Company Tax Returns, iXBRL and what filing software has to do

acrenvo.com/guides/company-tax-return-ixbrl-software, last reviewed 12 October 2026

Amara is fictional. She runs a small catering company and has always sent its Company Tax Return herself. Now that HMRC’s free service has closed, she has opened HMRC’s list of commercial software. In it she has found a page of products and a word she does not recognise: iXBRL, short for Inline eXtensible Business Reporting Language. Before she looks at company filing software and what it costs, she wants to understand what the return is made of, what format HMRC wants it in, and what she should check on the list. This guide covers those three questions and stops there. Preparing the accounts and the tax computation is a separate job, and it is not covered here.

What a Company Tax Return is made of

It is easy to think of the return as one form, but GOV.UK describes it as a bundle. A Company Tax Return is made up of:

• the form CT600
• any supplementary pages
• company accounts and tax computations

The duty to send it starts with HMRC. If HMRC issues you a notice to deliver a Company Tax Return then you have a legal obligation to submit a return. You must still send a return if you make a loss or have no Corporation Tax to pay.

The return is also where the tax is worked out. When you file your tax return, you work out your:

• profit or loss for Corporation Tax (this is different from the profit or loss shown in your annual accounts)
• Corporation Tax bill

The format HMRC asks for

HMRC has asked for returns online for a long time. Company returns must be filed electronically for any accounting period ending after 31 March 2010. What matters for Amara is the format of the parts that sit alongside the form. The computations and accounts must be in Inline eXtensible Business Reporting Language (iXBRL).

iXBRL is the same format Companies House will require for accounts. Its guidance describes it like this: This is a worldwide technical standard that combines HTML with machine readable ‘tags’. It allows documents to be read by both machines and humans. GOV.UK is also clear about how the return gets there. You must use proprietary third party software, and HMRC’s own list notes that commercial software suppliers also provide services to produce Inline eXtensible Business Reporting Language (iXBRL) accounts and computations.

If your company files micro-entity or small company accounts, the guide to micro-entity and small company accounts after the change explains what iXBRL means for the accounts you send to Companies House.

The computation is the one part where the format is still changing. HMRC’s directions for online returns tie it to the format HMRC publishes: Computations showing the derivation of the return entries from the accounts, must be prepared in accordance with the format published in Corporation Tax technical specifications in XBRL and iXBRL format. Alongside that, HMRC is developing a standardised format for the computation. It has published the first specifications, for accounts adjustments and capital allowances, but it is not asking for the new format yet. In HMRC’s words: We will not require computations to be submitted in this format until phase 2 of the work to develop the requirements has been completed, and a timetable for the changes has been confirmed. No timetable has been published, so for now Amara has no date to plan around.

What to check on HMRC’s list

HMRC’s list does not rank products or show prices, and being on it means HMRC will take a return the product sends. HMRC accepts returns filed using any of the products shown in this document. It does not mean every product does every part. These commercial software suppliers can produce one or more elements of a Company Tax Return with HMRC. You should consider which elements you need and make sure the software you choose allows you to file:

• a CT600
• a Corporation Tax computation
• your company accounts

• HMRC’s list of commercial software for Company Tax Returns

Sending the return

Whoever sends the return confirms it is right. The person making the return must include a declaration that it is correct and complete to the best of their knowledge and belief. Once it is sent, HMRC acknowledges it. When you submit your online return to HMRC you will receive an acknowledgment of receipt. The acknowledgment does not mean that HMRC has agreed the figures provided in the return.

Deadlines and late filing penalties

The return and the tax have separate deadlines, and the tax comes first. GOV.UK says to pay Corporation Tax 9 months and 1 day after your ‘accounting period’ for Corporation Tax ends, and to file the return 12 months after your accounting period for Corporation Tax ends.

Companies House charges its own penalties for late accounts. The guide to company accounts and Company Tax Return deadlines sets out both bodies’ penalties and how to appeal one.

If you find a mistake after filing, there is a window for putting it right. You must usually make any changes (‘amendments’) within 12 months of the filing deadline. The guide to filing after HMRC’s service closed lists the ways GOV.UK gives for making a change.

Questions about Company Tax Returns and iXBRL

You may also find these pages useful: Ayiza and Easy Digital Filing.