden?
- 04
Will the new footings sit deeper than the old ones?
- 05
Is a shared wall involved, and has the neighbour been told?
- 06
Is underpinning priced as a real line item, not a provisional guess?
Extending an old Centennial Park house
Why does my old house need test pits?
Because nobody recorded how 19th-century footings were built. Digging beside the wall shows depth, condition and bearing ground, and that shapes the engineering.
Will the extension crack where it joins the house?
Not if the junction is articulated. A designed movement joint lets old and new settle separately, so any movement stays in a planned line.
What is underpinning?
Deepening an existing footing to firmer ground, dug and poured in short sections so the wall is always supported.
Is a CDC possible for a Centennial Park extension?
Rarely. Almost every street sits inside a heritage conservation area, so most additions go to Woollahra Council as a development application.
Sources and references
Costs are from the Rawlinsons handbook with the local cost index applied; rules and timeframes are checked against the sources below.
Old house, new wing, no cracks.
A Centennial Park extension engineered from the footing up.