CT600 Box 450: Double Taxation Relief
If your company earns income from abroad, you may end up paying tax on that income twice — once to the foreign tax authority and again to HMRC when you file your company tax return. Double Taxation Relief (DTR) prevents this by allowing you to offset the foreign tax paid against your UK Corporation Tax bill. CT600 Box 450 is where you record that credit.
This guide explains what Box 450 is, who needs to complete it, and how the relief is calculated.
> Looking for marginal relief? Box 450 is often confused with marginal relief, but the two are completely different. Marginal relief is recorded in CT600 Box 435. Box 450 applies only to companies with foreign income.
What Is CT600 Box 450?
Box 450 appears in the Tax Reliefs section of the CT600 form. HMRC describes it as:
> "Enter any Double Taxation Relief claimed against Corporation Tax chargeable. Exclude any amount entered in box 500."
The box is completed only by companies that have received income from overseas — for example, interest from foreign bank accounts, dividends from foreign subsidiaries, profits from overseas branches, or royalties paid by foreign licensees.
If your company operates entirely within the UK and has no foreign income, Box 450 will be blank.
What Is Double Taxation Relief?
When a UK company earns income from another country, the country where the income arises will often tax it first — deducting withholding tax before the payment reaches the UK. HMRC then taxes the same income again as part of the company's UK profits. Without relief, the company would pay tax twice on the same money.
DTR solves this problem by granting a credit for the foreign tax paid, which is set against the UK Corporation Tax charged on that income.
Relief is available in two forms:
- Treaty relief: The UK has double taxation agreements with more than 100 countries. These treaties set out which country has taxing rights over particular types of income and what credit is due.
- Unilateral relief: Where no treaty exists, UK domestic law (the Taxation (International and Other Provisions) Act 2010) still allows a credit for foreign tax paid on most types of income.
Who Needs to Complete Box 450?
You would complete Box 450 if your company:
- Receives dividends from overseas subsidiaries on which foreign withholding tax has been deducted
- Has overseas branch profits taxed in the foreign jurisdiction
- Earns interest or royalties from abroad subject to foreign withholding tax
- Has made foreign chargeable gains on which tax has been paid overseas
How the Relief Is Calculated
The credit available in Box 450 is limited to the lesser of:
- The foreign tax actually paid on the income, and
- The UK Corporation Tax attributable to that same income
Worked example:
A company earns £50,000 of interest from a US bank account. The US deducts 15% withholding tax, so the company has paid £7,500 in US tax. In the UK, this interest income is taxed at the 25% main rate, giving a UK tax charge of £12,500 on that income. The DTR credit is limited to the lesser of £7,500 (foreign tax paid) and £12,500 (UK tax on that income), so the company enters £7,500 in Box 450.
The Cap: Box 470 Cannot Exceed Box 440
All tax reliefs are totalled in Box 470 (Total Reliefs and Deductions in terms of tax). HMRC's guidance is explicit: the figure in Box 470 cannot exceed the figure in Box 440 (Net Corporation Tax Chargeable).
In practice, this means your Double Taxation Relief credit — along with any other reliefs — cannot reduce your Corporation Tax liability below zero. HMRC will not repay tax that was never owed.
Related Boxes
Several other boxes relate to Box 450:
| Box | Purpose |
|---|---|
| 450 | Double Taxation Relief against Corporation Tax |
| 455 | Tick if Box 450 includes an underlying rate relief claim (where your company controls ≥10% of a foreign company's voting power and claims credit for underlying tax paid by that company) |
| 460 | Tick if Box 450 includes an amount carried back from a later accounting period — computations must identify those periods |
| 470 | Total Reliefs and Deductions (sum of boxes 445, 465, 480) |
Box 450 vs Box 500: What Is the Difference?
Box 450 must exclude any amount entered in Box 500 — this is a requirement stated directly in the HMRC CT600 guide.
Box 500 covers a group of separate, specialist taxes:
- Controlled Foreign Companies (CFC) tax (Box 490)
- Bank Levy (Box 495)
- Bank Surcharge (Box 496)
- Residential Property Developer Tax (RPDT) (Box 497)
What Must the Computations Include?
HMRC requires detailed supporting computations attached to the CT600. For Box 450, the computations must include:
- Full calculations of the DTR credit claimed
- Details of the foreign income and the foreign tax paid
- Details of any underlying tax relief claims (if Box 455 is ticked)
- Identification of any amounts carried back from later periods (if Box 460 is ticked)
Is Box 450 Supported by TinyTax?
TinyTax is designed for UK companies with straightforward Corporation Tax positions: trading profits, property income, and basic loss relief. Double Taxation Relief typically arises in companies with international income structures requiring specialist handling.
Box 450 is not currently supported in TinyTax. If your company needs to claim DTR, you will need to work with a qualified accountant or use specialist CT600 software.
Summary
CT600 Box 450 records Double Taxation Relief — a credit that prevents your company being taxed twice on the same overseas income. The relief is limited to the lesser of the foreign tax paid and the UK Corporation Tax on that income, and total reliefs cannot exceed your overall Corporation Tax liability.
For companies with straightforward UK-only income, Box 450 does not apply. To understand how Box 450 fits within the wider tax calculation, read our guide to CT600 boxes explained or explore how marginal relief is calculated in Box 435 for companies with profits in the marginal relief band.