Yearly Dates
5 April. This is the official end of the tax year, with the new tax year starting on 6 April.
31 October. If you're still filing paper self-assessment returns, this is the last day to file for the tax year that ended on 5 April.
31 January. This is the last day to file your online self-assessment return for the previous tax year. It is also the deadline to pay any tax that you owe to HMRC, regardless of the method you used to file.
If you need to revise and resubmit your self-assessment from the previous tax year, it must be filed now as well. For example, if you need to revise your submission from the tax year ending 5 April 2018, it should have been filed by 31 January 2020. It is still possible to make corrections after this deadline has passed, but it must be done in writing to HMRC.
Monthly Dates
The 5th. The end of the CIS and payroll month.
The 19th. If you use subcontractors in the construction industry, this is your CIS filing deadline. If you pay CIS or PAYE by cheque through the post, this is also your payment deadline.
The 22nd. If you don't pay by cheque, this is the PAYE and CIS payment deadline.
Deadlines
14 days. Your EC Sales List is due 14 days after the end of each month if you file a paper version.
21 days. If you're submitting your ESL online, you have some extra time.
1 calendar month + 7 days. VAT returns must be submitted after each quarter, but you'll have this much time to get them ready for submission - even if you've got nothing to report.
9 months. If you run a limited company, you have 9 months after your business year end to file your annual accounts at Companies House.
9 months + 1 day. This is the deadline for paying your limited company corporation tax.
12 months. This is technically the deadline for filing your limited company tax return, but since you've got to pay your corporation tax bill after nine months and a day, that effectively becomes your real deadline.
Immediate Filings
Capital gains should be reported immediately through the 'real time' Capital Gains tax service provided by HMRC.
Additionally, RTI returns must be filed each payday for those of you running a payroll.