What a duplex development costs in Australia

A duplex costs about $2,300 to $3,500 per square metre to build in Australia in 2026 excluding GST, so $415,000 to $630,000 per 180 m2 side before land. A knock-down-rebuild on a 700 m2 Brisbane lot bought for $1 million lands near $2.5 million total development cost, and the land price per dwelling decides whether it makes money.

A duplex is the smallest project that gets treated like a development: two attached dwellings on one lot, usually a knock-down-rebuild of a post-war house, subdivided into two titles at the end and sold or held. Most are done by an individual, a couple with equity in their home, or a small builder rather than a developer, and the numbers differ from a townhouse site because only two dwellings share the land cost. This guide sets out 2026 build rates from Rawlinsons-adjusted Sydney data and Melbourne and Perth builder guides, the demolition, subdivision and council charges by state, what duplex sides sell for, a worked feasibility on a 700 m2 Brisbane lot, and how the build is funded.

What a duplex costs to build per square metre

The tightest published Sydney figures are Buildana's May 2026 update of Rawlinsons Australian Construction Handbook rates: a medium standard attached duplex at $2,295 to $2,475 per square metre in brick veneer and $2,385 to $2,565 in full brick, and a high standard finish at $3,215 to $3,525. The step from medium to high specification adds roughly $880 to $945 per square metre. In Melbourne, Infinity Built's August 2026 guide puts dual occupancy construction at $2,600 to $3,400 per square metre, or $680,000 to $880,000 for two 130 m2 dwellings. Boda Building Group's 2026 guide has the national range at $2,000 to $2,500 low, $2,600 to $3,100 mid and $3,200 to $3,800 high, with Perth at $2,300 to $2,700, $2,800 to $3,500 and $3,600 to $4,450, and Yellow Pages brackets the pair at $700,000 to $1.5 million before land.

An attached pair shares a party wall, which Infinity puts at an 8 to 12% saving on the whole build against two detached dwellings, and a project builder's standard duplex plan is priced like a volume home. The site pulls it the other way: reactive clay footings add $10,000 to $40,000 per dwelling, a sloping block with retaining walls $30,000 to $80,000, and rock excavation $15,000 to $50,000. Turnkey pricing runs 15 to 30% above construction-only quotes, and Boda's recommended contingency for 2026 budgets is 8 to 12%.

Demolition, site works and the cost of two titles

Almost every duplex is a knock-down-rebuild, so the first cheque is demolition. Buildana's September 2026 Sydney figures are about $80 per square metre for brick and tile and $58 for fibro: $10,000 to $15,000 for a 150 m2 single storey home, $14,000 to $20,000 for a 200 m2 two storey. Add asbestos removal of $5,000 to $25,000 on pre-1985 houses, service disconnection $3,000 to $6,000, tree removal $2,000 to $8,000 and tipping $3,000 to $7,000. Melbourne demolition runs $18,000 to $40,000 in Infinity's guide with site preparation another $12,000 to $45,000.

External works are the line most owners forget: Infinity has $45,000 to $85,000 for driveways, fencing and landscaping on the pair, $12,000 to $24,000 for stormwater detention and $23,000 to $46,000 for service connections, because a duplex needs two of every meter and junction. The cost unique to a duplex is turning one title into two, and the method changes the sale price. Buildana's April 2026 comparison sets out the two paths for Sydney.

Torrens costs about twice as much, but Buildana's data has a Torrens duplex selling 12 to 18% higher than the same floor area on strata, with buyers financing at 80 to 90% LVR against 70 to 80% for a strata unit at some banks. Melbourne and Brisbane run $12,000 to $25,000 for a two lot plan. Minimum lot size decides whether it is available at all: ZoneScout's July 2026 summary has NSW at 450 to 600 m2 in R2 zones, Victoria around 500 m2, Queensland 400 to 800 m2 by council, South Australia 300 m2 per dwelling, and Western Australia by R-code (about 600 m2 at R30, 440 m2 at R40). A 700 m2 lot qualifies in most of them.

Council charges, consultants and approvals by state

Statutory charges on a duplex fall on the one net new dwelling, because the existing house earns a credit almost everywhere. Feasly's contributions guide and the 2026 South East Queensland schedule give the following.

Consultants on a two dwelling project cost $45,000 to $85,000 in Melbourne according to Infinity; Boda allows $18,000 to $30,000 for architecture and engineering plus $6,000 to $12,000 for permits per side, and Buildana budgets $5,000 to $15,000 for the DA or CDC and $5,000 to $8,000 for geotechnical and survey work. The list is a townhouse job at smaller scale: designer, town planner, engineers, surveyor, geotech, energy assessor, and a quantity surveyor if the lender wants cost-to-complete reports.

The approval pathway sets the holding cost. In NSW a complying development certificate takes 4 to 6 months against 12 to 18 months for a development application, and estate.sydney's Parramatta feasibility carries $108,000 of holding cost over 18 months at 6.5% because of it. In Queensland a dual occupancy is code assessable in most schemes, and Victoria's VicSmart pathway clears a compliant one in 10 business days.

What duplex sides sell for

A new duplex side sells between the suburb's unit median and its house median, and closer to the house median on Torrens title with four bedrooms. Cotality's 1 September 2026 index has median house values at $1,494,878 in Sydney, $920,432 Melbourne, $1,180,552 Brisbane, $1,043,478 Perth and $999,091 Adelaide, with units at $878,176 Sydney, $629,054 Melbourne and $733,223 Perth. Brisbane's July index put houses at $1,207,039 and units at $875,135, both up 14 to 17% over the year.

Sydney gives the cleanest read on where sides settle. estate.sydney's 2026 guide has the Parramatta duplex unit median at $950,000 against a $1,200,000 median site purchase, and a Canterbury-Bankstown case study bought at $950,000 in June 2024 that sold its two 90 m2 sides for $880,000 and $910,000. Western Sydney two-for-one sites bought around $1 million with a $1.1 million build are ending at $1.9 to $2.1 million combined. Infinity notes a front and back configuration sells the rear dwelling 5 to 15% cheaper than the front, so side-by-side pairs on wide frontages are worth more than a battleaxe layout. Two adjustments matter for a feasibility: the valuer prices the sides against completed duplex and townhouse sales, not detached houses, and new residential premises carry GST, so a $1.35 million contract nets less than a $1.35 million resale.

Worked example: knock-down-rebuild duplex on a 700 m2 Brisbane lot

The figures below are indicative and assembled from the sources above: a flat 700 m2 middle-ring Brisbane lot with a post-war house, bought for $1,000,000 against a Cotality house median of $1,180,552, replaced with a side-by-side pair of four bedroom two-storey dwellings of 180 m2 each, built by a project builder at $2,500 per square metre ex GST, subdivided into two freehold lots and both sold. Finance is a 70% loan to cost non-bank facility at 8.5% p.a. over 15 months. All costs are ex GST on the basis the owner registers and claims input credits.

Revenue side: two new four bedroom 180 m2 freehold sides at $1,350,000 each gives a gross realisation value of $2,700,000 including GST, a 12% premium to the suburb house median for brand new stock. Under the margin scheme GST is one eleventh of the sale price less the land cost, so ($2,700,000 less $1,000,000) divided by 11 is $154,545 payable, and net realisation is $2,545,455. Against $2,512,500 of cost that is a profit of about $33,000, or 1.3% on cost, and any lender will call that a loss once overruns and a slower sale are priced in.

That result is normal. The site costs $500,000 per dwelling, against roughly $250,000 on a six townhouse site, and GST, selling costs and finance take $402,000 before the owner sees a dollar. estate.sydney's Parramatta feasibility has the same shape, a loss at median land and build prices and 7.7% with cheaper land and a $500,000 build, and Infinity's Melbourne rule is that end value must clear total cost by at least 15% before a dual occupancy is worth starting.

The sensitivities show what has to be true. Buying the same lot at $850,000 lifts the margin to about 8.2%. Holding the build to $2,300 per square metre on a project builder's base specification gets 4.7%. Both together, land at $800,000 and build at $2,300, lands at 14.6%, which is where most duplexes that go ahead sit. Selling at $1,450,000 a side returns 8.4%. The other route is not to sell: Little Fish Properties' July 2026 example is a pair with an all-in cost of $2.37 million, a completion valuation of $2.78 million, an equity uplift of about $414,000 and rent of $850 a week per side, refinanced and held. No GST is paid on a hold, though input credits on the build are not claimable either.

How a duplex build is funded

Intention decides the road. If you will keep both sides or live in one, most major banks write it as a residential construction loan: 10 to 20% deposit on land plus the fixed price contract, land equity counting as deposit, an as-if-complete valuation of the pair, progress draws at slab, frame, lock-up, fixing and completion, and rates near an ordinary construction loan, about 7.0 to 7.5% p.a. in 2026. The second dwelling's rent counts at 70 to 80% in serviceability, and most lenders stop at two dwellings on one title.

If the plan is to sell on completion, or the borrower is a company or trust, it is a development facility. Major banks rarely look below about $10 million of cost, so a duplex sits with non-bank and private lenders: 70 to 80% of cost or 65 to 75% of value at 8.5 to 13% p.a., private senior at 10.75 to 14%, establishment 1 to 3%, 12 to 24 month terms, interest capitalised. Until the titles register the valuer prices the pair as one property at a 10 to 20% discount, and the facility is capped near 65% of that with no presales or 70 to 75% with one presold. A gap is filled by a second mortgage at 14 to 22% p.a. or by cross-collateralising the family home. Private and mezzanine lending is typically business purpose and not consumer regulated; all finance is subject to lender assessment. Equity release off land you own sits under property finance or commercial property finance, the build under construction finance, and a working capital shortfall under property-backed business loans. To get a number from Ventas, send the site address and lot size, DA or CDC status, the builder's fixed price quote, comparable duplex sales, whether you will sell or hold, and your land equity and cash. Compare the numbers on our townhouse development cost guide, and see development finance for how Ventas structures the whole stack.

This is general information only and not financial, credit, or tax advice. Figures are indicative market data from the sources listed, not Ventas offers, and move with the market. Consider your own circumstances and speak to a professional. All finance is subject to lender assessment and approval.

This article is general information only and not financial, credit, or tax advice. Ventas Asset Lending is a finance broker, not a lender. Approvals are subject to lender assessment. Consider your own circumstances and speak to a qualified professional, including your accountant for any tax questions.