Extending on the Gold Coast is shaped by three things that do not apply in most Australian cities: a minimum habitable floor level across a large share of the residential land, a wind classification that makes structure more expensive, and ground conditions on the coastal plain that mean piled footings rather than a simple slab. Get those three right at concept stage and the rest of the project is straightforward. Get them wrong and you redesign.
This guide covers what extensions cost here, which approach suits which housing stock, how the approval process runs, and the design moves that earn their cost in this climate.
What Extensions Cost
Ground-floor extension
Budget $4,200 to $6,200 per square metre for a well-built extension on a reasonably straightforward site.
- 30 square metres — a living room or main suite: $130,000 to $190,000
- 45 square metres — open-plan living, dining and kitchen: $190,000 to $280,000
- 65 square metres — living, dining, kitchen, main suite and ensuite: $275,000 to $400,000
- 90 square metres — a substantial addition with two bedrooms and a bathroom: $380,000 to $560,000
These include footings, structure, roof, cladding, windows and doors, insulation, internal linings, basic services, painting and flooring. They exclude kitchen joinery, bathroom fit-out, landscaping, professional fees and approvals.
Second-storey addition
Expect $6,500 to $10,000 per square metre. A 70 square metre upper level therefore runs $455,000 to $700,000. The premium comes from:
For more detail on this part of the project, see What Is the Average Renovation Cost per Square Metre?.
- Footing upgrades, which on the coastal plain usually means piling
- Structural steel and additional bracing for Region B wind loading — a two-storey house on a flat coastal site carries substantially more tie-down and bracing than the equivalent inland
- Removing and rebuilding the entire roof, with weather protection for months in a climate with intense summer rainfall
- A staircase consuming 8 to 12 square metres of existing ground floor
- Moving out for three to six months
The Gold Coast cost drivers
- Footings. Fill over soft compressible soils across the coastal plain means screw piles or bored piers rather than a strip footing. On a 45 square metre extension, the difference between a conventional slab and an engineered piled system is commonly $20,000 to $45,000.
- Acid sulfate soils. Excavation below the trigger level requires a management plan — testing, neutralisation, controlled disposal. $5,000 to $30,000 depending on volume.
- Minimum habitable floor level. If the extension must sit above the existing floor, you are building a suspended floor and dealing with a level change through the house.
- Corrosion specification. In C4 and C5 zones, grade 316 stainless connectors and marine-grade coatings add 15 to 30 per cent to external work.
- Termite management, installed and certified as part of the work.
- BAL requirements in the hinterland, adding $5,000 to $8,000 at BAL 12.5 and five figures at BAL 29 and above.
Approvals
An extension increases gross floor area, so it will require a building approval through a private certifier in every case, and a development approval from the City of Gold Coast unless it meets every accepted-development criterion in the City Plan.
What pushes an extension into code assessment
- Setbacks — side, rear and, where relevant, from a waterway
- Site cover, where the existing house already occupies a large share of the lot
- Building height above the zone limit, typically 9 to 15 metres depending on the zone
- Private open space provision and its dimensions
- Car parking
- Any overlay requirement not met by the design
The overlays that shape the design
- Flood and storm tide inundation — the minimum habitable floor level, which governs the extension’s floor. Confirm it before briefing a designer.
- Coastal erosion hazard — setbacks from the seaward vegetation line on beachfront sites, and a requirement that excavation demonstrate no adverse erosion effect on neighbours. A coastal engineering assessment is commonly needed.
- Acid sulfate soils — triggered by excavation depth.
- Bushfire — hinterland sites, driving BAL construction.
- Landslide hazard and steep land — geotechnical assessment and engineered retaining.
- Vegetation and environmental significance — protected trees on and adjoining the site.
- Airport environs — height limits and noise-attenuated construction near Gold Coast Airport.
QBCC requirements
Any extension will exceed the threshold, so the builder must hold a current QBCC licence in the correct class and pay the Home Warranty Scheme premium before work starts. A written contract is mandatory, with regulated deposit limits. Verify the licence and the insurance policy on the QBCC register yourself rather than relying on a number printed on a quote.
A realistic approval program
- Design and documentation: 8 to 16 weeks
- Investigations — soil test, survey, engineering, energy assessment, and any coastal, flood, geotechnical or bushfire report: 3 to 6 weeks in parallel
- Development application, code assessable: 6 to 14 weeks
- Building approval through a certifier: 2 to 6 weeks
- Tender and contract: 5 to 10 weeks
- Construction, 45 square metre extension: 5 to 9 months
What Works on Gold Coast Housing Stock
The 1970s and 1980s canal or beach house
Low-set or split-level brick, living areas facing the street, small openings to the water or the view, dark internal spaces.
- What works: extend toward the water or the north with a large open living space, a generous covered outdoor area at the same floor level, and glazing sized to frame the view rather than fill the wall.
- The detail that matters: the floor level. If the flood or storm tide level sits above the existing slab, the new area is a suspended floor and the transition needs to be designed rather than improvised.
- Budget note: piled footings, corrosion specification and termite management are all in this project whether you planned them or not.
The 1980s and 1990s brick-and-tile family house
Compartmentalised plan, concrete tile roof, small windows, dark kitchen, minimal outdoor connection.
- What works: a rear extension combined with removing one or two internal walls, plus a substantial covered alfresco. The result reads as one house rather than an old house with a new part attached.
- The detail that matters: the roof. A tiled roof at forty years is porous and the battens are often undersized. Doing the roof at the same time as the extension is far cheaper than doing it separately.
- Budget note: asbestos is common in eaves and wet area linings in the earlier part of this range.
The high-set or split-level house
- What works: building in under the existing house where the floor level permits. Substantially cheaper per square metre than a new addition because the roof and structure already exist.
- The constraint: the minimum habitable floor level. On many Gold Coast sites, the under-house area sits below it and cannot become habitable space. Check this first — it is the single most common disappointment in this project type.
- Also check: existing ceiling height, which needs to meet the Code for habitable rooms, and whether the existing footings and posts can carry enclosed use.
The hinterland acreage property
- What works: extending toward the view and the north, with deep verandahs and cross-ventilation.
- The constraint: BAL requirements, steep land, geotechnical assessment, access for trades and materials, and vegetation protection.
- Budget note: add 15 to 30 per cent over the coastal equivalent and expect a longer program.
Design Moves That Earn Their Cost Here
- Put living areas to the north or north-east. Good light, manageable heat, and it faces the prevailing sea breeze.
- Build a genuinely generous covered outdoor area. On the Gold Coast this is used year-round and buyers price it as living space. Roughly 20 to 30 square metres minimum for a family, at the same floor level as the interior where the flood level allows.
- Design for cross-ventilation. Openings on both sides of the plan and high-level openings to release hot air. The north-easterly is a free cooling resource for most of the year and it is routinely designed out by sealed additions.
- Shade the west with something adjustable. Fixed eaves do not stop low afternoon sun, and on a west-facing water frontage the view and the heat arrive together.
- Keep the eaves generous — 600 to 900 millimetres — which shades walls, protects openings from driving rain and reduces maintenance.
- Oversize the gutters, downpipes and stormwater for local rainfall intensity, with overflow provisions that discharge away from the building.
- Duct every exhaust to outside. In this humidity, venting into the roof space is how you grow mould in the ceiling.
- Specify grade 316 stainless connectors anywhere structural and exposed in the coastal strip. This is not a finish decision; it is a structural one.
- Insulate and ventilate the roof space, and choose a light roof colour.
- Match or deliberately step the ceiling height between old and new. A 2.4 metre ceiling attached to a house with 2.7 metre ceilings reads as cheap.
Connecting Old and New
The most common failure in Gold Coast extensions is that the finished house reads as an old house with a new part attached. A handful of decisions prevent it.
- Carry one floor material through the junction where levels permit
- Match floor levels exactly or step them decisively. A 40 millimetre difference reads as an error; a 300 millimetre step reads as a design.
- Resolve the ceiling height deliberately rather than defaulting to the minimum
- Carry skirting, architrave and door heights through from the original, or change them completely — half-matching is worse than either
- Upgrade the retained rooms to the same standard. A well-finished new addition attached to rooms with original wiring, no insulation and tired paint makes both halves look worse.
- Repeat one material or colour from the original in the addition, which ties them together without imitation
Living Through It
- Staying put works where the extension is genuinely separable and can be built as a shell before the connecting wall is opened. It adds 15 to 25 per cent to the program because the builder is working around you.
- Plan to be elsewhere for the connection phase, when the existing wall is opened and services are cut over.
- Moving out costs $30,000 to $70,000 for a six to eight month project in rent, storage and moving twice, and saves program time.
- Take a flexible lease end date, never a fixed one against a construction program.
- Plan around the wet season. Excavation, footings and roofing all lose days through the summer storm months, and a program that assumes otherwise is wrong.
An extension on the Gold Coast is a well-understood project once the floor level, the footings and the corrosion specification are settled. Those three are settled cheaply at concept stage with a soil test, a look at the council mapping tool and an hour with a certifier — and expensively at any point after that.
For more detail on this part of the project, see Do You Need Council Approval to Renovate on the Gold Coast? The 2026 Rules Explained.
If the extension arithmetic looks marginal, run the alternative in knockdown rebuild versus renovate on the Gold Coast, and check your position against the 2026 approval rules.
Building In Under an Existing House
A large share of the Gold Coast’s older stock is high-set or split-level, and the space underneath is the cheapest floor area available anywhere in the city — when it works. It is also the project most often abandoned after concept design, for one avoidable reason.
Check these before anything else
- The minimum habitable floor level. On a great many Gold Coast sites the under-house area sits below it, and it cannot become habitable space at all. This is the single most common disappointment in this project type and it is checkable in ten minutes.
- Ceiling height. Habitable rooms need to meet the Code minimum, and many under-house areas are 100 to 300 millimetres short. Excavating to gain height means underpinning, retaining and drainage, which removes the cost advantage entirely.
- Whether the existing footings and posts can carry enclosed use and the additional loads, and whether they were engineered for current wind requirements.
- Drainage and moisture. An open under-house area drains and ventilates itself. Enclosing it creates a room that needs a damp-proof floor, drainage, tanking where it is cut into ground, and mechanical ventilation.
- Termite management, because you are now creating concealed cavities in the highest termite pressure zone in the country.
- Whether the existing stair can serve two levels or a new internal stair is needed, which consumes floor area upstairs.
What it costs when it works
- Basic enclosure — walls, floor, doors, minimal services: $1,800 to $2,800 per square metre
- Full habitable fit-out with bedrooms and a bathroom: $2,800 to $4,200 per square metre
- Where excavation is required for height: add $30,000 to $120,000 for underpinning, retaining and drainage, at which point compare it honestly against a ground-floor extension
At $2,800 to $4,200 per square metre against $4,200 to $6,200 for a new extension, building in under is a genuine saving. The saving evaporates the moment excavation or underpinning enters the scope.
Opening the House to the Water or the North
The most valuable structural move available on Gold Coast housing stock, and the one most extensions get half right.
What the move actually involves
- Removing a structural wall, which on a masonry house needs an engineer, a steel beam and adequate footings — commonly piled on the coastal plain. $12,000 to $28,000 per opening, more where an external wall and wind loading are involved.
- Large glazed openings in a system engineered for the wind pressure on the frontage, in marine-grade finish with 316 stainless hardware. Lead times of eight to sixteen weeks.
- A covered outdoor area beyond the glazing, at the same floor level where the flood level permits, deep enough to be genuinely usable and to shade the glass behind it.
- A resolved threshold — flush where possible, draining properly, keeping out driving rain, meeting the waterproofing standard and satisfying the floor level. This detail is the whole project.
Getting it wrong
- Glazing everything. A full wall of glass on a west-facing frontage is unusable from 3pm to 7pm all summer. Frame the view; do not fill the wall with it.
- No shading beyond the glass. The covered area is not an optional extra — it is what makes the glazing work.
- A step down to the outdoor area that breaks the connection you spent the money to create.
- Standard residential window systems on an exposed frontage, which fail under wind pressure and corrode within a few years.
- Removing the wall without opening the ceiling. A 2.4 metre ceiling over a large open space feels lower than it did in two smaller rooms. Raking or lifting the ceiling over the new opening transforms it.