What is a Corporation Tax Self Assessment in 2026/27?

For corporate practitioners operating in the 2026/27 financial year, the compliance burden on companies requires strict navigation of overlapping statutory deadlines, updated tax rates, and mandatory digital software requirements. When advising corporate clients, clarifying exactly what a Corporation Tax Self Assessment (CTSA) entails is foundational to keeping them compliant.

The CTSA regime places the legal responsibility squarely on the company to calculate, report, and pay its own tax liability without waiting for HMRC to issue a demand. This guide breaks down the statutory definition, the 2026/27 tax rates, the dual deadlines, and the crucial April 2026 software mandate you must implement.

The Legal Definition of a CTSA

Under the Corporation Tax Self Assessment regime, a company must proactively quantify its own tax burden. Schedule 18 to the Finance Act 1998 strictly defines this requirement.

Every company tax return for an accounting period must include an assessment (a ‘self-assessment’) of the amount of tax which is payable by the company for that period.

The company must deliver this self-assessment on the standard CT600 return form.  Submitting this return creates a legally binding charge to tax.  HMRC does not issue a preliminary determination to finalise the figure; instead, the self-assessment stands unless HMRC opens a formal compliance enquiry into the return.

Calculating the CTSA Liability: 2026/27 Rates

Before submitting the CTSA, you must calculate the tax due using the rates active for the relevant financial year. For the 2026/27 period (Financial Year 2026), the government has maintained the tiered Corporation Tax structure.

Under section 13 of the Finance Act 2025, the main rate of Corporation Tax is 25%.  However, section 3 of the Corporation Tax Act 2010 subjects this main rate to a tiered relief system based on the company’s augmented profits:

• Small Profits Rate (19%): Companies with augmented profits below £50,000 calculate their CTSA using a 19% rate.
• Marginal Relief: If a company’s profits fall between the £50,000 lower limit and the £250,000 upper limit, you charge the 25% main rate but deduct marginal relief, creating a blended effective rate.
• Main Rate (25%): Companies with profits exceeding £250,000 calculate their entire liability at the strict 25% rate.

Dual Deadlines: Payment vs. Filing

A defining characteristic of a Corporation Tax Self Assessment is the misalignment between the payment deadline and the filing deadline. You must manage two distinct dates.

1. The Payment Deadline (9 Months and 1 Day)

For standard companies, the statutory due date to pay the Corporation Tax liability arrives before the deadline to file the return. Corporation Tax is normally due and payable exactly nine months and one day from the end of the accounting period.  Example: If a company’s accounting period ends on 31 March 2026, you must ensure the company pays its self-assessed tax by 1 January 2027.

2. The Filing Deadline (12 Months)

Under paragraph 14(1) of Schedule 18 to the Finance Act 1998, the statutory filing date for the CT600 company tax return is generally twelve months from the end of the period for which the return is made.  Example: Following the previous example, while the tax payment was due on 1 January 2027, you have until 31 March 2027 to actually file the CT600 return.

The April 2026 Shift: Software Mandates and Penalty Increases

If you are filing a Corporation Tax Self Assessment in the 2026/27 tax year, you must adhere to strict new technological and penalty frameworks.

Mandatory Commercial Software: HMRC has closed its free “File your accounts and Company Tax Return” (CATO) online service.  Consequently, you can no longer manage a company’s statutory filings using standard free government portals. From April 2026, directors and practitioners must use third-party commercial software to submit the CT600 return and accounts in the mandatory iXBRL format.

Doubled Late Filing Penalties: Failing to submit the CTSA by the 12-month filing deadline now triggers severe financial consequences. From 1 April 2026, HMRC doubled the late filing penalties for Corporation Tax.

• The initial flat-rate penalty for missing the deadline increased from £100 to £200.
• If the company fails to file on time for a third successive accounting period, the flat-rate penalty jumps to £1,000.

Summary Table: CTSA Elements for 2026/27

For completeness, audit your clients’ current accounting software immediately to ensure it supports the new mandatory iXBRL formats required for the April 2026 filing mandate, preventing any accidental exposure to the newly doubled £200 late filing penalty.