UK Self-Assessment: Deadlines and How to Avoid Penalties

Key dates to know

Registering for Self-Assessment (if new) should happen by 5 October following the end of the tax year in which you need to file. Paper returns are due 31 October; online returns and payment are due 31 January.

Why returns get filed late

The most common causes are disorganized expense records, uncertainty about what is deductible, and simply not having a bookkeeping system running throughout the year rather than scrambling in January.

How to stay ahead

A monthly bookkeeping cadence (rather than an annual scramble) means your return is largely ready before the deadline window even opens.