Extensions are one of the most misquoted jobs in Australian residential construction. Homeowners get a $200,000 figure from a mate at the pub, then a $480,000 quote from a registered builder, and have no idea why the gap is so wide. The honest answer: extensions are complex renovation work bolted onto an existing structure, and the structure usually has surprises waiting. This 2026 guide gives you real per-square-metre numbers, worked examples for common extension sizes, the hidden costs nobody warns you about, and how to compare quotes properly so you don’t get burned.

Extension vs New Build: When Each Makes Sense

Before you commit to extending, do the maths on a knockdown rebuild. In 2026, the extension premium per square metre is often higher than building new because you’re working around what’s already there. New build per m2 sits around $2,400-$3,800/m2 in most capital cities, while extensions are $2,800-$5,500/m2 depending on type.

Extensions make sense when:

• Your existing home is structurally sound and reasonably modern (post-2000 build)
• You love your suburb and the land value justifies investment
• You only need 30-80m2 more space, not a full reimagining
• Council restrictions (heritage overlay, character zones) prevent demolition

Knockdown rebuilds win when the existing home has asbestos, failing stumps, outdated wiring throughout, or when you need more than 100m2 of extra space. The knockdown cost itself is typically $25,000-$45,000 including disposal, and you start with a clean site that’s far cheaper to build on.

Run both scenarios through our property cost calculator before committing — most homeowners are shocked at how close the numbers get once hidden extension costs are factored in.

Cost Per Square Metre by Extension Type (2026)

Here’s where the real money goes. These rates assume standard finishes, metropolitan location, and reasonable site access. Premium finishes add 25-40% on top.

Extension Type	Cost per m2 (2026)	Typical Project Range	Build Time
Ground floor extension	$2,800 – $4,500	$85k – $360k	4-7 months
Second storey addition	$3,200 – $5,500	$160k – $440k	6-10 months
Granny flat (turnkey)	$2,200 – $4,200	$130k – $250k	3-5 months
Garage conversion	$1,800 – $3,200	$45k – $95k	2-4 months
Wraparound extension	$3,000 – $4,800	$180k – $480k	5-9 months
Knockdown rebuild	$2,400 – $3,800	$480k – $950k	9-14 months

Why second storeys cost more: engineering. You’re loading the existing footings, often needing pier reinforcement, structural steel transfer beams, temporary roof removal, and crane hire. Add scaffolding for 4-6 months and the per-m2 figure climbs fast.

Why granny flats look cheap per m2: they’re standalone, single-storey, simple roof lines, and most are built off pre-engineered designs. The catch — site costs and connection fees can add $30,000-$60,000 if your block isn’t easy to service.

Real Worked Examples (Sydney/Melbourne 2026)

Example 1: 30m2 Kitchen and Dining Extension

Standard rear addition opening into existing living. Single storey, brick veneer to match, colorbond roof, mid-range kitchen.

• Build: 30m2 x $3,400/m2 = $102,000
• Kitchen joinery and appliances: $35,000
• Demolition of existing rear wall: $4,500
• Council DA + plans + engineering: $9,000
• Site costs (services, drainage): $12,000
• Total: $162,500

Example 2: 50m2 Master Suite Ground Floor Addition

Bedroom, walk-in robe, ensuite with double vanity, study nook. Mid-to-high finish.

• Build: 50m2 x $3,800/m2 = $190,000
• Ensuite fixtures and tiling premium: $18,000
• Joinery (WIR, study): $14,000
• Permits and design: $11,000
• Site and services: $15,000
• Total: $248,000

Example 3: 80m2 Second Storey Addition

Three bedrooms, family bathroom, void over stairs. Existing single storey weatherboard.

• Build: 80m2 x $4,400/m2 = $352,000
• Footing reinforcement and structural steel: $38,000
• Stairs and void: $22,000
• Roof removal/crane/scaffolding: $26,000
• Permits, engineering, energy reports: $14,000
• Owner accommodation (12 weeks): $18,000
• Total: $470,000

If those numbers shock you, you’re not alone — most homeowners budget 30-40% short. Use our construction cost calculator guide to build realistic numbers from the start.

Hidden Costs Nobody Warns You About

The headline per-m2 rate is the easy bit. These are the items that blow extension budgets:

• Foundation surprises: $8,000-$25,000 for underpinning, replacing failed stumps, or upgrading footings to match new loads. Reactive clay sites in Adelaide and parts of Melbourne are notorious.
• Services rerouting: $5,000-$18,000 to relocate sewer, stormwater, gas mains, and electrical. NBN and Telstra pit relocations alone can cost $4,000-$8,000.
• Asbestos removal: $3,500-$15,000 depending on quantity. Almost guaranteed in any home built pre-1990 — eaves, cladding, vinyl tiles, fence sheets all qualify.
• Council fees and contributions: $4,000-$22,000 including DA, CDC, Section 7.11 contributions (NSW), open space levies, and BASIX/NatHERS reports.
• Structural matching: matching old brick, render textures, or roof tiles often requires custom blends or import. Budget $3,000-$12,000 extra.
• Temporary accommodation: if you can’t live through the build, $1,500-$3,500/week rent for 3-6 months.
• Driveway and landscape repair: $5,000-$15,000 to fix what trucks, scaffolding, and crews chew up.

State-by-State Cost Variation in 2026

Geography matters more than people think. Same 50m2 extension can swing $80,000 between states.

• NSW (Sydney metro): highest at $3,400-$5,500/m2. DA timelines and S7.11 contributions add $8,000-$25,000. Regional NSW drops to $2,800-$4,000/m2.
• Victoria (Melbourne): $3,200-$4,800/m2. Reactive clay drives footing costs up. Heritage overlays in inner suburbs add design fees.
• Queensland (Brisbane/Gold Coast): $2,800-$4,200/m2. Cyclone tie-down requirements north of Bundaberg add 8-12%. Termite barrier mandatory.
• WA (Perth): $2,900-$4,300/m2. Sandy soils make footings cheaper, but trade availability post-mining boom keeps labour rates high.
• SA (Adelaide): $2,700-$4,000/m2 — generally the cheapest capital. Stone matching on old homes adds cost.
• Tasmania: $3,000-$4,500/m2. Material freight surcharges of 8-15%. Energy compliance (7-star NatHERS) tougher in cold climate zones.
• ACT: $3,300-$4,900/m2. Stricter energy requirements and tighter compliance regime.
• NT: $3,500-$5,200/m2. Cyclone rating, freight, and trade scarcity all push prices up.

Structural Decisions That Drive Cost

Two extensions of identical floor area can cost wildly different amounts based on three structural choices:

Roof line integration: matching pitch, eaves, and gutter lines into the existing roof is engineering work. A skillion roof butted against the existing fascia is cheap. A continuous gable that ties into the original roof structure can cost $15,000-$30,000 more.

Spans and openings: every metre of unsupported opening you want in a load-bearing wall costs steel. A 4m structural opening into a kitchen extension might need a $4,500 steel beam. A 7m wall-to-wall sliding door opening can hit $14,000 just for the steel.

Concrete slab vs suspended floor: ground floor extensions usually use slab-on-ground, which is cheaper. But if you’re matching a raised existing floor level on a sloping block, suspended floors with steel posts add $8,000-$20,000. Use our concrete volume calculator to get accurate slab estimates from your plans.

Owner Liaison and Disruption Costs

If you stay in the home during the build (most do), there are real costs:

• Increased electricity bill from temporary heating/cooling: $200-$500/month
• Take-out and meals while kitchen is offline: $400-$900/week for a family of four
• Storage hire for furniture: $250-$450/month
• Lost productivity if you work from home: hard to quantify but real

Owner-builders need to factor in their own time. Project managing an extension well takes 8-15 hours per week for the duration of the build. If your trade is quoting you out at $90/hour and you spend 10 hours a week for 24 weeks, that’s $21,600 of opportunity cost.

How to Get Accurate Quotes From Three Builders

The biggest mistake homeowners make is asking for quotes on a sketch and a vibe. To get genuinely comparable numbers:

• Get a full design and engineering set first. Spend $8,000-$15,000 on architect or building designer plans plus engineering. Builders quoting from full drawings are within 5-8% of each other. Quotes from concept sketches vary by 40%.
• Provide a fixed inclusions schedule. List every fixture, fitting, finish, and brand. “Bathroom” is not a quote line. “Caroma Liano vanity, Methven Aurajet shower, 600×600 matte porcelain floor tile” is.
• Insist on a trade breakdown. Lump-sum quotes hide everything. Demand sub-totals for excavation, frame, lockup, fitout, external works.
• Check the PC sums. Prime cost allowances for tiles, taps, kitchens are where builders lowball quotes. A $4,000 PC for a kitchen is fantasy. Realistic is $25,000-$60,000.
• Verify the builder’s licence and insurance. Domestic Building Insurance (Vic), Home Building Compensation (NSW), QBCC (Qld) — they’re mandatory above thresholds.
• Check three recent extension references. Not new builds — extensions specifically. The skill set is different.

For builders pricing the work yourselves, our guide to quoting residential builds walks through margin, contingency, and PC sum strategy properly.

Built Simple Property Cost Calculator

Plugging real per-m2 rates into a spreadsheet is fine, but most homeowners forget half the line items. Built Simple’s property cost calculator takes your project type, floor area, location, and finish level and outputs a defensible budget range — including site costs, permit fees, and the hidden items above. It’s the same logic builders use internally to scope projects, repackaged for owners and owner-builders.

For full project workflow — quoting, scheduling, variations, payments, client communication — explore the broader Built Simple platform built specifically for Australian builders.

Frequently Asked Questions

How much does a 40m2 extension cost in Australia in 2026?

For a standard ground-floor extension at 40m2, expect $135,000-$200,000 all-in including permits, site costs, and mid-range finishes. Sydney and Melbourne sit at the top of that range.

Is it cheaper to extend or build a second storey?

Per square metre, ground floor extensions are cheaper ($2,800-$4,500/m2) than second storeys ($3,200-$5,500/m2). But second storeys don’t consume backyard space, which has its own value. If your block is small, second storey often wins on total project value.

Do I need council approval for a house extension?

Almost always yes, unless it qualifies as exempt development (very small additions in some states). Most extensions need either a Complying Development Certificate (faster) or a Development Application (slower, more flexibility). Budget $4,000-$22,000 in fees and 8-20 weeks for approval.

How long does a house extension take to build?

Ground floor extensions typically run 4-7 months on site, second storeys 6-10 months. Add 3-6 months for design and council approval before site work begins. So a realistic total is 10-16 months from first sketch to handover.

Can I live in my house during an extension?

Yes for most ground floor extensions if your kitchen and main bathroom stay functional. No for most second storey additions during the roof-off phase (typically 4-8 weeks). Budget temporary accommodation if needed.

What’s the cheapest type of extension?

Garage conversions ($1,800-$3,200/m2) are cheapest because the shell exists. Granny flats are next ($2,200-$4,200/m2 turnkey) due to standardised designs. Custom architectural extensions are most expensive.

How much should I budget for contingency?

Minimum 10% on new builds, 15-20% on extensions due to unknowns once walls open up. On a $300,000 extension, hold $45,000-$60,000 contingency.