Knockdown Rebuild Tax Australia 2026-27: CGT and Costs Guide

Short answer

Knocking down a house and rebuilding does not trigger capital gains tax by itself — there is no disposal, so there is no CGT event. But every dollar you spend on the demolition and the rebuild joins your property’s CGT cost base, which directly shrinks the taxable gain when you eventually sell. For your own home, a special four-year construction rule can keep the main residence exemption alive while the block sits bare. For an investment property, the news is harsher: interest, rates and insurance during the rebuild are generally not deductible — they get added to the cost base instead. And from 1 July 2027, the CGT reform draws a hard line that hits knockdown rebuilds: a like-for-like rebuild is not a “new build”, so it misses both the CGT discount choice and the negative-gearing exemption.

This guide walks through the demolition, the build, the holding years and the eventual sale — with the real 2026-27 numbers, the deductions most owners miss, and the traps the builder brochures never mention.

How a knockdown rebuild changes your tax picture

Think of a knockdown rebuild as three tax phases, not one project. Each phase has its own rules, and mixing them up is where the expensive mistakes happen.

1. Demolition. No CGT event, but demolition costs become part of your cost base. Any leftover depreciation on the demolished building (Division 43 capital works) can be claimed as a one-off scrapping deduction.
2. Construction. For an investment property this is a tax dead zone: holding costs are generally not deductible and instead accumulate in the cost base. For your home, the four-year rule protects the main residence exemption if you follow it precisely.
3. The eventual sale (or rental). CGT event A1 happens on the contract date. Your gain is sale price minus a cost base that now includes purchase price, demolition, construction, denied holding costs and selling costs — less any scrapping deduction you already claimed.

Before the detail, get a feel for the dollars with our CGT calculator — try the worked example later in this article (a $900,000 purchase, $40,000 demolition and $450,000 rebuild) to see how demolition and build costs shrink the taxable gain.

Demolition itself is not a CGT event — but the costs count

This surprises people: the day the bulldozer arrives, nothing happens for tax. A CGT event needs a disposal or some other defined trigger, and flattening a building you still own is neither. You keep the same CGT asset — the land — and its cost base simply grows.

The ATO’s technical view (confirmed in ATO Community technical answers, citing the cost-base rules) is that your cost base after a knockdown rebuild includes:

• the original purchase price of the property (land plus old building),
• demolition costs — the knockdown, asbestos removal, site clearing,
• construction costs of the new dwelling — the build contract, plus architect, engineering and council fees,
• holding costs incurred while the property couldn’t produce income (interest, rates, insurance, land tax during the build) — the “costs of owning” element,
• selling costs when you eventually dispose of it.

One common confusion, worth clearing up early because forums are full of it: your mortgage balance has nothing to do with CGT. The gain is sale price minus cost base. Whether you owe the bank $100,000 or $800,000 against the property doesn’t change the tax by a cent.

The family-home path: the four-year construction rule

If the property is (or will be) your main residence, section 118-150 of the tax law gives you a powerful concession: you can treat the land as your main residence while the old home is gone and the new one is being built — but only if every condition is met:

Miss any one of these and the exemption breaks — the period the old home was gone becomes partly taxable, apportioned by days. The classic failure is renting somewhere else and nominating that rental as your main residence during the build, or letting the project drag past four years.

What if you demolish your home, rebuild, live in it for a while, then rent it out and sell years later? Then a different rule bites: when the home first starts producing income, its cost base is reset to market value at that date (section 118-192). Your demolition and construction costs from before that point are effectively replaced by the market-value figure — which is usually good news, since the rebuilt home is worth more than it cost. Keep a valuation from the date it first earns rent.

Investment property: what you can and can’t claim while you rebuild

Here’s the part that stings. From 1 July 2019, the tax law denies deductions for holding vacant land — and the ATO treats your block as vacant land from the day the old house becomes uninhabitable until the new dwelling can be lawfully occupied and is rented or genuinely available for rent (see the ATO’s vacant-land guidance and Taxation Ruling TR 2023/3).

During the rebuild, these are generally not deductible for individuals, trusts and SMSFs:

• interest on the loan over the land,
• council rates and land tax,
• insurance and maintenance of the site.

They are not lost, though — denied holding costs join the third element of your cost base (costs of owning), so they still reduce your eventual capital gain. Keep every receipt; the paperwork you file away during the build is worth real money at sale time.

One nuance worth knowing: interest on a separate construction loan is arguably not a “holding cost of the land”, so it escapes the vacant-land ban — but the general deduction rules still require the borrowed money to be connected to producing assessable income, which is doubtful before the property can be rented. Most advisers treat construction-period interest as cost-base material too. If your structure is unusual (a company or a genuine development enterprise), get specific advice rather than assuming either treatment.

And the mirror-image question from PropertyChat: once the new dwelling is finished, leased, and producing rent, normal investment-property deductions resume — interest, rates, insurance, depreciation on the new build, and repairs.

The scrapping deduction most people miss

Buried in Division 43 is one of the best deals in the knockdown rebuild: when you demolish a building that had undeducted capital works, you can claim the entire remaining undeducted amount as a deduction in the year of demolition — even though the demolition was voluntary (ATO guidance QC40267 on the balancing deduction; the method statement is in section 43-250).

The conditions:

• the building (or extension) must have been constructed after 18 July 1985, when Division 43 started,
• it must have been used to produce assessable income during the income year of demolition — even partly (the deduction is pro-rated to income-producing use),
• you need the original construction expenditure figure — a quantity surveyor’s depreciation schedule is the usual evidence.

A concrete example: a $20,000 garage built 25 years ago, written off over the standard 40-year life, has $7,500 of undeducted capital works left. Demolish it and that $7,500 is deductible in the demolition year. On bigger original structures the figure can run into the tens of thousands — real money, claimed a decade or more before any sale.

The sting to remember: capital works amounts you claim (or could have claimed) as deductions reduce your cost base. You can’t have the same dollars in both places — the scrapped amount comes out of the cost base it would otherwise have sat in.

When the CGT clock actually stops: contract date, not settlement

When you sell the rebuilt property, CGT event A1 happens when you enter into the contract — the date on the contract, not settlement day. The contract date decides which tax year the gain falls in, whether the 12-month discount clock is satisfied, and — critically now — whether the pre- or post-1 July 2027 CGT rules apply to the gain. You generally don’t include the gain in your return until settlement occurs; if you’ve been assessed earlier, request an amendment (shortfall interest is usually remitted if you amend promptly after settlement). Read the full timing rules in our contract date vs settlement guide.

Worked example: the numbers on a knockdown rebuild

Take an investor who bought a Sydney house in 2023 for $900,000, demolished it ($40,000), rebuilt ($450,000), held it as a rental throughout, and sells for $2,100,000 with the contract signed in 2026-27. Purchase costs like stamp duty are ignored here for simplicity — in real life, add them to the cost base too.

Step 1 — cost base. $900,000 (purchase) + $40,000 (demolition) + $450,000 (construction) + $44,000 (selling costs, roughly 2% agent fees plus legal) = $1,434,000.

Step 2 — gross capital gain. $2,100,000 − $1,434,000 = $666,000.

Step 3 — the 50% CGT discount. The property was held well over 12 months and the contract predates 1 July 2027, so the discount applies: $666,000 × 50% = $333,000 net capital gain added to assessable income. (Try the same inputs in our CGT calculator to see how the discount changes the result.)

Step 4 — the tax. Suppose the investor’s other income already puts them at the top marginal rate: 45% plus the 2% Medicare levy. Tax on the gain = $333,000 × 45% = $149,850, plus Medicare levy $333,000 × 2% = $6,660 — a total of $156,510.

Two caveats that matter. First, the actual rate depends on total taxable income in the sale year — a $333,000 gain can easily push someone into a higher bracket than their salary alone would. Second, if the contract date falls on or after 1 July 2027, this maths changes completely: the 50% discount is gone for post-1 July 2027 gains, replaced by CPI indexation of the cost base plus a 30% minimum tax. The date on the contract is doing real work now.

The trading-stock trap for serial rebuilders

The single biggest risk in knockdown-rebuild land — and the one builder guides never mention — is being treated as carrying on a property development business. If you buy, demolish, rebuild and sell repeatedly, or with a profit-making purpose from the start, the ATO can treat the property as trading stock rather than a CGT asset.

The consequences are brutal: the profit is ordinary income, not a capital gain — so no 50% CGT discount, no main residence exemption, no indexation. The property is deemed disposed of at market value when it becomes trading stock, and every later sale is taxed in full at your marginal rate. One project done cleanly is an investment; a pattern of demolish-rebuild-sell starts to look like a business. If you’re on your second or third cycle, get advice before you sign the next demolition contract.

Selling the new build: GST and the margin scheme

CGT is only half the sale story if you’re rebuilding to sell. If the seller is registered (or required to be registered) for GST and the property is new residential premises — which explicitly includes “a new building built to replace a demolished building on the same land” — the sale is a taxable supply: GST of 1/11th of the price applies.

The margin scheme (Division 75 of the GST Act) can shrink that bill: GST becomes 1/11th of the margin (sale price minus the original purchase price — construction costs and stamp duty don’t reduce the margin). But two conditions are strict: the seller and buyer must agree in writing to use the margin scheme before the contract is signed, and the buyer gets no GST credit. Miss the written agreement and the full 1/11th applies.

Practical mechanics: the purchaser withholds GST at settlement and pays it straight to the ATO — 1/11th of the price normally, or 7% under the margin scheme. And the escape hatch: after 5 continuous years of renting the new dwelling out, a later sale is input-taxed (no GST). Mum-and-dad investors selling their own rebuilt home — not as an enterprise — are generally outside the GST system altogether.

Land tax while the block sits bare

Land tax doesn’t pause for construction, and the rules are state-specific. In NSW, there is a genuine concession: land intended as your principal place of residence — including land where a building is to be demolished and rebuilt — can be exempt for up to 4 tax years (extendable to 6 in exceptional circumstances), provided you genuinely intend to occupy the finished home and don’t claim another PPR exemption (Revenue NSW ruling LT 082v6). But a rebuild-to-sell investment pays full land tax on the unimproved value above the threshold (frozen at $1,075,000).

Victoria has no general rebuild concession — land tax is payable during construction, though the PPR exemption can continue while a home is uninhabitable under specific absence provisions. In every state, check the revenue office’s position before you budget: a two-year build on valuable land can produce a land-tax bill nobody warned you about.

The 1 July 2027 reform: why like-for-like rebuilds lose out

The 2026-27 Federal Budget rewrote the CGT rulebook from 1 July 2027, and the legislation (Treasury Laws Amendment (Tax Reform No. 1) Act 2026) is now law. The ATO’s capital gains tax reform page sets out the new system:

• The 50% CGT discount is replaced by CPI indexation of the cost base for gains accruing on or after 1 July 2027 (resident individuals, trusts, partnerships) — only real, above-inflation gains are taxed.
• A 30% minimum tax applies to certain net capital gains accruing from that date — you can no longer bank on selling in a low-income year (retirement, a career break) to have the gain taxed at a tiny marginal rate.
• Gains accrued before 1 July 2027 keep the 50% discount. You’ll need a market valuation (or the ATO’s apportionment formula) of the property as at 1 July 2027 to split the gain.
• The main residence exemption is unchanged.

Here’s the knockdown-rebuild sting. To encourage housing supply, genuinely new residential builds can choose between the old 50% discount and the new indexation-plus-30% system. But a like-for-like knockdown rebuild — one house demolished, one house built — is explicitly not a new build. Only demolition replaced by a greater number of dwellings (a duplex or townhouses replacing a house) qualifies. The same “genuinely new” test gates the negative-gearing exemption from 1 July 2027: rental losses on established properties bought after 12 May 2026 are quarantined, and a like-for-like rebuild doesn’t escape that either.

The planning implication is sharp: if you’re demolishing one dwelling and building one dwelling as an investment, contract dates and the 1 July 2027 valuation matter enormously. Get that valuation done — the ATO will provide tools, but the obligation to evidence the split is yours.

Common mistakes

• Assuming holding costs are deductible during the build. Since 1 July 2019 they’re generally not — they belong in the cost base. Claiming them as deductions invites an amendment.
• Forgetting the scrapping deduction. The Division 43 balancing deduction at demolition is free money most owners never claim because nobody tells them it exists.
• Breaching the 4-year rule by accident. Nominating another dwelling as your main residence during construction, or letting the project run past four years, breaks the exemption.
• Losing the paper trail. Demolition invoices, build contracts, quantity surveyor schedules, holding-cost receipts, the 1 July 2027 valuation — the cost base is only as good as your records.
• Ignoring GST on a rebuild-to-sell. The margin-scheme agreement must be in writing before the contract. After settlement it’s too late.
• Missing the trading-stock line. Repeat demolish-rebuild-sell cycles can convert capital gains into fully-taxed ordinary income.

Tips

• Commission a quantity surveyor’s depreciation schedule before demolition — it’s the evidence for both the scrapping deduction and the construction costs in your cost base.
• If the new home will ever be rented, get a market valuation for the date it first produces income (section 118-192 reset) and another as at 1 July 2027 (reform transition).
• For a rebuild-to-sell, settle the margin scheme in writing with the buyer before contracts are exchanged, and confirm the GST withholding amount with your conveyancer.
• Model the sale under both the pre- and post-1 July 2027 rules with our CGT calculator and the income tax calculator — the contract date can be worth tens of thousands.
• Keep a single “cost base file” from demolition day: every invoice, loan statement, rates notice and valuation in one place. Your future self (or your accountant) will thank you.

Frequently asked questions

Do I pay CGT when I demolish my investment property?

No. Demolition alone is not a CGT event because you haven’t disposed of anything — you still own the land. The demolition costs are added to the property’s cost base, reducing your taxable gain when you eventually sell.

Can I claim a deduction for knocking down a rental property?

The demolition cost itself isn’t deductible — it goes into the cost base. But check the scrapping deduction: any undeducted Division 43 capital works on the demolished building can be claimed in full in the demolition year, provided the building was used to produce income that year.

Can I claim interest and rates while my investment property is being rebuilt?

Generally no. Since 1 July 2019, holding costs of vacant land aren’t deductible, and the ATO treats the block as vacant during construction. Add them to your cost base instead — they’re not wasted, just deferred until sale.

If I knock down my home, rebuild, then rent it out and sell later, what is my cost base?

When the home first starts earning rent, the cost base resets to its market value at that date — your earlier demolition and build costs are effectively replaced by that valuation. Get it documented.

Does the 1 July 2027 CGT reform affect my knockdown rebuild?

Yes if it’s an investment. Gains accruing from 1 July 2027 lose the 50% discount (replaced by CPI indexation plus a 30% minimum tax), and a like-for-like rebuild doesn’t qualify as a “new build” for the discount choice or the negative-gearing exemption. You’ll need a 1 July 2027 valuation to split pre- and post-reform gains.

Do I pay GST if I sell the newly rebuilt property?

Only if you’re in the GST system (registered or required to be) and it’s new residential premises — which includes a new building replacing a demolished one. The margin scheme can reduce the GST to 1/11th of the margin, but it must be agreed in writing before the contract. Private sellers of their own home are generally outside GST.

The bottom line

A knockdown rebuild concentrates a decade of tax consequences into one project: demolition costs that quietly build your cost base, a scrapping deduction hiding in the rubble, a construction period where almost nothing is deductible, and a sale where the contract date, the GST treatment and the 1 July 2027 reform all collide. The owners who do well are the ones who treat the paperwork as part of the build — valuations at the right dates, every invoice kept, and the margin-scheme and main-residence choices made deliberately, not discovered at settlement. For the property-tax fundamentals behind all of this, see our capital gains tax on property guide and the main residence exemption explained.