CT600 Box 800: The CT600 Losses Section Explained

Your company's CT600 has a dedicated section for losses — covering situations where your company makes less than it spends in a given accounting period. Box 800 sits within this section, though the vast majority of small companies will never use it. This guide explains what Box 800 is, which loss boxes actually matter for most UK companies, and how trading and property losses work on your company tax return.

What Is the CT600 Losses Section?

The CT600 is the company tax return that every UK limited company files with HMRC. The losses section — covering broadly Boxes 780 to 815 — handles situations where your company makes a loss in its trading, property, or financial activities.

Even if your company makes a loss and owes no corporation tax, you must still file a CT600. The deadline is 12 months after the end of the accounting period covered by the return. Filing in a loss year is important because it formally records your losses with HMRC, creating the carry-forward pool you will draw on when your company returns to profit.

For more on your filing obligations generally, see our guide on whether you need to file a CT600.

Box 780: Trading Losses of This Accounting Period

Box 780 is the loss box most trading companies will encounter. It records the amount of trading loss your company made during the accounting period — that is, where your allowable trading expenses exceed your trading income.

If your tax-adjusted trading profit is negative, the absolute value of that loss goes into Box 780. Your profits chargeable to corporation tax will be zero in a loss year, since losses cannot create a negative tax liability.

Example:

Trading income: £42,000
Allowable trading expenses: £58,000
Tax-adjusted loss: £16,000
Box 780 = £16,000

This loss can be used in two ways:

Against other income in the same period (such as rental or investment income) via Box 275
Carried forward to offset trading profits in future accounting periods

There is no time limit on carrying trading losses forward. They can be held for as long as it takes for your company to become profitable again.

Box 800: Non-Trade Deficits on Loan Relationships

Box 800 applies to a much more specific scenario: non-trade deficits on loan relationships and derivative contracts. This covers losses arising from money lending, borrowing outside your main trade, or derivative contracts used outside trading activity.

Examples where Box 800 might apply:

A company that lends money to a related party and earns (or loses) interest outside its normal trade
Derivative contracts entered into for non-trading purposes that result in a net deficit

Box 800 does not cover trading losses (Box 780) or property losses (Box 805). The vast majority of small trading companies and property companies will never need to complete Box 800. If your company has complex financial arrangements involving loan relationships or derivatives, specialist advice from a tax professional is recommended.

Box 805: UK Property Business Losses Carried Forward

For companies whose main income is rental income from UK property, Box 805 records UK property business losses carried forward to the next accounting period.

If your allowable property expenses — such as repairs, property management fees, and allowable finance costs — exceed your rental income, the net deficit is a property business loss. This is recorded in Box 805 and carried forward to future years, where it can reduce property income when your company is in profit.

Box 805 = current year property loss + any unused property losses brought forward that remain after the current period's offset.

Important distinction: Box 805 (property losses) and Box 780 (trading losses) are entirely separate pools. Trading losses cannot be used to offset property income, and property losses cannot be used to offset trading profits. Each income stream has its own dedicated loss track.

The CT600 Losses Section at a Glance

Here is a summary of the key loss-related boxes across the CT600:

CT600 Box	Description	Applies to
160	Trading losses brought forward applied this period	Trading companies with prior-year losses
250	UK property business losses brought forward	Property companies with prior-year losses
275	Trading losses used against other income in this period	Trading companies with a current-year loss
780	Trading losses of this accounting period	Trading companies making a loss this year
800	Non-trade deficits on loan relationships	Companies with specific financial instruments
805	UK property business losses carried forward	Property companies making a loss this year

How Loss Carry-Forward Works in Practice

When a company makes a trading loss and carries it forward, the process works as follows in subsequent years:

You enter the losses brought forward amount when completing your following year's CT600
The losses are applied to reduce your taxable trading profit
Any remaining losses (if your profit is less than the losses available) carry forward again
The process continues until the losses are fully used

Example:

Year 1: Trading loss of £20,000 — carried forward
Year 2: Trading profit of £12,000 — £12,000 offset by losses; remaining £8,000 carries forward
Year 3: Trading profit of £25,000 — £8,000 of losses used; £17,000 is taxable profit

For Year 3, corporation tax applies only to the £17,000 remaining profit. If that figure falls below £50,000, the small profits rate of 19% applies. Profits above £250,000 would attract the main rate of 25%. This means the timing of when losses are used can be particularly valuable when your profit level is near a threshold.

Tax-Adjusted Profit vs Accounting Profit

One common source of confusion when completing the CT600 is that your tax-adjusted profit (which flows into the losses section) may differ from your accounting profit shown in your profit and loss account.

Some expenses are allowable for accounting purposes but not for tax — for example, depreciation is replaced by capital allowances in the tax computation. Some income items are also treated differently. The CT600 always uses the tax-adjusted figure, not the headline accounting profit.

This distinction means a company can show an accounting profit but a tax loss, or vice versa. Always work from the tax computation rather than the accounts alone when completing loss boxes.

Mixed Companies: Trading and Property Income Together

If your company has both trading income and UK property income, HMRC treats these as entirely separate streams for loss purposes:

Trading losses (Box 780) can only offset trading profits — not property income
Property losses (Box 805) can only offset property income — not trading profits

This means a company with profitable trading but loss-making property reports the property loss in Box 805 to carry forward. It cannot be used to reduce the trading profit immediately.

Filing a CT600 in a Loss Year

A frequently asked question: do you still need to file a CT600 if your company made a loss and owes no tax?

Yes. HMRC requires a CT600 to be filed whenever it issues a notice to deliver. A loss year does not exempt you from the obligation, and late filing penalties still apply if you miss the 12-month deadline.

Completing your CT600 in a loss year is also the formal mechanism by which HMRC accepts your losses on record. Without a filed return, the losses cannot be carried forward.

For more context on upcoming changes to how companies file, see CT600 filing after March 2026.

Summary

Box 800 on the CT600 covers non-trade deficits on loan relationships — a niche area that most small companies will never need. The loss boxes that matter for most companies are Box 780 for trading losses and Box 805 for UK property business losses carried forward. Both allow you to carry losses forward indefinitely to reduce future taxable profits. You must file a CT600 in every year HMRC issues a notice — including years where you make a loss.

CT600 Box 800: Losses Summary