Self Assessment for plumbers

Register, file, pay

1. Register for Self Assessment if your gross trading income is over £1,000 in a tax year. Deadline to tell HMRC is 5 October after that year ends (5 October 2026 for 2025/26).
2. Keep income and expense records as you go. HMRC can ask for them for at least five years after the 31 January filing deadline.
3. File the online return by 31 January 2027 for 2025/26 (paper was 31 October 2026). Pay the balancing payment the same day.
4. If last year’s bill was over £1,000, Payment on Account is due 31 January and 31 July, each 50% of that bill.

Full deadline calendar and penalty ladder.

What goes on the self-employment pages (SA103)

Turnover is the full invoice value (labour plus materials you billed), not the net after CIS. Expenses as usual. On the tax calculation, CIS deducted for the year is set off against Income Tax and Class 4 due. If CIS exceeds the bill, you should get a repayment. That only happens if the statements are on the return.

Deadlines and penalties

Miss the online deadline and the late-filing penalty is £100 immediately, even if you owe no tax. After three months it is £10 a day (capped at £900). At six and twelve months HMRC add 5% of the tax due or £300, whichever is greater. Late payment is a separate 5% at 30 days, six months and twelve months, plus interest.

Mistakes plumbers make on the return

• Putting net-of-CIS bankings as turnover.
• Putting CIS in expenses, then also claiming it as tax paid (double).
• No mileage log, van claimed on a guess.
• Materials bought for home used as if they were job costs.
• Ignoring the second Payment on Account because “CIS already takes tax”. CIS may not cover January.

Register for CIS as a subcontractor if you work on construction. Registration is how you get the 20% rate instead of 30%. It is not a substitute for Self Assessment registration.

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