Extended Accounting Period: Why You Have Two CT600s and How They Are Submitted

Why do I have two CT600s? If your company's accounting period is longer than 12 months, you'll need to file two CT600 returns with HMRC. TinyTax handles this automatically — here's how it works.

Why Does This Happen?

HMRC has a strict rule: no corporation tax return can cover more than 12 months. This is set out in the Corporation Tax Act 2009.

Extended periods typically occur when:

• Your company was incorporated mid-year and chooses a different year-end
• You've changed your accounting reference date
• It's your company's first accounting period

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How TinyTax Detects Extended Periods

When you select an accounting period longer than 12 months, TinyTax shows this banner:

The banner includes:

• The total number of days in your period
• The dates for each CT600 return
• How the split works

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How the Split Works

TinyTax automatically splits your extended period into two:

Return	Coverage	Typical Example
CT600 #1	First 12 months	1 Jun 2023 to 31 May 2024 (366 days)
CT600 #2	Remaining period	1 Jun 2024 to 31 Mar 2025 (304 days)

Your profits and allowances are apportioned automatically based on the number of days in each period.

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What You Need to Enter

Here's the good news: you only enter your figures once.

Enter the totals for the entire extended period:

• Total turnover for the whole period
• Total expenses for the whole period
• Balance sheet at the period end

TinyTax calculates the split for each CT600 automatically.

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How Profits Are Apportioned

For a period of 670 days split into 366 + 304 days:

Item	Total	Period 1 (366 days, 55%)	Period 2 (304 days, 45%)
Trading profit	£50,000	£27,313	£22,687
Interest income	£1,000	£546	£454
Tax @ 19%	£9,690	£5,289	£4,401

The apportionment is done on a day count basis - the most accurate method accepted by HMRC.

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What About Accounts?

Unlike CT600 returns, your accounts submission to Companies House covers the entire extended period - no splitting required.

Filing	Period Covered
CT600 #1	First 12 months
CT600 #2	Remaining days
Companies House Accounts	Full extended period

TinyTax generates all three documents automatically.

Reconciling the dashboard view after filing

After an extended-period filing is accepted, your dashboard shows three separate rows — two CT600 rows and one Accounts row — and the figures across them won't sum the obvious way at first glance. That's expected, and it's what HMRC and Companies House each receive:

• The two CT600 rows show apportioned turnover, profit and tax for each of the two split periods. Add the two CT600 rows together to get the totals HMRC has on file for the full year.
• The single Accounts row shows the combined turnover and profit reported to Companies House for the full extended period — equal to the sum of the two CT600 turnovers, give or take rounding.

The total Corporation Tax payable to HMRC is the sum of the two CT600 tax amounts, not the figure on any single row. If you're reconciling against an HMRC statement of liability, add Period 1 + Period 2 to get the full-year total.

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The Submission Process

When you reach the preview page, you'll see both CT600 returns ready:

1. Review Period 1 - Check the apportioned figures for the first 12 months
2. Review Period 2 - Check the apportioned figures for the remaining period
3. Submit CT600 #1 - This goes to HMRC first
4. Submit CT600 #2 - This follows immediately after
5. Submit Accounts - To Companies House (full period)

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Tax Rates Across Periods

If tax rates change during your extended period, TinyTax applies the correct rate to each portion:

Period	Tax Rate	Applies To
Pre-1 April 2023	19%	Profits in this portion
Post-1 April 2023	19-25%	Depends on profit level

The marginal relief calculations are done separately for each CT600, using the apportioned profits and the rates in force during that period.

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Common Questions

"Can I avoid splitting by choosing different dates?"

No. If your accounting period is longer than 12 months, it must be split for corporation tax purposes. This is a legal requirement, not a TinyTax limitation.

However, you can:

• Shorten your accounting period by changing your accounting reference date (Companies House form AA01)
• Choose dates that create a standard 12-month period going forward

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"Why are my two CT600 returns showing different tax amounts?"

This is normal. The apportionment is based on days, so:

• Period 1 (12 months) typically has more profit apportioned
• Period 2 (remaining days) has less profit
• Tax is calculated separately on each portion

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"What if I have losses?"

Losses are also apportioned between periods. If you have a loss in the overall period:

• Each CT600 shows the apportioned loss
• Loss relief is calculated separately for each period
• Losses can be carried forward from Period 1 to Period 2

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"Do I pay tax twice?"

No. You're paying tax on the same profits, just split across two returns. The total tax across both returns equals what you'd pay if HMRC allowed a single return.

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"The per-period figures don't match my own accounts / my accountant's figures"

This is expected, and it's correct. HMRC's standard basis for splitting an extended period is set out in the Corporation Tax Act 2009 (section 52): you compute the result for the whole period once, then apportion it across the two returns on a time basis — by the number of days in each. TinyTax does exactly that.

So if your trade was uneven across the year — busier in one part than another — the apportioned figures on each CT600 will differ from a month-by-month breakdown of what actually happened in each sub-period. That's normal. The totals are identical: add the two periods together and you get back to your accounts.

A few things worth knowing:

• HMRC does not expect two separate actual profit-and-loss accounts for the sub-periods. You prepare one set of accounts for the full period (filed once at Companies House), and the tax result is time-apportioned across the two CT600s. Using the actual figures that fell in each sub-period is only appropriate in the rare case where time-apportionment would give an unreasonable result — an uneven spread of income on its own is exactly what time-apportionment is designed to handle.
• There's usually no difference to the tax. Where one sub-period is a loss, that loss carries forward automatically and covers any profit in the other, so the outcome is the same either way.
• TinyTax always time-apportions income and expenses — there isn't a screen to enter separate actual figures for each sub-period (only capital allowances can be split by actual date — see the capital allowances question above). If your accountant specifically needs to file on the actual, non-apportioned split, that isn't something TinyTax produces.

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"What about capital allowances?"

Capital allowances like the Annual Investment Allowance (AIA) are pro-rated across the two sub-periods:

Period Length	AIA Available
12 months	£1,000,000
6 months	£500,000
18 months (split)	Period 1: £1,000,000 + Period 2: £500,000

By default TinyTax splits your AIA, Writing Down Allowance and balancing charges by day-ratio across the two sub-periods. If your capital expenditure landed mostly in one sub-period, you can override the split:

• AIA — the AIA line shows your total claim with a slider beneath it. Drag the slider, or click either £ amount to type a value directly. The two amounts must sum to your total claim within £1.

• WDA and balancing charges — each pool (main, special rate, zero-emission cars, other) gets the same total + slider treatment, so you can allocate the correct amount to each sub-period:

If you're loading an existing extended-period filing, you may see a banner asking you to reconfirm the per-period split — a one-off check that previously day-ratioed values still match your records.

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"My first period is very long - is that a problem?"

Common for new companies. A first period of 18 months means:

• CT600 #1: First 12 months
• CT600 #2: Remaining 6 months

This is completely normal and TinyTax handles it correctly.

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Extended Period Deadlines

Both CT600 returns have the same filing deadline - 12 months after the end of the full accounting period.

Extended Period	Deadline
1 Jun 2023 to 31 Mar 2025	31 Mar 2026

Payment deadlines may differ - corporation tax is due 9 months and 1 day after the end of each CT600 period:

• Period 1 payment: 9 months after Period 1 end
• Period 2 payment: 9 months after Period 2 end (which is the full period end)

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Troubleshooting

"The split dates don't look right"

TinyTax always makes the first period exactly 12 months (or 12 calendar months ending on the day before the anniversary of the start date). This is the correct method per HMRC rules.

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"My tax calculation seems too high/low"

Check that you've entered the full period figures, not just one portion. TinyTax needs the complete picture to calculate the split correctly.

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"I've already filed the first CT600 elsewhere — how do I submit only the remaining days?"

If you've already submitted CT600 #1 through another service (for example HMRC's own online filing service), you can use TinyTax to file just CT600 #2. The Period 2 dates won't appear in the period dropdown — you enter them yourself:

1. Start the return as normal (Start Filing → CT600 only, or Edit Form from the row's menu if you've already started it)
2. On the form, just under the Accounting Period, click Need to file for different dates? and pick Part of a longer period of account — not "Other reason". This tells TinyTax that your statutory accounts already cover the whole period elsewhere, so it won't generate and attach a conflicting short-period set of accounts to this return. (Picking "Other reason" here attaches a fabricated short-period accounts set and ticks the wrong accounts-period box on the CT600 form.)
3. Enter the Period 2 dates only — the start date is the day after CT600 #1's end date, and the end date is your full period end. The extended-period notice disappears, because you're now filing a single short return
4. Enter just the figures for that shorter period — the amounts not already reported on CT600 #1
5. Submit as normal

Because this return covers a different period from the one already filed, HMRC treats it as a new original return — it will not clash with your earlier submission.

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"One of my two CT600 returns was rejected by HMRC"

When you file an extended period, TinyTax sends two CT600 returns to HMRC — one for each half of the period. They are independent submissions. HMRC can accept one and reject the other.

> First, check the rejection message. If the short half was rejected with Error 3001 "original return already received for this period", do not resubmit or amend it — re-filing just repeats the error. This is a known split-period pattern: your main return was accepted, and HMRC's records already show the short period. See HMRC Error 3001 ("Split Period" section) for what to do. The steps below apply to rejections for other reasons (figure or document errors), where correcting and resubmitting the half works.

On the dashboard:

• The merged row for the period shows Rejected if either half failed.
• The Actions menu exposes Review & Submit / Edit Form for the rejected half. The accepted half stays accepted at HMRC — you do not need to refile it.

To resubmit the rejected half:

1. Open the merged 2023 (or relevant year) row on your dashboard.
2. Use Edit Form if you want to correct figures, then continue through to Review & Submit.
3. Click Submit. Only the previously-rejected half is sent to HMRC — the accepted half is skipped automatically, so there is no risk of a duplicate filing.

If the dashboard does not show a Resubmit option and instead offers Amend CT600, that is a stale state from before our 2026-05 fix. Reply on your support thread with a screenshot and we will reopen the rejected half for you.

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Can I Use Zero Income for One Period if All Activity Was in the Other?

If your income and costs genuinely all fell in the first sub-period — for example, your VAT returns for the short second period show zero — you may use actual results rather than time-apportionment. Under this method you report what actually happened in each period: zero income for the second period, and all figures in the first.

A few important points:

• Actual results must genuinely reflect activity. You cannot choose a split arbitrarily or for tax convenience — the figures need to correspond to what really happened in each period. Supporting evidence (such as VAT returns, management accounts, or bank statements) should confirm the picture.
• Time-apportionment is the default and is always acceptable. If you are unsure, HMRC will always accept time-apportionment — you never need to justify choosing the default method.
• TinyTax automatically uses time-apportionment. When TinyTax splits your extended period into two CT600s, it apportions your entered figures by the number of days in each period. If you want to use actual results instead, you should either adjust the figures manually in the form, or (where the two periods were set up separately as original + amendment) enter the actual figures for each period independently.
• Professional judgment applies. The choice between time-apportionment and actual results is ultimately an accounting decision. If the amounts are significant or the position is unusual, discuss it with your accountant before submitting.

"Why is Box 3 (UTR / Tax Reference) blank in the downloaded CT600 PDF?"

The downloaded draft CT600 is generated before you enter your HMRC credentials, so Box 3 shows blank in the preview. Your UTR is entered at submission time via the credentials popup — once you submit, it is included correctly in the electronic CT600 XML sent to HMRC. There is nothing you need to do to pre-fill Box 3 in the draft; the submitted version will contain it.

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"Why is Box 50 ticked on both CT600 returns? Is it supposed to say 'Making more than one return for this company now'?"

Yes — Box 50 is automatically ticked on both CT600 returns in a split-period filing. HMRC requires this checkbox whenever two returns are submitted simultaneously for the same company; it signals to HMRC's system that a sibling return exists. This is correct and mandatory.

Box 50 appears near Box 55 ("This return contains estimated figures") on page 1 of the CT600. TinyTax does not tick Box 55 — the apportioned figures in a split-period return are a time-based calculation, not provisional estimates. If you see a single ticked checkbox near the bottom of page 1, it is Box 50.

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"Why does Box 85 ('I attach accounts and computations for a different period') appear instead of Box 80?"

Box 85 is the correct treatment for split-period returns. Your statutory accounts cover the full extended accounting period (for example, 13 months from July 2024 to July 2025), while each CT600 covers only a sub-period of up to 12 months. Box 85 confirms to HMRC that the attached iXBRL accounts were prepared for a longer period than the CT600 being submitted. Box 80 ("I attach accounts for the period to which this return relates") is only correct when the accounts and CT600 cover exactly the same dates — which never applies to a split-period filing. TinyTax sets Box 85 automatically.

Still Have Questions?

Extended periods can be confusing. If you're unsure about your specific situation, we're here to help.

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Last updated: February 2026