Key Points
- The tax base is the amount the tax authority will allow as a deduction (for assets) or will tax (for liabilities) in future periods.
- A temporary difference arises whenever the carrying amount of an asset or liability differs from its tax base, triggering deferred tax at the applicable rate.
- IAS 12.7 defines the tax base of an asset as the amount deductible against taxable income when the entity recovers the carrying amount.
- Incorrectly determining the tax base is the most common root cause of deferred tax errors on non-Big 4 engagements.
What is Tax Base?
IAS 12.7 defines the tax base of an asset as the amount that will be deductible for tax purposes against any taxable economic benefits flowing to the entity when it recovers the carrying amount. If those economic benefits will not be taxable, the tax base equals the carrying amount (producing no temporary difference). IAS 12.8 applies the mirror logic to liabilities: the tax base of a liability is its carrying amount less any amount that will be deductible for tax purposes in future periods.
The concept exists because IFRS measurement and tax measurement frequently diverge. A machine might have a carrying amount of EUR 400,000 under IAS 16 but a tax written-down value of EUR 280,000 because the tax authority allows accelerated depreciation. That EUR 120,000 gap is a taxable temporary difference under IAS 12.15, producing a deferred tax liability. Revenue received in advance might have a carrying amount of EUR 50,000 as a liability, but if the cash has already been taxed on receipt, the tax base is zero (IAS 12.8 deducts the future-deductible amount of zero from the carrying amount). The EUR 50,000 difference becomes a deductible temporary difference, producing a deferred tax asset.
The auditor applies ISA 540.13(a) to evaluate whether management's method for identifying tax bases is appropriate. That means tracing every material balance sheet line to the relevant tax computation, not accepting a spreadsheet that simply assumes tax base equals carrying amount.
Worked example
Client: German engineering company, FY2025, revenue EUR 28M, HGB reporter also preparing an IFRS reporting package for the group. Hoffmann acquires a CNC milling machine in January 2025 for EUR 600,000. Under IFRS (IAS 16), Hoffmann depreciates the machine straight-line over 10 years, giving a carrying amount at 31 December 2025 of EUR 540,000. German tax law allows degressive depreciation at 25% in year one, giving a tax written-down value of EUR 450,000. The corporate tax rate is 30% (trade tax and corporate income tax combined).
Step 1 — Determine the carrying amount
Under IFRS, accumulated depreciation is EUR 60,000 (EUR 600,000 / 10 years). The carrying amount at year-end is EUR 540,000.
Documentation note: record the IFRS depreciation policy, the useful life, and the year-end carrying amount per the fixed asset register. Cross-reference to IAS 16.73(a).
Step 2 — Determine the tax base
Under German tax law, degressive depreciation at 25% on the cost of EUR 600,000 produces a year-one deduction of EUR 150,000. The tax base at 31 December 2025 is EUR 450,000.
Documentation note: record the tax depreciation method, the applicable rate, and the resulting tax written-down value. Cross-reference to the entity's corporate tax return or tax computation workpaper.
Step 3 — Calculate the temporary difference
The carrying amount (EUR 540,000) exceeds the tax base (EUR 450,000) by EUR 90,000. This is a taxable temporary difference per IAS 12.15 because future recovery of the carrying amount will generate taxable income exceeding the remaining tax deductions.
Documentation note: record the temporary difference amount, classify it as taxable, and note that it arises from differing depreciation rates between IFRS and tax. Reference IAS 12.15(a).
Step 4 — Recognise the deferred tax liability
The deferred tax liability is EUR 90,000 multiplied by 30%, producing EUR 27,000. IAS 12.47 requires measurement at the rate expected to apply when the temporary difference reverses. Hoffmann applies the enacted 30% rate.
Documentation note: record the deferred tax liability of EUR 27,000, the tax rate used, and confirm that the rate is the enacted (not proposed) rate at the reporting date per IAS 12.47. Include a rollforward from prior year.
Conclusion: the deferred tax liability of EUR 27,000 is defensible because the carrying amount and tax base are each independently verified, the temporary difference classification follows IAS 12.15, and the tax rate is the enacted combined rate at the balance sheet date.
Why it matters in practice
Teams frequently fail to identify the tax base of liabilities. IAS 12.8 requires a specific calculation (carrying amount less amounts deductible in future periods), but practitioners often skip this step for provisions, deferred revenue, and accrued expenses. The result is incomplete identification of deductible temporary differences and understated deferred tax assets. ISA 540.18 requires the auditor to evaluate whether the entity has identified all relevant inputs to the deferred tax calculation.
The tax base of items with no future tax consequence is sometimes left blank or set to zero rather than set equal to the carrying amount. IAS 12.7 specifies that when economic benefits are not taxable, the tax base equals the carrying amount (producing no temporary difference). Setting the tax base to zero creates a phantom temporary difference and an erroneous deferred tax entry.
Tax base vs. carrying amount
| Dimension | Tax base (IAS 12) | Carrying amount (IFRS) |
|---|---|---|
| Determined by | Tax legislation applicable to the entity | IFRS measurement requirements (cost, fair value, amortised cost) |
| Depreciation method | As prescribed or permitted by tax law (often accelerated) | As required by IAS 16 or IAS 38 (reflecting the pattern of economic benefit consumption) |
| Purpose | Calculates future tax deductions or taxable amounts | Represents the asset or liability in the financial statements |
| Changes when | Tax law is amended, or the entity's expected recovery method changes | Remeasurement, impairment, depreciation, or revaluation occurs under IFRS |
| Mismatch creates | A temporary difference, triggering deferred tax recognition under IAS 12.15 | No standalone consequence; the mismatch with the tax base is what matters |
The distinction matters because the entire deferred tax balance on the balance sheet flows from comparing these two figures for every material item. If an auditor accepts management's assertion that carrying amount equals tax base without testing, the deferred tax line could be materially misstated in either direction.
Related terms
Frequently asked questions
How do I determine the tax base when an asset has never been claimed for tax?
If an asset's cost is not deductible against taxable income (for example, non-deductible goodwill in certain jurisdictions), the tax base is zero. The entire carrying amount is a taxable temporary difference. IAS 12.7 is clear: the tax base is the amount deductible against future taxable economic benefits. Where no deduction is available, that amount is nil. However, IAS 12.15(a) and IAS 12.24 include a recognition exception for goodwill, so the deferred tax liability on non-deductible goodwill is not recognised.
Does the tax base change when tax rates change?
No. The tax base is determined by the tax rules governing the deductibility or taxability of the item, not by the rate. What changes is the measurement of the resulting deferred tax asset or liability. IAS 12.47 requires the entity to measure deferred tax at the rate expected to apply when the temporary difference reverses, using rates enacted or substantively enacted at the reporting date. A rate change does not alter the tax base itself, but it changes the deferred tax balance.
When do I need to reassess tax bases during the audit?
Reassess at every reporting date. Tax law amendments, new rulings, or changes in how the entity expects to recover an asset (sale versus use, per IAS 12.51A–51B) can all change the tax base. ISA 540.13(b) requires the auditor to evaluate whether the data and assumptions used in measuring deferred tax remain appropriate. If a client changed its recovery method for an investment property from use to sale, the tax base may shift significantly.