Knockdown-rebuild in Australia: when the numbers actually work

A knockdown-rebuild only makes sense when the house you're flattening is worth less than the land it sits on by a wide margin. That sounds obvious. It isn't how most buyers run the math. They fall in love with a street, find a tired weatherboard, and start sketching open-plan living before anyone has asked whether the existing dwelling is actually depreciated to scrap.

Skip that question and you can spend $600,000 to destroy $400,000 of usable house. Get it right and KDR is one of the cleanest paths to a brand-new home on a street you couldn't otherwise buy into.

The land-to-house ratio that decides everything

Rule of thumb: KDR is worth modelling when land value is roughly 60-70% or more of the total property value, and the existing dwelling is past economic life (1960s-70s build, structurally dated, expensive to renovate properly). On a $1.5M Melbourne property where the land is worth $1.3M and the house contributes $200k, demolishing is barely destroying anything. On a $1.5M property where the house is a sound 1990s build worth $700k of the price, swinging an excavator deletes serious value.

How do you find land value? State Valuer-General site values are public, council rates notices show them, and any decent agent can pull comparable vacant-land sales on the same street. If land is 50% of the price tag and the house is liveable, renovate. If land is 75%+ and the house is a lost cause, the question shifts to whether you can afford to build, not whether to demolish.

The full cost stack, no surprises

Pricing a KDR off a builder's headline figure is how people end up $150k over budget. The line items most quotes gloss over:

• Demolition:$20,000 to $40,000 for a standard suburban house. More if there's asbestos, a pool, or restricted access.
• Site prep, soil test, survey:$5,000 to $15,000. Reactive clay sites in Melbourne's east push the slab spec up and add real money.
• Build cost:$2,500 to $4,500 per square metre for project homes. $4,500 to $8,000+ per square metre for architectural builds. A 220m² project home lands $550k-$990k in the build line alone.
• Council DA or CDC fees: $3,000 to $10,000, plus Section 7.11 contributions in NSW which can run another $10k-$30k depending on council.
• Service connections: water, sewer, power, gas, NBN. Allow $10,000-$25,000 if any need upgrading.
• Landscaping, driveway, fencing: $20,000 to $60,000 to finish the property properly.
• Contingency: 10-15% on the build line. Non-negotiable.

A typical 3-4 bedroom project home rebuild on an existing suburban block lands $400k to $700k all-in once the extras are honest. A custom architectural build with high-end finishes comfortably passes $1.5M.

Timelines, and the carry costs nobody quotes

Plan for 18 to 24 months from contract to keys. That breaks down roughly into 3-6 months of design and council approval (a Complying Development Certificate is faster than a full DA but only available on conforming lots), and 9-18 months of build depending on complexity and how generously the builder is pricing programme.

During that time you're paying rent or holding a temporary property, often $2,500-$4,500 a month for a family-sized rental. You're paying construction-loan interest on progressive drawdowns, which adds up faster than people expect. You're paying council rates on land that's producing nothing. Twenty months of $3,500 rent is $70,000. That's a kitchen.

Construction loans drawdown in stages (slab, frame, lockup, fixing, completion), so interest accrues progressively rather than on the full balance from day one. The mortgage calculator will let you sanity-check what the eventual fully-drawn repayment looks like once the build settles into a standard loan.

A worked Melbourne example

Take a $1.5M established 1960s home in a middle-ring Melbourne suburb. Land $1.2M, house $300k. The land share is 80%, the house is end-of-life, the street's recent new builds sell in the $2.3M-$2.5M band. KDR is on the table.

Numbers:

• Purchase price: $1,500,000
• Stamp duty (VIC, non-first-home): roughly $82,500
• Demolition: $30,000
• Build (220m² quality project home at $2,500/m²): $550,000
• DA, contingency, landscaping, connections: $80,000
• Total invested: roughly $2,242,500
• Comparable new builds selling at $2.3M-$2.5M
• Pre-selling-costs margin: $60k to $260k

Compare that to a $250,000 cosmetic-plus renovation on the existing house. Likely finished value $1.85M, total in $1.83M including stamp duty, margin around $20k, and you're left with a 65-year-old structure dressed in new clothes. The KDR margin is wider, the end-product is a 50-year asset, and the depreciation schedule on a brand-new build adds meaningful tax benefit if you ever rent it out.

Now run the same example with land at $900k and house at $600k on the same $1.5M price. KDR spend stays $660k, total in $2.24M, but you've thrown out a $600k house. The renovation path on a sound dwelling probably finishes around the same $1.85M for a $250k spend, leaving renovation as the obviously cleaner play. Same headline price, opposite answer.

The over-capitalisation trap

Every suburb has a ceiling price, the upper bound where buyers stop appearing regardless of how nice the house is. Build a $1.6M finished home on a street where the highest 2025 sale was $1.4M and you've manufactured a loss-making asset. The market doesn't reward the build cost, it rewards comparable sales.

Research the ceiling before locking specs. Pull every sale on the street and the next two streets over from the past 18 months. Filter for similar lot sizes. The top of that range is your hard cap, not the builder's display home brochure. The same public-data shortlisting framework used for buying works just as well for sense-checking a rebuild's exit value.

Tax, finance, and depreciation worth knowing

Stamp duty is paid once, on the original land-and-house purchase. The new build itself isn't separately dutied, you already paid on the way in. The stamp duty calculator will show that line item by state and price. Construction loans are progressive-drawdown facilities that convert to a standard P&I loan on completion; lenders usually want fixed-price building contracts and 20% deposits, sometimes more on architectural builds.

If the rebuild ever becomes an investment property, the depreciation schedule on a new build is materially better than on a renovation. Capital works deductions on the structure run 2.5% per year for 40 years, and plant-and-equipment depreciates aggressively in the first five. A quantity surveyor will quote a depreciation schedule for $700-$900 once the build is finished, and on a $600k construction cost the first-year deduction can comfortably clear $20k.

When to walk away

Walk if the existing house is worth more than 35-40% of the total price. Walk if the suburb's ceiling is too close to your finished cost. Walk if you can't carry 24 months of rent plus interest without stress. Walk if you're relying on capital growth during the build to make the numbers work, because that's a bet, not a budget.

KDR done well delivers a brand-new home on land you couldn't afford to buy as vacant lots in any established suburb. The ratio decides it. Run yours on Burbfinder before the emotional attachment to a specific block makes the math impossible to read clearly.