We cannot work out the Corporation Tax for this return yet: what each message means

Short version. The “We cannot work out the Corporation Tax for this return yet” panel is a decision, not an error. It appears when we refuse to compute a figure we cannot check. Nothing is lost when it appears. Your figures stay saved, and the live estimate returns the moment the cause is fixed. The same rules block submission, so each message below is also telling you what would stop you filing. Find your message, read why we show it, and apply the fix.

Your AIA claim is above the limit

“The claim is declared as all Annual Investment Allowance, but the amount is above the AIA limit for this accounting period. The whole amount cannot be AIA, and this product cannot calculate or report the remainder as a writing-down allowance without the pool figures.” The panel shows your amount and the limit for your exact dates.

Why. The Annual Investment Allowance has an annual limit, and a claim above it is refused rather than capped. The excess would be a writing down allowance computed from pool figures we do not hold, so capping the claim would file a claim you never made.

Short accounting periods have a reduced limit. For whole calendar months, HMRC accepts daily or monthly calculations. Its January to June 2023 example allows £500,000 using six twelfths of £1 million. Other period shapes need a daily calculation. A period crossing a rate change also has transitional expenditure restrictions. Our date-based check is an outer ceiling, not proof that every item qualifies. Enter your calculated claim, not a suggested maximum. We show guidance rather than asking for an extra confirmation or expenditure breakdown. You remain responsible for applying the transitional restrictions in your calculation. See HMRC's AIA examples and timing rules.

The fix. Check the amount first, because the usual cause is a typo such as an extra zero. If the figure is wrong, correct it in the wizard and the estimate returns. If the company genuinely spent more than the limit on qualifying equipment, the return needs pool computations we do not produce, so please use an accountant for this period. Our capital allowances article explains what belongs in the AIA field.

A trade and a property business, one claim

“Company carries on both a trade and a property business and has a capital-allowances claim that has not been confirmed as belonging wholly to the trade. We file one figure against the trade, which can overstate the tax if part of it relates to the property business.”

Why. A trade claim and a property-business claim go in different boxes on the Corporation Tax return, and the split changes the tax. We do not guess the split for you.

The fix. Answer the “Does the whole claim belong to the trade?” question in the wizard. If part of the claim relates to the property business, see capital allowances for landlords for how the two claims are kept apart.

An old draft still shows a whole-period message

“A structures and buildings allowance accrues only while the building is in qualifying use, and we have not been told it was in qualifying use for the whole period. We cannot check a part-period claim, so we cannot file this return.”

Why. This message comes from returns saved under our older structures and buildings questions, which asked you for the allowance amount and a whole-period use confirmation. We now work the allowance out from the dates on your allowance statement instead, and part-year use is handled by the calculation, so the question no longer exists for new claims.

The fix. Open the return in the wizard. Your earlier figures are carried into the new structures and buildings entry, and we ask for the statement dates the old questions never collected. Add the dates and the claim files, with the part-year arithmetic done for you. See the structures and buildings allowance article for how part-year use and empty spells work now.

Your figure uses the enhanced 10% special tax site rate

“The enhanced 10% special tax site allowance needs the CT600M supplementary pages with the return, and we do not produce those pages, so we cannot file this return. Your claim may be valid, but we have not checked it and cannot file it here, so please ask an accountant rather than reducing it to the ordinary rate to get past this.”

Why. This one is not about your figures being wrong. It is about the form. A structure in a genuinely qualifying special tax site can earn 10% a year instead of the ordinary 2%/3%, and HMRC requires an extra supplementary page, CT600M, with any return that claims it. We do not generate that page, so filing your return through us would send an incomplete claim. You will see this message because you chose the enhanced rate on that building’s rate question, not merely because the building is inside a tax site’s boundary. Location alone does not create the enhanced claim: HMRC also needs the construction, first use and spending to have happened while the site was designated.

The fix. Take the return to an accountant. What matters most here is what not to do: do not reduce your claim to the ordinary rate to get past the message. Until 4 August 2026 this claim was refused for being over the ordinary rate, which invited exactly that, and it would have cost you two thirds of the relief. If your claim is actually the ordinary 2%/3% amount, and the building only happens to be inside a tax site, choose the ordinary rate instead. See the structures and buildings allowance article for the full conditions the enhanced rate needs.

A first period over 12 months with a claim

“This accounting period is filed as two Corporation Tax returns, and you have not yet confirmed how your capital allowances divide between them. You can confirm the split in the tax adjustments step of your return, which shows both returns with a suggested division you can change. Do not simply leave the claim out, because unclaimed allowances can be lost.”

Why. A company’s first accounts often cover more than twelve months, and HMRC requires two Corporation Tax returns for that year. An allowance belongs to the return whose dates cover the purchase, and a balancing charge to the return covering the sale. We do not hold your invoice dates, so we cannot place them for you. You can, and that is all this message is asking for.

The fix. Go back to the tax adjustments step. Under your capital allowance figures you will see both return periods with their dates, and two boxes for each amount you entered. We prefill a split based on the number of days in each return, which is only a starting point. Check your purchase and sale dates, change the amounts to match, then confirm. The two boxes on each row have to add up to the total you entered. See what to enter for capital allowances for a walkthrough with a screenshot, and filing a long first accounting period for how the two returns work.

Do not delete the claim to get past this message. The Annual Investment Allowance can only be claimed in the period of the expenditure, and an unclaimed structures and buildings allowance can be lost for good, as HMRC explains in CA94650. Removing the claim would clear the panel and cost the company relief it is entitled to.

The structures and buildings allowance is not part of this split. You do not divide it, because we calculate each return’s own share from the dates on the allowance statement, and the building cost is disclosed once on the first return the claim accrues in. Only the amounts you entered yourself, such as equipment allowances and balancing charges, need the confirmed split.

The same message for a charitable donation

“This accounting period is filed as two Corporation Tax returns, and you have not yet confirmed which return your charitable donation belongs to. A donation goes on the return covering the date you paid it. You can confirm this under the Charitable donations field in the tax adjustments step, which shows both returns. Do not leave the donation out, because the relief cannot be carried to another year.”

Why. A company deducts a charitable donation in the accounting period it was paid in, not spread across the year. A long period is two accounting periods, so the donation belongs to whichever return covers the payment date. We do not hold that date, so we ask you.

The fix. In the tax adjustments step, directly under the Charitable donations field, a panel shows both returns with their dates and a box for each. We start with the whole donation in the first return, because most long periods are twelve months and a few weeks. If any of it was paid after the first return ends, move that part to the second return. If you give regularly, add up the payments whose dates fall inside each return. Then confirm the split. See charitable donations and Corporation Tax relief for the full rules.

The same message for dividends, gains and capital losses

“This accounting period is filed as two Corporation Tax returns, and you have not yet confirmed which return your dividends received, chargeable gains and capital losses belong to. You can confirm the division under the Dividends received and Chargeable gains fields of your return. Each shows both returns with a suggested split you can change.”

Why. A dividend is received on a day, and a gain or a capital loss arises on the day of the disposal. A long period is two accounting periods, so each amount belongs to whichever return covers its date. We do not hold those dates, so we ask you.

The fix. Each amount has its own panel directly under the field where you entered it. Dividends received are divided under the Dividends received field in the profit and loss section. Chargeable gains and capital losses are divided under the Chargeable gains fields in the tax adjustments step. For dividends and gains we prefill a split by the number of days in each return, and for a capital loss we start with the whole amount in the first return. Check the actual dates, change the amounts to match, then confirm each division. A loss from a disposal in the second return cannot reduce a gain in the first.

Stops we have retired

Older versions of the service stopped some capital-allowance claims with messages you can no longer see. A property company’s claim was blocked until you answered a question about what the equipment is used for, and a plant claim inside a let home was refused outright. A 100% low-emission car claim was also refused for a period outside the relief’s dates.

Those stops no longer exist. We now file the capital-allowance figures you supply, and we do not confirm that a claim qualifies, so the checking is yours to do before you enter a figure. The eligibility rules, including HMRC’s dwelling rule for landlords, are explained in capital allowances for landlords, and the low-emission car dates are covered in company cars and capital allowances.

Other messages

Three further messages appear less often:

After you fix it

The estimate recomputes live as you edit, so there is nothing to resubmit and no button to press. The moment the cause is fixed, the panel disappears and the Corporation Tax figure comes back on its own.

Messages that appear at the filing stage, after you send the return to HMRC, are a different thing with a different page. See CT600 filing errors for those.

This article is general guidance, not tax advice. Where a message points you to an accountant, that is a genuine boundary of what we can check rather than a formality, and an accountant can file the claim we cannot. If you are not sure which message you are seeing, get in touch and we will look at it with you.