Thinking about adding a bedroom, second storey or open-plan living area to your home rather than going through the stress of moving? You're not alone. House extensions remain one of the most popular ways for Australian homeowners to get more space without leaving the suburb, school zone or garden they love.
But before you get too far into Pinterest boards and floor plans, you need a realistic idea of cost. Extension pricing in Australia varies enormously depending on where you live, the type of extension, site access, finishes and how much of the existing house is affected. This guide walks through the numbers, what drives them and how to keep your project on budget in 2026.
House Extension Cost Per Square Metre in 2026
Builders and quantity surveyors typically talk about extension costs in dollars per square metre, as it gives a rough starting point before a detailed quote. Treat the figures below as broad, indicative ranges only, actual quotes can sit outside them depending on your site and finish level.
Extension Type
Budget/Basic Finish
Mid-Range Finish
High-End Finish
Ground-floor extension
~$2,000–$2,800/m²
~$2,800–$3,800/m²
~$3,800–$5,500+/m²
Second-storey addition
~$2,800–$3,500/m²
~$3,500–$4,500/m²
~$4,500–$6,500+/m²
Kitchen/bathroom-inclusive extension
~$3,000–$4,000/m²
~$4,000–$5,000/m²
~$5,000–$7,000+/m²
Second-storey (upstairs) additions generally cost more per square metre than ground-floor extensions because they involve structural upgrades to the existing house, temporary roof removal, scaffolding and often relocating stairs. Ground-floor extensions built on a new slab tend to sit at the lower end of the range, especially for simple rectangular footprints.
Single-Storey vs Double-Storey Extensions
Whether you build out or build up has a big impact on total cost, and on your block's suitability.
Single-storey extensions are usually the more affordable option per square metre, particularly if you have space to build outward. They avoid the cost of strengthening existing footings and walls to carry an upper level, and construction is generally faster with fewer trades needed on site at once.
Double-storey (second-storey) additions can be a smart choice on smaller blocks where there's limited room to extend outward, or where you want to preserve garden and outdoor living space. Expect a higher overall spend due to structural engineering, temporary roofing, and the logistics of building above an occupied home.
As a very rough guide, a modest single-storey addition might land somewhere in the order of $150,000–$300,000, while a full second-storey addition (adding several bedrooms and a bathroom) could realistically run from around $250,000–$500,000 or more, again, always confirm with quotes based on your specific home and location.
What Drives the Cost of a House Extension?
Two extensions of the same size can vary in price by tens of thousands of dollars. Key cost drivers typically include:
Size and footprint – larger extensions generally cost more in total but can achieve a lower cost per square metre, as fixed costs (site setup, connections) are spread over more space.
Finish level – stone benchtops, engineered timber flooring, feature cladding and high-end tapware can add significantly more than budget-grade equivalents.
Structural complexity – irregular shapes, cantilevers, large spans of glazing or removing load-bearing walls all add engineering and labour cost.
Existing home condition – older homes may need rewiring, replumbing or asbestos removal before or during the build, which can add unexpected cost.
Site access and slope – covered in more detail below, this can materially change excavation and delivery costs.
Location – labour and material costs vary between capital cities, regional areas and remote locations, and can shift with market conditions.
Cost by Extension Type
Different rooms come with different cost drivers, largely because of plumbing, structural spans and finish expectations.
Extra bedroom/study – generally the most affordable type of extension, as it usually needs no plumbing and simple structural requirements. Roughly $40,000–$90,000 for a single room, depending on size and finish.
Living/family room extension – often involves larger open spans, bigger windows or bi-fold doors and higher ceilings, which pushes cost up. Roughly $60,000–$150,000+ is a reasonable planning range for a mid-size addition.
Kitchen extension – plumbing, electrical, joinery, appliances and stone benchtops make kitchens one of the pricier square-metre spends. Many homeowners budget somewhere from $70,000 upward, with high-end kitchen extensions reaching well beyond $150,000.
Second-storey addition (multiple rooms) – as noted above, a full upper level with bedrooms and a bathroom is typically the largest single investment, often $250,000–$500,000+ depending on scope.
Bathroom-inclusive extensions – adding a bathroom or ensuite within an extension adds waterproofing, plumbing and fixtures, typically adding tens of thousands compared with a dry room of similar size.
These figures are general planning ranges, not quotes. Always get at least two or three itemised quotes from licensed builders before locking in a budget.
Site and Access Factors That Affect Price
Your block itself can add or remove tens of thousands of dollars from a build, even before a wall goes up.
Sloping sites often need retaining walls, extra excavation or engineered footings (such as piers or a suspended slab), which can add meaningfully to cost compared with a flat block.
Restricted access for trucks and machinery, common on narrow inner-city blocks or laneway homes, can mean manual handling of materials, smaller equipment hire or even crane lifts, all of which add cost.
Soil type revealed in a geotechnical/soil test can change footing design; reactive clay soils, for example, often require deeper or more robust footings.
Existing services – the distance to stormwater, sewer and electrical connections, and whether they need relocating, can be a hidden cost driver.
Trees and vegetation – protected or significant trees near the build zone may require arborist reports and design workarounds.
A site inspection and soil test early in the process helps avoid nasty surprises once contracts are signed.
Council Approval and Design Costs
Before building, most extensions need either a Development Application (DA) approved by your local council, or a Complying Development Certificate (CDC)/private certification pathway where the project meets set criteria, requirements vary by state and by council, so check locally.
Design and drafting – working drawings from a draftsperson or architect typically cost somewhere in the order of a few thousand up to tens of thousands of dollars, depending on complexity and whether you engage a full architectural service or a simpler drafting service.
Council/certifier fees – DA lodgement fees, construction certificate fees and inspection fees vary by council and project value, but are worth budgeting a few thousand dollars for in most cases.
Engineering reports – structural engineering, and sometimes soil testing, bushfire attack level (BAL) or energy efficiency (NatHERS) reports, add further professional fees.
Timeframes – approval pathways can take anywhere from a few weeks (fast-tracked complying development) to several months for a standard DA, so factor this into your overall project timeline.
Ways to Keep Extension Costs Down
There are several practical ways Australian homeowners commonly manage extension budgets without compromising on the outcome:
Keep the footprint simple – a rectangular or near-rectangular shape is cheaper to build than a highly articulated design with lots of corners and roof junctions.
Reuse existing plumbing and wet-area locations where possible, extending a kitchen or bathroom near its existing location can avoid costly pipe runs.
Match roof lines and levels to the existing house rather than introducing new roof pitches or split levels.
Stage the project – some homeowners complete the structural shell first and finish interiors over time as budget allows.
Choose mid-range fixtures and finishes in less visible areas, and reserve the higher-end spend for feature elements that have the most visual impact.
Get multiple detailed quotes so you're comparing like-for-like inclusions, not just a bottom-line number.
Lock in a fixed-price contract where possible, with a clearly defined provisional sum and variations process, so cost blowouts are easier to spot and manage.
Hidden Costs to Budget For
Beyond the headline build cost, it's worth setting aside a contingency for items that often get missed in early budgeting:
Contingency allowance – many builders and industry bodies suggest budgeting an additional buffer (often cited around 10–20%) for unforeseen items, particularly on older homes.
Temporary accommodation – if your kitchen or bathroom is out of action, or the works are extensive, you may need to budget for short-term rental or alternative living arrangements.
Removal and storage of furniture and belongings from the work zone.
Landscaping and reinstatement – restoring gardens, fencing, driveways or paths disturbed during construction.
Utility connections and upgrades – such as upgrading the electrical switchboard or hot water system to service the new space.
Furnishing and fit-out of the new space – new furniture, window coverings and styling for the finished extension.
GST – confirm whether quotes are GST-inclusive, as this affects your final bottom line.
Do You Need a Builder, Architect or Both?
Most extensions involve at least a licensed builder, and often a design professional as well, though the right combination depends on your project's size and complexity.
Draftsperson – a cost-effective option for straightforward extensions that don't need a highly customised design, producing working drawings for council approval and construction.
Architect – worth considering for more complex sites, heritage constraints, tricky slopes, or where you want a distinctive design outcome. Architects can also help manage the build and tender process, which adds professional fees but can add value through better design and cost control.
Builder – a licensed, insured builder is essential for the construction itself. Many builders also offer design-and-construct packages that bundle drafting and building under one contract, which can simplify the process for straightforward extensions.
Structural engineer – required for most extensions to certify footings, beams and any structural changes to the existing house, regardless of who else is involved.
Whichever path you choose, check licensing, insurance (including home warranty insurance where applicable) and ask for references or examples of similar completed projects before signing a contract.
Next steps and further reading
Get a ballpark for your own project with our renovation cost calculator, then refine it with builder quotes.
Is it cheaper to extend or knock down and rebuild?
It depends on the condition of your existing home and how much of it you're keeping. Extending can be more cost-effective if your existing structure, roof and services are in good condition, since you're not paying to demolish and rebuild everything. A full knockdown-rebuild can make more sense if the home needs extensive work throughout. A builder or architect can help assess which option suits your specific property.
How long does a typical house extension take to build?
Timeframes vary widely with size and complexity, but as a general guide, a single-room ground-floor extension might take a few months on site, while a second-storey addition with multiple rooms can take six months to a year or more. This is on top of design and approval time, which can add several more months before construction even starts.
Do I need council approval for a small extension?
Often yes, though some small or low-impact extensions may qualify for a faster complying development pathway rather than a full Development Application, depending on your state, council and how the extension complies with local planning rules. It's worth checking with your local council or a certifier early in the planning process, since this affects both cost and timeline.
How much of my budget should I set aside as contingency?
Many industry professionals suggest allowing an additional buffer on top of your build quote, often cited in the order of 10–20%, particularly for older homes or sites where the condition of existing structures isn't fully known until work begins. This helps absorb surprises like unexpected wiring, plumbing or structural issues uncovered during construction.
Can I live in my house while it's being extended?
In many cases yes, especially for extensions that don't affect existing kitchens, bathrooms or bedrooms. However, extensions that involve significant structural work, second-storey additions, or renovations to essential living areas may require you to move out temporarily. Discuss this with your builder during planning so you can budget for temporary accommodation if needed.
Will a house extension add value to my home?
Well-planned extensions, particularly those adding bedrooms, bathrooms or living space in line with what buyers in your area expect, can add value, though the return varies by location, market conditions and the quality of the build. It's worth researching comparable renovated homes in your suburb, or speaking with a local real estate agent, before committing to a large-scale extension purely as an investment decision.
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