Cost guide
What drives the cost of an extension.
An extension is a small new building attached to an old one, and the attaching is where most of the surprises live. This guide explains what moves the price of a ground-floor or second-storey extension, what you control, and the mistakes that turn a good idea into a redesign. No figures: your own estimate is free.
Two jobs in one
Every extension has two parts. The first is new building work: a slab or suspended floor, walls, roof, windows and doors, services and finishes, priced the way a new home is, by quantities first and then a price on each. The second is joining it to the house you already have, and that part has no equivalent in a new build.
The join starts with demolition and making good. An external wall becomes an internal opening, often with a beam to carry what the wall held up. Roofs meet, which means flashing, gutters and sometimes a box gutter where two roofs drain into each other. Floors meet at a level that must match. Services that ran through the old wall are rerouted. Each of these is a line in the estimate, and none of them adds floor area, which is why extensions cost more per square metre than the house they extend.
Up or out is the first big decision. Building out onto the garden means a new slab or floor, new footings and losing outdoor space, but the existing house is barely touched. Building up means a second storey on a house that was not designed for one: the existing walls and footings must be checked and often strengthened, the roof comes off, a stair takes space from the floor below, and the family usually moves out while it happens. The engineering and the disruption are real costs.
The site matters as much as it does for a new home. Access to the back of a block through a side passage limits what machinery can reach the work and how materials arrive. A sloping garden means retaining and a stepped floor. Trees, boundaries and the neighbours' windows shape what can be built where.
Matching is a cost people underestimate. Bricks that match the existing house, a roof covering that lines up, windows in the same style and floor boards that continue through from the old rooms all cost more than the plainest option, and sometimes the match is simply not available and a deliberate contrast is the better design.
Finally the rooms you add decide the services. A bedroom and a living room are mostly structure and finish. A kitchen, bathroom or laundry brings plumbing, drainage, electrical, tiling and cabinetry into a small space, and may need the house's existing drainage and switchboard upgraded to serve it.
The cost factors, and what you control
| Factor | Why it moves the price | What you control |
|---|---|---|
| Out or up | Building up means checking and strengthening the existing structure, removing the roof and adding a stair | The direction, decided with the site and the engineer |
| The join | Demolition, beams, flashing where roofs meet, floors matched at the same level, services rerouted | A design that meets the old house simply |
| Access | A side passage limits machinery and deliveries; everything takes longer by hand | Where the work sits on the block |
| Existing structure | Old footings, walls and drainage may need upgrading to carry or serve the new work | An inspection before you design |
| Matching materials | Bricks, roofing and windows that match cost more than the plainest option | Match, or a deliberate contrast |
| Rooms added | Wet rooms concentrate plumbing, electrical, tiling and cabinetry; dry rooms are mostly structure | What the extension is for |
| Openings | Large doors onto the garden are among the dearest elements | Number, size and performance |
| Finish level | Runs across every visible surface and fitting in the new rooms, and the old ones you refresh to match | Economical, Standard, Good or Advanced, then product by product |
| Living through it | Moving out, staging the work or protecting the rooms you keep using all cost time and money | How the work is sequenced |
Finish levels explained
On Modern GBQ a finish level, Economical, Standard, Good or Advanced, sets the default product on every line that needs one: flooring, wall and ceiling finishes, cabinetry, tapware, doors, hardware and lighting. Economical is the plainest listed product that does the job; Standard the mid-range most homes are built with; Good a step up in material and brand; Advanced premium products and the labour some of them need.
Extensions bring a question new homes do not: the new rooms sit next to old ones. A finish level well above the existing house looks odd at the doorway and tempts you into refreshing the old rooms too, which is a second project. Many people set the level to match what they have and spend the difference on the one room the extension is really for.
Every default can be swapped for a listed alternative, with a photo and a short reason recorded, and re-priced as a new version. The finish level never changes the structure or the join: those are decided by the design and the house.
Common mistakes
- Pricing it like a new home per square metre. The join, the demolition and the making good add cost without adding area.
- Designing before the engineer looks. Whether the existing footings and walls can take a second storey decides the whole budget. Find out first.
- Forgetting what the extension displaces. A stair takes a room's worth of space from the floor below; a rear extension takes the garden and may move the laundry, the drainage or the hot water system.
- Assuming the services can cope. Older drainage, a small switchboard or an undersized hot water system may need upgrading to serve the new rooms.
- Chasing a perfect match. Exact bricks and roofing may be unavailable or dear; decide early whether to match or contrast.
- Leaving the old rooms out of the plan. If the old living room becomes a hallway, its flooring and paint are part of the job.
- Comparing builders on different scopes. Approve one scope and have every builder price that, so the cheapest quote is not the one that left the join out.
Your own figure
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Suburb, floor area of the new rooms, project type and finish level. We email you a link that opens your free account with the project already filled in.
Extension questions.
Is building up cheaper than building out?
Not usually, and not always. Up saves the garden but adds engineering, strengthening, roof removal and a stair. Out adds footings and a new floor but leaves the house alone. The estimate prices the quantities of each so you can compare them rather than guess.
Does the estimate include the demolition and the join?
Describe the extension as its own building or storey on the project and the estimate prices its structure, services and finishes. Work on the existing house that you add as quantities is priced too; anything you leave out is listed under what is not in the price, so nothing is silently assumed.
Can I see what the finish level does to the total?
Yes. Re-price after changing it and put the two versions side by side. Every version is kept.
Will builders price the same thing?
Yes, once you approve a Design or Detailed estimate. Every invited builder prices that scope, including the lines you excluded, and you read their responses blind.
Price the extension before you meet a builder.
Free to start. Every line explained.