Most CIS subcontractors think about the tax year ending on 5 April and don't do anything until January 31st — the filing deadline nine months later. That gap is expensive. The decisions you make in the final weeks of the tax year have a direct bearing on your refund, your compliance record, and whether you start the new year in a clean position or carrying unresolved problems from the one just ended.

Here's the thing about 5 April: once it passes, it's done. Any expense you forgot to record, any tool purchase you meant to log, any mileage you didn't write down — it's gone from that tax year. You can't add it to a return filed nine months later. The window closes at midnight, and what's on the other side of it is the return you're going to file.

These are the six things worth doing before that window closes.

• 1
  Chase Down Every Missing CIS Statement

  Your CIS refund calculation is only as accurate as the deduction statements behind it. Every month you worked under CIS this year, your contractor should have issued a payment and deduction statement showing the gross amount paid and the tax withheld. If you're missing any of these — a contractor went quiet, changed systems, or simply hasn't sent them — now is the time to chase them.

  After 5 April, HMRC can use their own records to verify figures if you can't get the statements, but this slows things down and creates more room for error. Getting every statement in your hands before year end means your Self Assessment can be filed quickly and accurately the moment the window opens on 6 April.

• 2
  Do a Final Sweep of Your Business Expenses

  Go through your bank statements, email receipts, and any paper receipts you've accumulated since April — month by month, not all at once. Look for anything that qualifies as a legitimate business expense: tools and equipment, protective gear, branded workwear, fuel for business journeys, materials, insurance, phone costs for business use, and any accountancy fees.

  The question to ask for each item is whether it was bought wholly and exclusively for work. If the answer is yes and it's in the current tax year, it belongs on your return. If you find expenses you haven't recorded yet, get them logged now — not in January.

• 3
  Log Your Full Year of Business Mileage

  Mileage is consistently the most under-claimed legitimate expense among CIS subcontractors. HMRC allows 45p per mile for the first 10,000 business miles in a tax year (25p per mile after that). On 10,000 miles, that's £4,500 in allowable expenses — worth £900 in tax relief at the basic rate.

  If you've been logging mileage loosely or not at all, do a proper reconstruction now while it's still in the current tax year. Use your calendar, job records, and bank statements to rebuild a reasonable mileage log for journeys to sites, suppliers, and other work-related destinations. After 5 April you can still include this, but reconstructing it gets harder the longer you leave it.

• 4
  Make Any Planned SIPP Contributions Before the Year Closes

  If you've been thinking about making a pension contribution this year, 5 April is your last chance to have it count for the 2025/26 tax return. Contributions made on 6 April or later fall into the following tax year and can only be claimed on the next return.

  This matters particularly if part of your income this year fell in the 40% higher-rate band. A SIPP contribution before year end reduces your Adjusted Net Income for the current year — potentially bringing you back below the higher-rate threshold and saving you additional tax. Even if you're a basic-rate taxpayer, a contribution before 5 April gets the government's 20% top-up into your pension pot while it still counts for this year's numbers.

• 5
  Check Your CIS Registration Is Still Active and Accurate

  Log into your Government Gateway account and confirm your CIS subcontractor registration is showing as active. Also check that the name and details on your registration match exactly what you've been putting on invoices and telling contractors — a mismatch in any of these is what causes verification failures and the 30% deduction rate.

  If you're considering Gross Payment Status and you've had a clean year — all returns filed on time, no missed payments to HMRC — the end of the tax year is a good moment to review whether you now meet the threshold. Your accountant can advise whether an application makes sense before the new year starts.

• 6
  File Your Return Early — Don't Wait Until January

  The Self Assessment window opens on 6 April — the day after the tax year closes. Filing in April or May rather than January means your CIS refund arrives in your bank account by early summer rather than March the following year. On a typical refund of £1,500–£3,000, that's a significant difference in when you actually have access to the money.

  It also means you file when everything is fresh — your CIS statements are in front of you, your expense records are organised, and you haven't spent nine months half-forgetting what you claimed. The work involved is identical whether you do it in April or January. The outcome isn't.

What These Six Steps Are Actually Worth

Here's a realistic picture of what a subcontractor earning £38,000 gross in a year could expect, comparing a rushed January return against one prepared properly before year end.

£700 is a realistic figure for what subcontractors leave on the table by rushing or leaving things too late. For some, it's more. The point isn't the exact number — it's that the money is yours, it's legitimate, and the only thing standing between you and it is a few hours of organised record-keeping before 5 April.

Frequently Asked Questions

Six Weeks, One Opportunity

The run-up to 5 April is one of the few moments in the tax year where a small amount of focused effort pays off directly in cash. Chase your CIS statements, sweep your expenses, log your mileage, think about a pension contribution if it makes sense for you, check your registration is clean, and then file as soon as the window opens on 6 April.

None of this is complicated. It's just a matter of doing it before the year closes rather than nine months later under pressure. The refund at the other end of a well-prepared return is the same money — you just get it sooner, and more of it.