Short answer:
You are automatically fined, even if you owe no tax — but the consequences escalate gradually, not all at once.
Missing the deadline is common.
The system is designed to penalise lateness mechanically, not emotionally.
The first thing that happens
If you miss the 31 January filing deadline for an online Self-Assessment return:
- You are issued an automatic £100 penalty
- This applies even if:
- You owe no tax
- You are due a refund
- The delay is only one day
There is no warning stage.
The penalty is triggered automatically.
What happens if you’re a bit late
If the return is still not filed after the first penalty:
After 3 months
- Daily penalties begin
- £10 per day
- Capped at £900
These accrue automatically until the return is submitted or the cap is reached.
After 6 months
- An additional penalty is charged
- This is usually:
- £300 or
- 5% of the tax due (whichever is higher)
Even if HMRC has estimated your tax, the penalty still applies.
After 12 months
- Further penalties may be added
- These increase if HMRC believes the delay was deliberate
At this point, the situation becomes more serious, but it is still procedural rather than personal.
What about late payment of tax?
Filing late and paying late are treated separately.
If you file on time but pay late:
- Interest accrues on the unpaid tax
- Late payment penalties may apply later
If you file late and pay late:
- Penalties stack
- Interest continues to accrue
The system tracks each failure independently.
Why this system exists
Self-Assessment relies on:
- Predictable deadlines
- Automated processing
- Cash-flow forecasting for government finances
Automatic penalties reduce the need for manual enforcement and keep the system scalable.
The system prioritises compliance, not flexibility.
What the system does
not
assume
Missing a deadline does not automatically mean:
- Fraud
- Evasion
- Deliberate avoidance
Those require separate evidence.
Most late filers are treated as routine cases and never interact with a human investigator.
Can penalties be appealed?
Yes, but only under specific conditions.
HMRC may cancel penalties if you had a reasonable excuse, such as:
- Serious illness
- Bereavement close to the deadline
- System failures outside your control
Forgetting, misunderstanding, or being busy are usually not accepted.
Appeals are assessed against defined criteria, not discretion.
The practical takeaway
Missing the deadline triggers penalties automatically, but the system gives multiple opportunities to stop escalation.
Filing late is usually worse than filing imperfectly.
Submitting the return — even with estimates — stops daily penalties from increasing.
One simple next step
If you’ve missed or may miss the deadline:
File the return as soon as possible, even if payment comes later.
That single action limits penalties and stops further escalation.
The system responds to timing, not intent.
Speed matters more than explanations.