I hit this on a client's return earlier this year. New plant, bought after New Year, plainly qualifying for the 40% first-year allowance that had been law since 1 January. I went to enter the claim and found the CT600 had nowhere to put it. That is still true today, and it stays true until April 2027.
Disclosure up front: I work at Xero. Read this as field notes from someone who uses the platform daily, not a neutral review.
TL;DR: The CT600 is running behind the statute book, and HMRC's fix is to file in a general box now and correct it later. Claim the 40% allowance in box 725 or 750, with box 760 for the expenditure. The bigger problem is section 455, where a correctly filed return has to be amended after April 2027.
Where do you claim the 40% first-year allowance on a CT600?
In box 725 or box 750 for the claim amount, and box 760 for the qualifying expenditure. That is HMRC's own instruction, not a workaround I invented: the guidance tells filers to "use: boxes 725 or 750 for claim amounts, box 760 for qualifying expenditure" and confirms that HMRC "will update the Corporation Tax online service in April 2027 to support the new allowance by including 2 new boxes on form CT600" (GOV.UK, 2026).
So the claim has a home. It just isn't a home that says what it is. Boxes 725 and 750 are the general first-year allowance boxes, which means a 40% main-rate claim currently sits in the same space as every other FYA a company might make.
What is the 40% first-year allowance, and when did it start?
It runs at 40% on new and unused plant or machinery that would otherwise get the main rate of writing-down allowance, for expenditure incurred on or after 1 January 2026. It sits in section 45U of the Capital Allowances Act 2001, excludes cars, and excludes second-hand assets (HMRC Capital Allowances Manual CA23195A, 2026). The remaining 60% drops into the main pool.
One date correction worth making, because I've seen it wrong in a few places: the 1 January 2026 start applies to income tax as well as corporation tax. It is the writing-down allowance cut, main pool from 18% to 14%, that splits on the usual lines of 1 April 2026 for companies and 6 April 2026 for unincorporated businesses (Deloitte TaxScape, 2026). If you want the rates in full, the firm's 2026 capital allowances explainer covers them properly and I'm not going to restate it here.
Why does a missing box matter if HMRC has told you what to do?
Because a box is a statement, and a general box is a quieter statement than a specific one. When a designated box exists and a return doesn't use it, that difference is visible. When it doesn't exist yet, every claim looks the same from the outside, including the ones that are wrong.
The practical risk is on the record-keeping side rather than the tax side. If you claim in 725 today and HMRC opens an enquiry in 2028, the return won't tell anyone which part of that figure was a 40% main-rate claim and which was something else. Your computations and your working papers have to carry that weight instead. I've started writing the split out explicitly in the tax comp rather than leaving it to be inferred from the fixed asset additions, which is roughly the same instinct that made me write what I check in an old accountant's files before taking a client on.
The section 455 lag is the one that actually bites
Here the timing problem stops being cosmetic. The section 455 charge on loans to participators rose from 33.75% to 35.75% for loans made, or benefits conferred, on or after 6 April 2026, and HMRC has confirmed the Corporation Tax online service won't reflect that until 6 April 2027 (ATT, 2026).
Read that twice, because it means a company with an overdrawn director's loan at the new rate that files its return before 6 April 2027 will need to amend it afterwards, not because anyone got it wrong but because the service could not accept the right answer at the time.
That's a genuine cost, and it's worth being clear about who carries it. HMRC's filing service is late, and the work created by that lateness lands on whoever prepares the return. Nobody is compensated for the second pass. The incentive runs the wrong way too, since the party that could have shipped the update sooner is not the party paying for the delay.
Did the EV charge-point allowance end in April 2026?
No. Both the 100% first-year allowance for zero-emission cars and the one for electric vehicle charge-points were extended by a year, to 31 March 2027 for corporation tax and 5 April 2027 for income tax (GOV.UK, 2026).
I flag it because the old end date was April 2026, and plenty of advice given in late 2025 said so in good faith. If you told a client that last year, it has since gone stale.
Which artefact version is your CT software on?
This is the question I would put to a vendor in writing. HMRC publishes the CT600 technical specification as versioned artefacts, and the version currently in the Local Test Service, the Third Party Validation Service and live is V1.994, dated 10 October 2025. V1.995 was published on 25 September 2026 with the status "Awaiting implementation into Local Test Service (LTS), Third Party Validation Service (TPVS) and live" (GOV.UK, 2026).
A specification that isn't yet in the live service isn't something you can file against, whatever your software shows you. Xero Tax, TaxCalc, IRIS and Sage all file CT600s in the UK, and none of them can get ahead of HMRC's live schema. So the useful question is not "does my software handle the 40% allowance" but "which artefact version are you on, and what is your plan for April 2027". Ask it now, while it's a calendar item rather than a deadline.
What I've changed in my own practice
Two things, both dull, both cheap. I keep a list of returns that will need an amendment once the service catches up, sorted by the date the amendment becomes possible rather than the date the return was filed. And I note in the tax computation, in plain words, which allowance a general-box claim represents.
Neither is clever. But the alternative is trusting that in eighteen months I will remember which returns had a section 455 balance at the new rate, and I won't. Same reason I'm careful about who is actually backing up the ledger: the systems are fine until you need them to have remembered something for you.
Frequently Asked Questions
Which box do I use for the 40% first-year allowance right now?
Box 725 or box 750 for the claim amount, and box 760 for the qualifying expenditure. HMRC will add two dedicated boxes to the CT600 when it updates the Corporation Tax online service in April 2027.
I filed a CT600 earlier this year with 40% first-year allowance spend. Do I need to amend it?
Not for the sake of the box, provided the claim amount was right and it went into 725, 750 or 760. What matters is that your computation shows what the figure represents. Amend if the claim itself was wrong or landed somewhere it doesn't belong, not simply because a better box now exists on paper.
Does the 40% allowance apply to sole traders and partnerships?
Yes. The 1 January 2026 commencement applies to income tax payers as well as companies. The separate cut in main-pool writing-down allowances from 18% to 14% is the one that starts on 6 April 2026 for unincorporated businesses.
Can I claim the 40% allowance on a second-hand machine or a car?
No on both. The allowance requires new and unused plant or machinery, and expenditure on the provision of a car is specifically excluded. Special-rate expenditure is also out, though it may qualify for the annual investment allowance or the 50% special-rate first-year allowance instead.
What happens if my section 455 charge is at the new 35.75% rate and I file before April 2027?
You file at what the service will accept, then amend the return after 6 April 2027 once it supports the higher rate. Diarise it at the point of filing. An amendment you have planned for is administration, and one you discover late is a penalty conversation.
If you have bought plant this year and you're not certain the claim has been recorded in a way that will still make sense in 2028, send me the computation and I will tell you what I would change. Same offer if you have a director's loan sitting at the new section 455 rate and want the amendment diarised properly.