Demolition, services and stormwater: the costs beside the build
The build price covers the two homes. A duplex also needs the site cleared, connected and drained, and those costs depend on your block more than your design. Here's what to plan for.
Demolition
Most duplex sites in Sydney start with an existing house. Demolition is usually approved as part of your DA or CDC, and the old house needs a hazardous materials check before it comes down. SafeWork NSW warns that homes built before the late 1980s may contain asbestos. A licensed removalist is needed for any friable asbestos and for more than 10 m² of bonded asbestos. Disconnecting old services and removing trees add to the cost.
Excavation and retaining
A level block keeps this simple. A sloping block may need cut and fill, retaining walls and extra drainage, and rock adds cost. A survey and a soil test early on show what you're dealing with.
Water and sewer
Each home usually needs its own water and sewer connection. Sydney Water needs to see the building plans, and your council or certifier will tell you if you also need a Section 73 compliance certificate. If a sewer main runs through your block, building over or near it needs Sydney Water's approval and can mean encasing the pipe in concrete or moving it.
Power and NBN
Each home usually needs its own electricity supply and meter, and the street supply sometimes needs an upgrade. NBN is arranged for each home.
Stormwater
Council sets how stormwater is handled. Some blocks need an on-site detention tank, and some need an easement over a neighbour's land to reach a drain at the back. Where the block falls away from the street, this can be one of the larger site costs.
Driveways and crossings
Two homes usually mean two driveways, or one shared driveway. Council sets the rules for vehicle crossings, and a second crossing isn't always allowed. In the Sutherland Shire, for example, a duplex normally gets one driveway per street frontage. Council may allow a second one where the frontage is at least 15 m wide and other conditions are met.
Fees and levies
Budget for council and certifier fees, the long service levy and, in some areas, development contributions. The NSW long service levy is 0.25% of the cost of building work of $250,000 or more, including GST. These are paid before or during approval, so they belong in your budget from day one.
How we handle site costs
Our free site assessment looks at the block before design starts, including the slope, the services, the stormwater route and the access. Our proposal lists the site costs we expect, so you can see what's in the price and what sits beside it.
General information only. Every site is different. Facts checked 9 October 2026 against SafeWork NSW, Sydney Water, the Sutherland Shire DCP and the NSW long service levy rules.