Attaching supporting documents to your CT600: what and how

In short: Yes. You can attach PDF documents to an online Company Tax Return (CT600), but only as extra information. HMRC wants the computation, and most companies' accounts, in iXBRL, with the explanation of the figures inside the computation (GOV.UK: Completing your Company Tax Return). Attach what HMRC must see, such as a signed group relief consent notice.

Take the 30-second check to see whether Taxley fits your company, or see why accounts go to HMRC as iXBRL, not PDF.

Key facts (checked on 30 September 2026)

Can you attach documents to a CT600 in 2026?

Yes. HMRC's online service accepts PDF files as "other" attachments to a Company Tax Return, next to the iXBRL accounts and computation, and refuses any other file type for them (error 1604 in HMRC's Corporation Tax online service validation rules). A PDF adds information to the return. It never replaces the computation, the accounts or a box.

HMRC's validation rules also look inside each file. Error 1605 rejects a file whose content doesn't match its type, such as a Word document renamed as a PDF, and error 5016 rejects an attachment that contains a virus. Error 1614 caps the whole submission, including every attachment, at 25 MB (HMRC: Corporation Tax online service validation rules). HMRC's online service guidance published on the same page says that a return can include only one iXBRL accounts file and one iXBRL computations file, and that all other documents forming part of the return must be PDF files. The computation rule matters most in practice: HMRC's guide to completing the return asks you to include all the information needed to explain the figures in the return within the single iXBRL computations file (GOV.UK: Completing your Company Tax Return). A PDF attachment therefore suits a document that has to exist separately, such as a signed notice, rather than workings that belong in the computation.

What should you attach to a CT600, and what should you leave out?

Attach a PDF only when HMRC needs to see a separate document with the return. A signed group relief consent notice is the clearest case; workings or a note that the computation can't carry come next. Leave out the accounts and the computation, R&D claim details, and records such as bank statements that HMRC hasn't asked for.

Taxley's decision table: which documents to attach as a PDF

An explanatory note is worth attaching when the figures would otherwise leave an HMRC officer with an obvious question, for example a one-off receipt or a position taken on an uncertain point. The law treats information contained in documents accompanying a return as made available to HMRC, which is one of the tests for whether HMRC can make a later "discovery" assessment once its time to open an enquiry has passed (FA 1998 Sch 18 para 44). HMRC's guide to completing the return also asks you to attach calculations of each chargeable gain and allowable loss, with full details of any claims or elections, whenever boxes 210, 215 or 825 are used (GOV.UK: Completing your Company Tax Return). Bank statements, invoices and documents full of personal details are different: keep them with the company's records, which HMRC may check in a compliance check (GOV.UK: Running a limited company).

How does a group relief consent notice reach HMRC?

As a PDF attached to the claimant's return. Every group relief claim needs the surrendering company's consent, given in a written notice to HMRC at or before the claim, and the claim is ineffective unless a copy of that notice accompanies it (FA 1998 Sch 18 para 70). Attaching the signed notice to the CT600 making the claim sends that copy.

Every notice must give the names of both companies, the amount surrendered, the surrendering company's accounting period that the surrender relates to, and both companies' tax references (their UTRs), or it is ineffective; it can't be amended, only withdrawn and replaced by a new notice (FA 1998 Sch 18 para 71). For carried-forward losses, that accounting period is the one the losses were carried forward to. A notice for carried-forward losses must also identify each loss surrendered by the provision of CTA 2009 or CTA 2010 it was carried forward under, such as CTA 2010 s.45A(4) for a trade loss. Where the surrendering company is owned by a consortium, the notice must also give the accounting period each loss is attributable to. A notice without these details is ineffective (FA 1998 Sch 18 para 71A).

HMRC's manual adds that a company in a simplified arrangement can claim without a copy notice if the authorised company countersigns the claimant's return (HMRC manual CTM97020). If the surrendering company has already had relief for the losses it gives up, it must amend its own return at the same time to reflect the notice, or the notice is ineffective (FA 1998 Sch 18 para 72). In Taxley, choose the signed notice in the Group relief section and Taxley produces a printable notice, filled in from the claim. For carried-forward losses, it lists each loss with the provision it was carried forward under and the period it was made in. Check it against those details before the other company signs. You attach the signed PDF under Supporting documents and choose it against that claim. Taxley won't take payment while any such claim lacks its notice.

How do you attach a supporting document to your CT600, step by step?

Six steps take about 15 minutes once the document exists: decide it belongs with the return, save it as an unlocked PDF, give it a plain name, attach it, link any group relief notice, then check the list before you file. HMRC accepts supporting PDFs as "other" attachments (HMRC: CT online validation rules).

Step 1: Decide whether the document belongs with the return

Run the document through the decision table: attach it only if HMRC needs to see it with this return, such as a signed consent notice or workings the computation can't carry. Don't attach the accounts or the tax computation, which go to HMRC as iXBRL (HMRC manual COM60040), or R&D project details, which go on HMRC's separate additional information form before or on the same day as the return (GOV.UK: R&D additional information). Some companies must also send an R&D claim notification online within 6 months of the end of the period of account, never as an attachment (GOV.UK: R&D claim notification). Keep bank statements and invoices with the company's records instead.

Step 2: Save it as an unlocked PDF of 5 MB or less

Export or print the document to PDF from the program that made it, because HMRC rejects a file whose content doesn't match its type, such as a renamed Word document (error 1605 in HMRC's validation rules). Remove any password: HMRC's manual tells staff to view and print each attachment individually (HMRC manual COM132067), and Taxley doesn't check for passwords. Taxley accepts PDFs of up to 5 MB each. Keep the combined size to a few megabytes, because HMRC's 25 MB limit covers the whole return, including the accounts and computation.

Step 3: Give the file a plain name

Rename the file using only letters, numbers, spaces, hyphens, underscores and full stops, for example "Consent notice Acme Ltd 2025.pdf". HMRC's CT600 schema allows only unaccented letters, numbers, spaces and some simple punctuation in an attachment's file name and description. It rejects £, #, $, €, ~, |, square brackets, accented letters, curly apostrophes and long dashes, and file names can't contain slashes, colons, asterisks, quotation marks or angle brackets (HMRC: CT600 RIM artefacts). Taxley checks the return against that schema before you pay, and a failing name stops it with a message that it did not pass HMRC's CT600 format check. Write amounts in the description as "12,000 pounds", not with a £ sign.

Step 4: Attach it under Supporting documents

On the return's filing page, find Supporting documents, just above HMRC submission. Choose the PDF, add an optional description of up to 200 characters saying what it is and why HMRC needs it, then press Attach PDF. Taxley stores the file encrypted and lists it with its name, size and description. A return can carry up to five supporting documents; a sixth is refused with a message that you can attach at most 5. HMRC's rules accept PDF only for these attachments (HMRC: CT online validation rules).

Step 5: Link a group relief consent notice to its claim

If the return claims group relief under a signed notice, open the Group relief section, choose "It signed a notice of consent", and use "Print a notice of consent to sign" to produce a notice filled in from the claim. For carried-forward losses, first list each loss under "The losses it's giving up", because the notice prints them. Once the other company has signed it, attach the signed PDF as in Step 4, then choose it under "Its signed notice of consent" for that claim. The copy must accompany the claim, or the claim is ineffective (FA 1998 Sch 18 para 70). Skip this step when the return claims no group relief.

Step 6: Check the documents are listed before you file

Check that each PDF appears in the Supporting documents list with the right name, size and description, and use Remove to take out anything that shouldn't go. Success looks like a short list of plainly named PDFs, none of them accounts or computations, and a Group relief section with no notice missing. Once the return has been sent to HMRC, the section reads "This filing can no longer be edited", and the PDF copy of the CT600 lists each supporting document by file name.

What happens to your supporting documents after you file?

They stay part of the return HMRC holds. HMRC's manual tells its staff to view and print each attachment individually when they need a paper copy, for example to carry out a risk assessment or pre-enquiry review (HMRC manual COM132067). Attaching a document doesn't replace keeping the company's records.

A company must keep the records it needs to deliver a correct and complete return, and preserve them until the sixth anniversary of the end of the period, or later while an enquiry is open (FA 1998 Sch 18 para 21). GOV.UK states the same 6-year rule and lists cases where records must be kept longer, such as a late return or a compliance check (GOV.UK: Running a limited company). Keep the originals of anything you attach, including a signed consent notice. To add, remove or replace a document once HMRC has the return, amend the return; amendments are usually made within 12 months of the filing deadline (GOV.UK: Company Tax Returns). An amendment in Taxley starts with copies of the accepted return's PDFs, which you can change before sending it. Read how to correct a CT600 after filing for the rest of that process.

Can Taxley send supporting documents with your CT600?

Yes, within limits. Taxley (taxley.co.uk), UK online software that prepares and files the Company Tax Return (CT600) with HMRC and the annual accounts with Companies House, sends up to five PDFs of up to 5 MB each inside the return, alongside the iXBRL accounts and computation it prepares (HMRC manual COM60040).

Taxley stores each document encrypted and sends it to HMRC only, never to Companies House with the accounts. It embeds the PDFs in the CT600 as "other" attachments with their file names and descriptions, and a period of account longer than 12 months, which becomes two returns, carries the same documents on both. The group relief flow produces a printable consent notice for signing and won't take payment until each claim relying on a notice has one attached. In agent mode, adding or removing a document after the client has approved the return voids that approval. Taxley checks each file's type and size, but not passwords or the combined size against HMRC's 25 MB limit, which five files at the 5 MB maximum would reach on their own (HMRC: CT online validation rules), so check those yourself. It doesn't support R&D claims. Take the 30-second check to see whether Taxley fits your company, or read how to file a CT600 yourself.

Frequently asked questions

Can I attach my company accounts as a PDF instead of iXBRL?

Not for most companies. HMRC requires the accounts and computations in an online return to be iXBRL (HMRC manual COM60040). Its format guidance accepts PDF accounts only in listed cases, such as accounts under a standard with no supported taxonomy (HMRC: format for accounts).

Do supporting documents go to Companies House too?

No. Supporting documents are part of the Company Tax Return, which goes to HMRC (GOV.UK: Company Tax Returns). Companies House gets the company's annual accounts, and Taxley files those there without any of the PDFs you attach to the CT600.

Will HMRC read every document I attach?

HMRC doesn't say it reads every attachment. Its manual tells staff to view and print each attachment individually when they need a paper copy, for example for a risk assessment or pre-enquiry review (HMRC manual COM132067). Write each attachment so it makes sense on its own.

Can I add a document after the CT600 has been filed?

Only by amending the return, usually within 12 months of the filing deadline (GOV.UK: Company Tax Returns). In Taxley, an amendment starts with copies of the PDFs you filed, so you can add, remove or replace documents before sending the amended return.

What happens if my attachments take the return over 25 MB?

HMRC rejects the return with error 1614 and asks you to reduce the size of the attachments or delete some before resubmitting (HMRC: CT online validation rules). Compress large scans or split out pages HMRC doesn't need.

General information, not personalised tax or accounting advice.