A house extension can be one of the best ways to gain more space without leaving the suburb, street or property you already know. It can also be one of the hardest residential projects to price from a simple cost-per-square-metre figure.
Unlike a new home on a clear site, an extension has to connect new work to an existing building - often one that was built decades ago, has been altered over time or does not perfectly match its original plans.
That is why two 40m² extensions can have completely different budgets.
A practical early budgeting range
For a first-pass feasibility conversation, it is more useful to think in bands than to pretend there is one universally accurate rate. The allowances below are construction-only feasibility figures and generally exclude professional, statutory and site-specific costs unless a later builder estimate expressly includes them.
| Extension type | Early feasibility allowance | Typical characteristics |
|---|---|---|
| Straightforward ground-floor addition | $3,000-$4,000/m² | Simple rectangular bedroom/living addition, easy access, limited wet-area work and straightforward connection to the existing house. |
| Mid-range extension with more integration | $3,800-$5,000/m² | Kitchen or bathroom work, larger structural openings, more services, higher finish level and more complex roof/floor tie-ins. |
| Complex / premium ground-floor work | $5,000-$6,500+/m² | Difficult site, extensive demolition, premium finishes, major structural alteration, difficult access or complex architecture. |
| Second-storey addition | $4,500-$7,000+/m² | Structural assessment of the existing house, temporary works, stair, roof removal/rebuild, additional engineering and more difficult construction logistics. |
These are deliberately conservative Shoreline-style early feasibility allowances informed by current Perth and Australian market guides. They are not construction quotes. Builder pricing should be obtained once the design is sufficiently developed.
Why extensions can cost more per square metre than new homes
A new home is designed and built as one coordinated structure. An extension has to work around what is already there.
- demolition and making good existing finishes;
- temporary support while structural walls or roof sections are altered;
- matching floor levels and ceiling heights;
- connecting old and new roof structures;
- extending plumbing, electrical and air-conditioning services;
- protecting occupied areas of the existing house;
- working with limited site access;
- matching or deliberately contrasting older materials; and
- unexpected conditions that are only discovered after work starts.
Small extensions can also have a high cost per square metre because expensive fixed items - such as engineering, mobilisation, demolition, plumbing and electrical work - are spread over a relatively small new floor area.
Wet areas change the budget quickly
A 20m² bedroom addition and a 20m² kitchen/bathroom extension should not be expected to cost the same.
Kitchens, bathrooms and laundries add cabinetry, appliances, plumbing, waterproofing, tiling, fixtures, ventilation and additional electrical work. Moving an existing kitchen or bathroom can also require substantial changes within the original house, not just within the new extension.
Ground floor or second storey?
Building out is generally simpler where the block has enough usable land. Building up can preserve the backyard but usually introduces more structural and construction complexity.
Before designing a second-storey addition, the existing structure needs to be understood. Engineering may be required to determine whether the existing walls, footings and roof arrangement can support the proposed work or whether strengthening is necessary.
The stair also consumes floor area on both levels, and the project may involve removal and reconstruction of substantial parts of the existing roof.
What costs sit outside the builder's headline $/m² rate?
This depends on how a builder presents their estimate, which is another reason quotes need to be compared carefully.
- feature / contour survey or site measure;
- building design and drafting fees;
- structural engineering;
- energy assessment;
- building surveyor / certification;
- development application and building permit fees;
- BAL, geotechnical or other specialist reports where required;
- asbestos or hazardous-material removal;
- upgrades outside the extension footprint;
- landscaping, fencing and external works;
- temporary accommodation if the home cannot be occupied; and
- a contingency for unknown existing conditions.
A useful feasibility budget has three parts: the likely construction cost, professional / statutory / external costs, and a realistic contingency. Looking at construction $/m² alone can understate the amount of money the project actually needs.
Design decisions that have the biggest cost impact
A large part of extension cost is determined before a builder arrives on site. Early design decisions can either simplify the construction or create expensive complexity.
| Usually helps control cost | Usually increases cost |
|---|---|
| Simple, compact geometry | Multiple projections, complex corners and highly articulated forms |
| Working with existing wet-area locations where practical | Relocating kitchens, bathrooms and major service runs |
| Rational roof forms and clear structural spans | Complicated roof junctions and very large unsupported openings |
| Standardised window and door sizes | Oversized custom glazing and complex stacking/corner systems |
| Retaining sound existing walls and finishes where they work | Demolishing large areas of otherwise usable house |
| Resolving the budget during concept design | Designing to the wish list first and discovering the budget problem at tender |
How much should I allow for contingency?
For extension work, a contingency is sensible because the existing building always contains some uncertainty. Around 10% is a common starting point for a well-understood project, while older homes, difficult renovations or early-stage budgets may justify a higher allowance.
The contingency is not a target to spend. It is protection against unknown conditions and changes that cannot be priced accurately before opening up the existing building.
Should I get a builder price before design?
An early builder conversation can be useful, but a meaningful construction price needs a meaningful scope.
Pricing a vague sketch forces the builder to make assumptions. That can produce a misleadingly low estimate, a heavily qualified estimate or a large provisional allowance.
A better process is to establish the project brief and budget together, develop a concept design, test the likely construction range, then produce enough documentation for the builder to price the same defined project.
When might extending not be the best option?
A good feasibility exercise should also be willing to conclude that an extension is not the strongest strategy.
That can happen where the existing house has major structural or layout limitations, the work needed to connect old and new becomes disproportionately expensive, or the proposed renovation cost approaches the value of a more comprehensive rebuild.
The answer is property-specific. The point is to compare the options early, before design and consultant costs make the chosen direction harder to change.
Quick questions
What is a realistic extension rate in Perth?
For early 2026 feasibility, Shoreline generally works with roughly $3,000-$4,000/m² for straightforward ground-floor additions, with more integrated or complex work moving materially higher. It is a planning allowance, not a construction quote.
Why is a small extension expensive per square metre?
Because fixed costs such as demolition, engineering, site establishment, plumbing and electrical work are spread across a small amount of new floor area.
Is a second-storey extension always more expensive?
It is usually more complex and often more expensive per square metre because the existing structure must be assessed, a stair introduced and the roof / structure altered. The actual difference depends on the house and site.
How Shoreline Design can help
Shoreline Design can help establish whether the extension makes sense before you commit to detailed documentation. That can include reviewing the existing house, measuring and documenting it where required, preparing concept options, considering the planning pathway and coordinating early consultant input.
Once the design is agreed, Shoreline can progress the project through planning and building documentation, council submission and consultant coordination in accordance with the agreed scope.
This article is general information for Perth property owners. Planning rules, fees and costs change, and every property is different, so check the requirements for your own property before you act on it.