Self-assessment for individuals:

Obligation	Deadline
Notification of chargeability (new source of income)	By 5 October following the end of the tax year in which the income first arises
Filing the tax return — paper	By 31 October following the end of the tax year
Filing the tax return — online	By 31 January following the end of the tax year
Payment of balancing payment	By 31 January following the end of the tax year
Payments on account (POAs)	31 January in the tax year and 31 July following the tax year (each = 50% of prior year's income tax + Class 4 NIC liability, less tax deducted at source)

Penalties for late filing:

1 day late: £100 fixed penalty (even if no tax is owed)
3 months late: Daily penalty of £10/day for up to 90 days (maximum £900)
6 months late: 5% of the tax due or £300 (whichever is greater)
12 months late: Further 5% of tax due or £300 (whichever is greater). If deliberate concealment: up to 100% of the tax due.

Penalties for late payment:

Penalties for inaccuracies in returns:

HMRC compliance checks (enquiries):

Enquiry window: HMRC can open an enquiry into a return within 12 months of the filing date (or of the date the return was actually filed, if late). For returns filed on time by 31 January: enquiry window closes 31 January the following year.
Discovery assessments: If HMRC discovers that tax has been lost due to careless or deliberate behaviour, they can raise an assessment outside the normal enquiry window — up to 4 years (careless), 6 years (careless — extended), or 20 years (deliberate) from the end of the tax year.

Taxpayer rights:

Right to appeal against an HMRC decision (to HMRC first, then to the First-tier Tribunal — Tax Chamber, then Upper Tribunal, Court of Appeal, Supreme Court)
Right to a review by a different HMRC officer before appealing to the tribunal
Record-keeping: Individuals must keep records for 5 years from 31 January after the tax year (self-employed: 5 years from the filing date). Companies: 6 years from the end of the accounting period.

Principles of Taxation