How Much Does It Really Cost to Build a Duplex in NSW in 2026?

If you search “how much does a duplex cost to build in NSW”, you’ll find numbers that range from $700,000 to $1.8 million.

That’s not helpful - and it’s why so many duplex projects blow out.

The real answer is: It depends on your site, approvals and engineering far more than the house itself.

Here’s what duplexes actually cost in 2026 - and what most people don’t budget for.

The Real Cost Range in 2026

For a standard two-storey, 2 x 3–4 bedroom duplex in NSW, most projects fall into this range:

Duplex Type	Typical Build Cost (ex GST)
Basic spec	$850,000 – $1.05M
Mid-range	$1.05M – $1.30M
High-end	$1.30M – $1.60M+

That’s for construction only not approvals, not design, not engineering, not site works, not council requirements.

Most people forget those.

What Actually Makes Duplexes Expensive

Two duplexes on the same street can differ by $300,000+ in cost.

Why?

Because duplexes are driven by site conditions, not floorplans.

The biggest cost drivers are:

1. Stormwater & OSD

If council requires onsite detention (OSD), pump-out systems or tanks, that can add $25,000 – $120,000.

This alone can kill feasibility.

2. Driveway & Levels

Steep blocks mean:

• Retaining walls

• Split-level slabs

• Piered footings

• More excavation

That can add $40,000 – $150,000.

3. Soil & Footings

Class P, H or E soil? Reactive clay? Fill?

Expect $20,000 – $80,000 just in footing upgrades.

4. Fire & Acoustic Separation

Duplexes must have:

• Fire-rated party walls

• Acoustic walls and floors

• Independent roof and framing

This is not optional and adds $25,000 – $60,000 compared to two normal houses.

5. Services Separation

Two meters Two sewer connections Two stormwater systems Two electrical feeds

Add $15,000 – $40,000.

The Hidden Costs People Miss

Most duplex feasibility spreadsheets are fantasy.

Here’s what usually gets forgotten:

Item	Typical Cost
Architect & design	$12k – $25k
Structural engineering	$8k – $20k
Hydraulic & drainage	$6k – $15k
BASIX	$2k – $4k
NatHERS	$2k – $4k
Bushfire (if triggered)	$3k – $8k
Surveyor	$3k – $8k
Certifier	$6k – $12k
Contributions (area dependent)	$0 – $60k+
Demolition (if knockdown)	$15k – $40k

These costs are real and unavoidable.

Why Cheap Duplex Quotes Are Dangerous

A $950,000 “duplex build” often becomes $1.25M once:

• Stormwater is added

• Footings are upgraded

• Fire walls are priced properly• Drainage gets approved

• Council conditions arrive

The difference is usually engineering, not the builder.

What a Real Duplex Budget Looks Like

Here’s what a realistic mid-range NSW duplex looks like in 2026:

Category	Typical Range
Construction	$1.05M – $1.30M
Design & engineering	$25k – $50k
Council & approvals	$10k – $30k
Site works & services	$40k – $120k
Total	$1.15M – $1.45M

This is why duplex feasibility matters.

The Only Way to Know Before You Build

You cannot guess duplex costs from floorplans or online calculators.

You need:

• Zoning & CDC check

• Flood & overland flow

• Bushfire mapping

• Stormwater rules

• Soil classification

• Driveway compliance

• Service locations

That’s what Plan Haus checks before anyone buys land or commits to a build.

Final Thought

Duplexes are one of the most powerful ways to create wealth in NSW.

But they only work when the engineering and approvals are understood before the design is drawn.

That’s the difference between:

“This site makes $400k profit” and “Why did this cost so much?”