Loss relief is a workflow, not a single box

A small company with a Corporation Tax loss still has filing work to do. The CT600 needs the right profit or loss figures, the computations need to explain the claim, and the reviewer needs to know whether the loss is being used in the same period, carried back, carried forward, or left unclaimed for now.

HMRC's trading loss guidance says a claim for trading losses forms part of the Company Tax Return. It also gives specific CT600 box references for some current-year loss claims, including box 155, box 275, and box 780. That is why loss relief should be handled inside the return workflow rather than as a note added after the return is prepared.

Start with the type of loss

Not every loss follows the same route. Trading losses, property income losses, non-trading loan relationship deficits, terminal losses, and capital losses have different rules and claim mechanics. This page focuses on the common small-company trading loss workflow, but the file should record the loss type before the CT600 is finalised.

The danger is treating every negative number in the accounts as a trading loss available for the same claim. A company with investment income, property activity, loan relationships, or capital disposals may need a more specialist review.

Current-year relief

A trading loss may be set against other profits or gains of the same accounting period. In practice, that means the reviewer should compare the trading result with other taxable sources and decide how much loss relief is being claimed in the current return.

The computation should show the starting accounting loss or profit, tax adjustments, capital allowances or balancing charges, and the loss relief used. If the company is claiming against total profits, the CT600 and computation should agree.

Carry-back claims

HMRC guidance says a company can choose to carry a trading loss back, subject to the relevant rules. A carry-back claim should not be treated as a casual repayment request. The return needs enough detail to support the claim, including the amount, the accounting period to which it is being carried back, and how it affects earlier Corporation Tax.

Practices should pay close attention where the loss arises after a profitable period. The client may expect an immediate repayment, but HMRC claim timing and processing can depend on the precise claim route and supporting detail.

Carry-forward relief

If a trading loss is not used in the same period or carried back, it will generally be carried forward to a future accounting period, subject to the rules. HMRC's carry-forward guidance says trading losses can be carried forward to deduct from future profits as long as the trade continues.

For small companies, the practical problem is continuity. If the loss is not stored clearly, the next CT600 preparation starts with uncertainty: what was the loss, what part has already been used, and is the same trade still continuing?

Review checklist for small companies

Identify whether the loss is trading, property, capital, terminal, or another category.
Reconcile the accounts result to the Corporation Tax computation.
Check add-backs, deductions, capital allowances, and balancing charges before deciding the tax loss.
Decide whether the loss is used in the current period, carried back, carried forward, or split between routes.
Record the accounting period receiving any carry-back claim.
Keep brought-forward and carried-forward schedules visible for the next return.
Explain the claim in the computation where needed.
Review whether specialist rules apply, such as terminal losses or capital loss restrictions.

Common filing mistakes

The first mistake is using the accounting loss as the tax loss without adjustment. The accounts are the starting point, but Corporation Tax adjustments can change the figure.

The second mistake is losing the loss schedule. A company that carries losses forward needs a durable record that survives staff changes, software migration, and the next accounting period.

The third mistake is claiming a carry-back without enough explanation. A clear computation and review note reduce the chance of confusion later, especially where the claim affects a prior period already filed.

How Robocount helps

Robocount is built for the CT600 package: return, computations, supplementary pages where needed, review trail, and filing readiness. For losses, the aim is to make the decision visible rather than burying it inside a final tax figure.

Structured CT600 workflow for current-year and prior-period loss questions.
Computation-led review so tax adjustments are visible before the return is filed.
Loss schedules that connect to the next accounting period's workflow.
Practice review trail for preparer, reviewer, client approval, and submission.
API and AI-assisted workflows that can treat loss claims as structured data, not free text.

FAQ

Does a company with no Corporation Tax to pay still need to file a CT600?

If HMRC has issued a notice to deliver a Company Tax Return, the company must file even if there is no Corporation Tax to pay. A loss-making period can still require a complete return package.

Can a small company carry a trading loss back?

Often, yes, subject to the current rules and the company's facts. HMRC's trading loss guidance explains that losses may be offset in the same period, carried back by claim, or carried forward where not otherwise used.

What should be retained for carried-forward losses?

Keep the original loss computation, the amount used in any later period, the remaining balance, and the basis for concluding that the trade continues where that matters.

Official references

This guide is general product and filing workflow information, not tax advice. Check current HMRC guidance and the company's facts before filing.

Corporation Tax losses for small companies: a CT600 workflow guide.