Self Assessment deadlines: a complete guide for agents

Self Assessment remains one of the highest-risk areas for client penalties — both for late filing and late payment. As an agent, understanding the full deadline structure is essential for managing client expectations and avoiding avoidable penalties.

Filing deadlines

31 October — Paper Self Assessment return deadline for the previous tax year
31 January — Online Self Assessment return deadline. Also the deadline for paying any tax owed for the previous year.

Almost all agent-filed returns are submitted online. The 31 January deadline is effectively the universal deadline for agent-managed clients.

Payments on account

Clients whose tax bill exceeds £1,000 (and where less than 80% was deducted at source) must make payments on account — advance payments toward the following year's liability:

31 January — First payment on account (50% of prior year's tax bill)
31 July — Second payment on account (the other 50%)

The balancing payment (final tax owed minus payments on account already made) is also due on 31 January.

⚠️Missing the 31 July payment on account is one of the most common errors. Clients often don't realise a payment is due in summer — they associate Self Assessment solely with January.

Late filing penalties

Penalties for late Self Assessment filing are structured as follows:

Day 1 late: automatic £100 penalty, regardless of tax owed
3 months late: £10 per day (up to a maximum of £900)
6 months late: greater of £300 or 5% of the tax liability
12 months late: a further greater of £300 or 5% of the tax liability (and potentially higher if deliberate non-compliance)

Late payment penalties and interest

Interest accrues from 1 February on any unpaid tax. Additionally:

30 days late: 5% surcharge on outstanding tax
6 months late: further 5% surcharge
12 months late: further 5% surcharge

Reasonable excuse and appeals

HMRC will consider a reasonable excuse appeal for late filing if there was a genuine reason outside the taxpayer's control — serious illness, bereavement, or system failures. Administrative errors or forgetting are not accepted. Appeals must be made within 30 days of the penalty notice.

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