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How to Price an Extension in 2026 (A Builder's Step-by-Step Method)
The Biggest Job on Your Board Is the Easiest to Get Wrong
An extension is a different animal to a bathroom or a kitchen. You are not renovating an existing space, you are building a new one and then marrying it to a house that was built decades ago, to a different standard, probably out of square, and full of surprises you cannot see until you break in.
That combination is why extensions catch out good builders. There is groundwork and structure, which carry real risk. There is the join between old and new, which is where the fiddly, unpredictable costs live. There is a longer program, which means more exposure to weather, price rises and delays. And there is a client living in the house the whole time, watching it happen.
Get the pricing method right and an extension is one of the most profitable jobs you can do. Get it wrong and it is the one that can take a year of profit with it. The difference is building the number properly rather than guessing at a big figure and hoping.
TL;DR
Extensions carry more risk than any renovation because of groundwork, structure and the join to the existing house
Price by stage from the ground up, and never work backwards from a number the client hoped for
Approvals (a DA or CDC in Australia, permits in the US) need to be costed and their responsibility made clear
The connection between old and new is where hidden costs hide, so price and exclude carefully
Longer programs mean more exposure to weather, price rises and delays, so build that in
Why a Per Square Metre Rate Is Only a Starting Point
People love a per square metre figure for extensions, and as a rough sanity check it has its place. As a guide, extensions in Australia commonly run somewhere around $4,000 to $6,500 per square metre depending on structure and finish level. But a headline rate like that hides everything that actually determines your price: the ground conditions, the structural work, how hard the old house is to connect to, the finish level and the access. Two extensions of identical size can differ wildly in cost. Use a rate to gut-check your final number, never to build it.
Builder and Extension Specialist
My first big extension I priced the new build nicely and completely underdid the join to the existing house. Cutting in the roof, matching the floor levels, patching where the old wall came out. That messy middle bit ate the job. Now I price the connection as carefully as the structure, because that is where extensions bleed.
The Step-by-Step Method
Step 1: Get the Scope, Drawings and Approvals Straight First
Extensions almost always involve drawings, engineering and approvals, and you cannot price accurately without them. Work from the plans and the engineer's details, not a conversation.
Be crystal clear on the approvals, because they carry cost and time. In most of Australia an extension needs either a Development Application (DA) through council, or a Complying Development Certificate (CDC) if it meets the fast-track criteria, followed by a Construction Certificate before work starts. In the US the equivalent is your building permit, and often a zoning or planning review, issued by the local authority. Whatever the jurisdiction, confirm in writing who is responsible for lodging and paying for these: the builder, the client, or the designer. Also confirm who is handling engineering, surveys, and any boundary matters.
Pricing an extension off a vague brief is how you end up carrying costs that were never yours. A defined scope of work built from real drawings and a clear approvals split is the only safe foundation for the number.
Step 2: Price the Groundwork and Foundations Carefully
This is the highest-risk part of the job, because it is underground and largely unknown until you dig. Site conditions, soil type, drainage, existing services and access all swing the cost heavily.
Price the excavation, the footings and slab or subfloor to the engineer's specification, and factor in the soil classification, because reactive or poor soils push footing costs up quickly. Be honest about access. A tight site where everything is barrowed by hand costs far more than one a machine can reach. Where ground conditions are genuinely unknown, this is a place to carry contingency and to exclude clearly, because rock, poor soil or unexpected services can blow a groundwork budget fast.
Step 3: Price the Structure to the Engineer's Details
The structural shell (framing, steel or timber beams, roof structure) should be priced directly from the engineering, not estimated. Steel in particular is expensive and specified precisely, so carry the real quantities and current supplier prices. Removing load-bearing walls to open the old house into the new space is common on extensions and needs propping, beams and making good, all of which cost real money and belong in the price.
Step 4: Price the Connection Between Old and New
Here is the part that separates builders who make money on extensions from those who do not. Joining a new structure to an existing house is fiddly, slow and full of surprises.
You are matching floor levels that are rarely the same, cutting the new roof into the old one and making it watertight, tying new walls into old ones, and blending finishes so the join does not look like a join. The existing house is old, out of square and built to yesterday's standards, so nothing lines up cleanly. Price this stage generously, because it almost always takes longer than it looks, and exclude the things you genuinely cannot see until you open the existing structure up.
Step 5: Price All the Trades for a Full Build
An extension runs the full trade sequence of a small build: groundwork, framing, roofing, windows and doors, plumbing, electrical, plastering, flooring, fit-off, painting and external works. Get current written quotes from each trade for the exact scope, and make sure they have priced the same job you are selling the client.
Do not forget the external work that extensions always involve: making good the surrounding ground, drainage, paths, and repairing whatever the build disturbed. It is easy to price the box and forget the mess around it.
Step 6: Build In the Longer Program
Extensions take months, not weeks, and a longer program carries costs a short job does not. More exposure to weather delays. More risk of material price rises over the life of the job. More site running costs, from skips and site toilets to temporary protection. And a client living alongside the build the whole time, which means dust protection, safety and communication.
Price your time to run a long job, and structure the payments as progress payments tied to stages so you are never funding months of work out of your own pocket.
Step 7: Add Overheads, Then Margin, Not Markup
Once your costs are built, add your overheads as a percentage of turnover, then apply your target margin correctly. On a job this size the markup versus margin trap is expensive, because the numbers are large. Adding 20 percent markup does not give you a 20 percent margin. If that is fuzzy, read markup vs margin once, because on an extension that error is not small change.
Step 8: Carry Real Contingency and Clear Exclusions
Extensions expose more unknowns than any other job because you are disturbing old ground and old structure. Carry a genuine contingency sized to the age of the house and the uncertainty of the site.
Then exclude the big unknowns in writing: unexpected ground conditions and rock, defects discovered in the existing structure once opened up, asbestos, upgrades required to bring disturbed parts of the old house up to current code, and anything outside the drawn scope. State clearly these will be handled as variations. On a job this long and this large, clear exclusions are what stand between you and an argument that costs you the whole margin.
Extension Pricing Checklist
Use this as a final sweep before your number goes out. It is the stuff that gets forgotten on extensions, which is exactly the stuff that eats the margin.
What to price | Notes and what gets missed |
|---|---|
Approvals (DA / CDC / permits) | Confirm in writing who lodges and pays. Council fees, certifier fees and lodgement costs add up |
Engineering and surveys | Structural engineering, soil test, contour or identification survey. Often assumed to be someone else's cost |
Site setup and access | Fencing, site toilet, waste, protection of the existing house. Tight access means everything is slower and dearer |
Groundwork and foundations | Excavation, footings, slab to spec. Soil classification and unknown ground are the big risks. Exclude rock |
Structure | Framing, steel, beams to the engineer's details. Removing load-bearing walls needs propping and making good |
Connection to existing house | Matching floor levels, cutting in the roof, tying in walls, blending finishes. Always slower than it looks |
Full trade sequence | Roofing, windows, plumbing, electrical, plastering, flooring, fit-off, painting. Price return visits |
External works | Drainage, paths, making good the disturbed ground. Easy to price the box and forget the mess around it |
Longer program costs | Weeks of site running costs, weather exposure, and the risk of material price rises over months |
Overheads and margin | Overheads as a percentage of turnover, then true margin, not markup |
Contingency | Sized to the age of the house and how much is unknown underground and behind old walls |
Clear exclusions | Rock, hidden defects, asbestos, code upgrades to the old house, anything outside the drawn scope |
If every row on that list has a number or a written exclusion against it, you have priced the extension properly. If any are blank, that is where the job will surprise you.
The Bottom Line
An extension is the most rewarding and the most dangerous job a smaller builder takes on. Price it from the ground up, stage by stage, straight off real drawings and engineering. Sort the approvals and who pays for them, give the groundwork and the old-to-new connection the respect they deserve, and build in the longer program. Add your overheads, apply true margin, and protect yourself with real contingency and clear exclusions. Do that and the biggest job on your board becomes the most profitable one, instead of the one that keeps you up at night.
Where Jobdocs Fits
An extension quote has a lot to carry: a detailed scope, an approvals split, a long list of inclusions and exclusions, staged payments and a realistic program. Jobdocs turns all of it into a clear, professional quote or proposal the client can actually understand, with your terms and payment stages built in, so nothing important gets lost in a messy document. You price the job properly. Jobdocs makes sure it is presented properly.
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