The build contract is the biggest number in the feasibility. It is never the only one. Owners who budget for the builder and nothing else are the ones who run out of money at lock-up.
The lines that make up a duplex
| Line item | Indicative range | Notes |
|---|---|---|
| Demolition and site clearing | $30k–$60k | Asbestos, tree removal and tip fees push the top end |
| Survey, geotech, arborist, stormwater | $10k–$20k | Done before design, not after |
| Architect and design | $40k–$80k | Two homes, not one. Council-ready drawings |
| Town planner, BASIX, Section 10.7, approval fees | $15k–$35k | Higher on a DA than a CDC |
| Certifier and inspections | $8k–$15k | Through to Occupation Certificate |
| Construction — two 4-bed homes | $1.6m–$2.2m | Site conditions and spec drive the spread |
| Siteworks: retaining, driveways, OSD | $60k–$150k | Slope is the variable. Belrose costs more than Dee Why |
| Service connections and utilities | $15k–$40k | Sydney Water, Ausgrid, NBN |
| Landscaping and fencing | $25k–$60k | Buyers judge the house from the street |
| Subdivision, linen plan, titles | $15k–$30k | Creates the two Torrens titles |
| Finance costs over the program | depends on structure | Interest capitalised over 14–20 months |
| Agent, marketing, legal at sale | 2–3% of sale price plus marketing | Two sales, two campaigns |
| Contingency | 5–10% of build | Non-negotiable in our feasibilities |
What moves the construction number
- Slope. A flat Forestville block and a sloping Belrose block with the same floor plan can be $100k apart before finishes.
- Spec. A duplex sold to a Northern Beaches family needs a Northern Beaches finish. Stone benchtops, engineered timber, quality tapware. Cheap finishes cost more at sale than they save at build.
- Access. Narrow street, no room for a crane, no laydown area. Every delivery costs more.
- Bushfire BAL rating. Mapped land brings compliant glazing, cladding and ember protection. Real money.
- Basement or garage under. A split level with a garage beneath is a different build to a slab-on-ground.
Why fixed scope matters more than fixed price
A builder's fixed price is only fixed for the scope it describes. Variations are where budgets die. Our contracts tie down scope line by line before settlement, with provisional sums kept to a minimum and allowances set at real selection prices. If a client wants to upgrade later, they pay for it knowingly. They do not find it in a variation at frame stage.
Where owners save and where they shouldn't
Save on: the things buyers never see. Over-engineering a slab the engineer did not ask for. Design changes after approval.
Don't save on: the street elevation, the kitchen, the master bathroom, the landscaping. These set the sale price.
The honest comparison
A renovation of your existing house: $300k–$500k, most of it gone at sale.
A duplex on the same block: everything above, funded by construction finance against the end value, with a gross profit that has typically been around $1.25m on our projects.
The second number is bigger because the second project creates two new assets. The first just improves one old one.
Get a real number
An indicative range is a start. A number you can take to a lender comes from a design and a builder's estimate on your block. That is what the free site assessment produces.