What Not to Do When Funding a Home Extension

Construction Loans for Extensions Work Differently to New Builds

Construction loans for extensions operate on a progressive drawdown structure, which means funds are released in stages as the build progresses, not upfront. You'll typically pay interest only on the amount drawn down at each stage, and most lenders charge a progressive drawing fee for each inspection and payment release. Your existing mortgage stays in place, and the construction loan sits alongside it until the extension is complete and both loans are consolidated.

In Canterbury, where many period homes are being extended rather than replaced, the build timeline matters more than you'd think. Council approval in Boroondara can take anywhere from six to twelve weeks depending on the scope of the development application, and most lenders require you to commence building within a set period from the loan approval date. If council delays push you past that window, you may need to reapply.

The Contract Type Determines How Funds Are Released

Lenders will only approve construction finance against a fixed price building contract with a registered builder. A cost plus contract, where you pay the builder's costs plus a margin, won't be accepted by most mainstream lenders because the final cost isn't locked in. The contract needs to include a progress payment schedule that aligns with the construction draw schedule, and those stages are usually broken into five or six instalments: base stage, frame stage, lock-up, fixing, and practical completion.

The payment schedule in your building contract should match the drawdown structure your lender uses. If your builder expects payment at different milestones to what the lender will release, you'll either need to renegotiate the contract or fund the gap yourself. We regularly see this mismatch cause cash flow problems halfway through a build, particularly when builders request payment for materials before a stage is technically complete.

Why the Valuation Includes Both Current and Completed Value

The lender will order a valuation that assesses your property twice: once at its current value, and again at its expected value once the extension is finished. The loan amount is based on the lower of those two figures, which means if your current home is worth $1.2 million and the completed value is estimated at $1.5 million, the lender will use $1.2 million as the baseline and lend against the construction cost from there.

Consider a scenario where a Canterbury homeowner wants to add a second storey to a single-level brick home. The current value sits at $1.3 million, and the completed value is estimated at $1.65 million. The extension costs $280,000 to build. The lender assesses serviceability based on the existing mortgage plus the construction loan repayments, and because the loan-to-value ratio stays under 80% once the build is done, no lenders mortgage insurance applies. The construction loan is approved, funds are released progressively as each stage is signed off by the bank's valuer, and once the final inspection clears, both loans roll into a single home loan at the standard variable or fixed rate.

What Happens If the Build Runs Over Budget

If your builder submits variations or the scope changes mid-build, the lender won't automatically increase the approved loan amount. You'll need to apply for additional funds, which means another valuation, another serviceability assessment, and potentially a delay in releasing payment to the builder. That's why a fixed price contract with a registered builder matters. It locks in the cost and removes the risk of the lender pulling back halfway through.

In our experience, the most common budget blowouts come from owners requesting changes after the contract is signed, or from unexpected structural work that wasn't flagged in the initial building inspection. If you're extending an older Canterbury home, get a pre-purchase or pre-construction inspection done by a structural engineer, not just a standard building inspector. Period homes in this area often have foundation issues or timber framing that doesn't meet current code, and those problems surface once the walls come down.

The Interest Rate Structure During Construction

During the construction phase, you'll be on an interest-only repayment schedule for the construction loan, and interest is calculated daily on the drawn balance. That means if $80,000 has been released across the first two stages, you're only paying interest on that $80,000, not the full approved amount. Once construction is complete and the loans consolidate, you'll move to principal and interest repayments unless you've specifically structured the loan to remain interest-only.

Some lenders offer a construction loan interest rate that's slightly higher than their standard variable rate during the building phase, then revert to the standard rate once the build is finished. Others keep the rate consistent throughout. It's worth comparing both the construction rate and the revert rate before you settle on a lender, because a lower headline rate during construction doesn't help if the revert rate is uncompetitive.

Canterbury-Specific Considerations for Extensions

Canterbury sits within Boroondara, which has strict planning overlays and heritage controls in certain pockets. If your property falls within a heritage overlay, your development application will take longer and may require specific materials or design elements that increase the build cost. The council is particular about streetscape consistency, so even if you're not in a heritage zone, expect some back-and-forth on façade design if your extension is visible from the street.

The suburb's proximity to Canterbury Road and the train line makes it popular with young families upgrading from apartments, and many of those buyers are extending within two or three years of purchasing. If that's your plan, make sure your current mortgage broker knows upfront so the loan structure can accommodate future construction finance without requiring a full refinance. Some lenders will pre-approve a construction top-up at the time of purchase, which saves time and paperwork later.

What Lenders Want to See Before Approving Construction Finance

Lenders will ask for council plans, a signed fixed price building contract, proof that your builder is registered and insured, and evidence that you can service both your existing mortgage and the construction loan. If you're planning to use equity from your current property to fund the build, the lender will calculate serviceability based on the combined debt, not just the construction loan in isolation.

You'll also need to demonstrate that you have enough cash or available equity to cover cost overruns. Most lenders hold back a portion of the approved amount as a buffer, and some require you to have an additional 10% of the build cost in accessible savings or redraw. That's not part of your deposit, it's a contingency in case the build takes longer than expected or variations push the cost up.

If you're working with an owner builder arrangement, your finance options narrow significantly. Most mainstream lenders won't touch owner builder projects because the risk sits with you, not a licensed professional. Specialist lenders do exist, but the rates are higher and the deposit requirements are stricter. Unless you're a licensed builder yourself, a fixed price contract with a registered builder is the only realistic path to competitive construction finance.

Timing the Loan Approval with Your Build Start Date

Construction loan approvals are usually valid for three to six months, and the clock starts from the formal approval date, not from when you submit the application. If your builder can't start within that window because of council delays or scheduling issues, the approval may lapse and you'll need to reapply. That's frustrating, but it's also a built-in safeguard. Lenders don't want to lock in a loan for a project that's sitting idle, because your financial situation or the property market could shift in the meantime.

Once the build starts, the lender will send a valuer or building inspector to each progress payment stage to confirm the work has been completed to the standard outlined in the contract. If the inspector flags incomplete work or variations that weren't pre-approved, the payment gets held until the issue is resolved. That can delay your builder and strain the relationship, so keep your broker in the loop if anything changes during the build. Small variations can often be absorbed without triggering a formal reassessment, but only if the lender knows about them before the inspection happens.

Call one of our team or book an appointment at a time that works for you. We'll walk through the contract, the drawdown schedule, and the serviceability assessment so your extension funding is locked in before you sign with the builder.

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