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What Records Do I Need to Substantiate My Cost Base?

Published 26 June 2026 · Last updated 26 June 2026

Quick Answer

To substantiate your cost base for CGT in Australia, keep records of purchase price, legal fees, stamp duty, improvement costs, and holding costs. These documents ensure you correctly calculate your CGT liability. Under the ATO's guidelines, maintaining accurate records for at least five years after the CGT event is crucial.

Capital Gains Tax (CGT) is a significant consideration for property investors in Australia, and understanding how to substantiate your cost base is crucial for accurately calculating your tax liability. The cost base includes the original purchase price, associated purchase costs, capital improvements, and certain holding costs. Proper documentation is essential to verify these amounts and ensure compliance with the ATO's requirements.

Under the ATO's guidelines, your cost base is essentially the total of all costs incurred in acquiring, holding, and improving a property. This includes the purchase price, legal fees, stamp duty, costs of improvements, and other related expenses. A common misconception is that only the purchase price matters, but ignoring other components can result in an overstated CGT liability.

To see how this plays out, consider a practical example: Imagine you purchased a 2-bedroom apartment in Melbourne in 2010 for $600,000. Over the years, you spent $50,000 on renovations and incurred $20,000 in legal and stamp duty fees. To substantiate your cost base, you need to keep records of these expenses, including receipts, invoices, and bank statements. When you sell the property for $850,000, these documents will help calculate your capital gain accurately. At a 37% marginal tax rate, proper documentation can significantly impact your tax liability, potentially reducing it by $7,400 in year one.

In our experience reviewing thousands of properties across Australia, we often see investors neglecting to keep comprehensive records of improvements and holding costs. This oversight can lead to an inflated tax bill. Many also forget that costs like council rates and loan interest during the ownership period can form part of the cost base if they are not claimed as deductions.

The answer can differ depending on your situation. For instance, if you acquired the property before 1985, it is exempt from CGT. Properties held in a trust or by a company may have different record-keeping requirements. Additionally, if you have made improvements that qualify as capital works under Division 43, you need to distinguish these from depreciating assets under Division 40.

When it comes to substantiating your cost base, individual circumstances can significantly affect the outcome. Engaging a Chartered Quantity Surveyor alongside your accountant can ensure all potential deductions are captured and accurately recorded, maximising your financial outcome.

  • Review your property acquisition documents and ensure all purchase-related costs are documented.
  • Gather and organise all receipts for capital improvements and repairs.
  • Maintain records of holding costs, distinguishing between those claimed as deductions and those added to the cost base.
  • Consult with a Chartered Quantity Surveyor to verify capital works and depreciating assets.
  • Regularly update your records to ensure they remain accurate and complete.
  • Schedule a meeting with your accountant to review your CGT strategy and documentation.
  • Frequently Asked Questions

    What documents are essential for substantiating my cost base?

    Essential documents include purchase contracts, receipts for renovations, legal fee invoices, bank statements for stamp duty, and records of holding costs like council rates.

    How long must I keep records for CGT purposes?

    You must keep records for at least five years after the CGT event, such as the sale of the property, to substantiate your cost base.

    Can I include loan interest in my cost base?

    Loan interest is typically deductible as an expense, not included in the cost base, unless it is not claimed as a deduction.

    Are there any state-specific variations in costs I can include?

    While the types of costs are generally consistent, the amounts, such as stamp duty, can vary by state. Check local regulations for specifics.

    How do I report my cost base on my tax return?

    Your accountant will use your substantiated cost base to calculate your capital gain and report it on your tax return, ensuring compliance with ATO requirements.

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    Written by Koste Team · Koste Chartered Quantity Surveyors · AIQS Member · RICS Member · TPB Registered · 1300 669 400 · info@koste.ai