Self Assessment · Tax Filing Strategy
The January Filing Trap: Why Waiting Until the Deadline Costs You Money
Most CIS subcontractors file their Self Assessment in January because that's the deadline. But for anyone expecting a refund, January is the worst possible time — and this post explains exactly why.
January 31st is the Self Assessment deadline — not the filing date. Most subcontractors treat it like a finish line, but for anyone expecting a CIS refund, it functions more like a bottleneck. Filing under pressure at the last minute means rushing calculations, missing legitimate expenses, and waiting months longer for money that could have been in your account since the spring. None of that is inevitable, and all of it is avoidable.
What Actually Gets Lost in the January Rush
Think about the last time you filed in January — did you check every single receipt, fuel log, and site parking record going back to April? Most people don't. When there's a deadline looming, small legitimate expenses get left behind: a few months of mileage recorded on your phone but never properly logged, a tool replacement in June you forgot to file, a small parking receipt from a site visit in September.
Individually, each of these is a few pounds. Across a full year of construction work, missed expenses can add up to several hundred pounds of legitimate deductions that simply don't make it onto the return because there wasn't time to find them.
There's also a practical problem specific to January: if you're waiting on a missing CIS statement from a contractor — they went quiet, changed systems, or just haven't sent it — filing in January gives you almost no time to chase it down and resolve the gap before the deadline hits.
Filing in January isn't just a deadline risk — it's a refund risk. The same pressure that causes people to miss the submission also causes them to miss the expenses that determine how much comes back.
CIS Tax Insights · 2026What a Single Missed Expense Actually Costs
Here's a simple, realistic example. A subcontractor rushes their return on January 30th and forgets to add 2,000 miles of legitimate site travel — not an unusual amount for a full year of CIS work.
| Mileage allowance value (2,000 miles × 45p) | £900 |
| Tax relief lost on that £900 (at standard 20% rate) | −£180 |
| Cash left unnecessarily with HMRC from one missed log | £180 |
£180 from one mileage log. Add a forgotten tool purchase, a few months of site parking, and a missed material receipt, and it's not hard to see how a rushed January return costs subcontractors £300–£500 compared to a carefully prepared early return — without any complicated tax planning involved.
If a contractor hasn't sent you a monthly CIS statement, your return may show less income — or the wrong deduction figures — than HMRC has on record. Filing early gives you months to identify and resolve these gaps. Filing in January means discovering the problem with no time to fix it before the deadline.
Filing Early vs. Filing in January
- Your CIS refund lands in your bank account months before Christmas
- Time to find missing receipts, mileage logs, and outstanding contractor statements
- No risk of portal downtime, submission errors under pressure, or deadline penalties
- Your accountant has proper time to review everything carefully
- Pressure to submit quickly increases the chance of overlooking legitimate expenses
- HMRC's online portal experiences heavier traffic and occasional slowdowns in late January
- Refund processing is slower when millions of returns land at the same time
- No time to resolve discrepancies with contractor statements before the deadline
The Self Assessment window for the 2025/26 tax year opens on 6 April 2026. You can file your return from that date — you don't have to wait until January. HMRC processes refunds within a few weeks once the return is submitted, so filing in April or May typically means receiving your CIS refund by June or July.
Your Pre-Filing Checklist
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Collect Every Monthly CIS Statement From Every Contractor Chase down any outstanding monthly deduction statements now, before the end of the tax year. If a contractor has closed or stopped responding, HMRC can use their own records — but you need to flag the gap early, not the night before the deadline.
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Go Through Your Receipts Month by Month, Not All at Once Trying to reconstruct a year's worth of expenses in one sitting is where things get missed. Go through receipts one month at a time when you have a clear head — tools, fuel, materials, parking, insurance, protective clothing. Anything bought wholly for work is worth checking against the HMRC allowable expenses list.
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Reconcile Your Bank Statements Against Your Business Expenses Go through your business bank account and flag any outgoing payments that might be legitimate expenses you haven't recorded yet. Trade accounts, online tool suppliers, fuel stations — you may have spent money on business costs that simply never made it onto a receipt pile.
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Check That Your Government Gateway Access Still Works Log into your Government Gateway account now and confirm your UTR, password, and authenticator app are working. Reset codes are posted by HMRC and take up to 10 days to arrive — discovering an access problem in late January leaves you in a very awkward position with almost no time to resolve it.
Frequently Asked Questions
Does filing early mean I pay tax sooner too?
Not necessarily. Filing your return early tells HMRC what you owe (or what they owe you) but doesn't change the payment deadline. Any tax you owe is still due by 31 January — filing in April doesn't accelerate that. What does accelerate is your refund: if HMRC owes you money, they can pay it out as soon as the return is processed, regardless of when the filing deadline falls.
What if I don't have all my documents yet when April arrives?
You don't have to file the day the window opens — the point is simply to file well before January, not necessarily on April 6th. If you're waiting on a contractor statement or working through your expenses, filing in May, June, or July still gets your refund months earlier than waiting until January, and gives you time to make sure everything is right.
What exactly happens if I miss the January 31st deadline?
An automatic £100 penalty applies immediately, regardless of whether you owe any tax or are actually due a refund. After three months, daily penalties of £10 begin and run for up to 90 days (up to £900 extra). After 6 and 12 months, further percentage-based penalties can apply on any tax owed. Missing the deadline is worth avoiding even if you don't think you owe much.
Can I amend my return after submitting if I find something I missed?
Yes — you can amend a submitted Self Assessment return within 12 months of the original filing deadline (so, for a 2025/26 return, until 31 January 2028). If you realise after filing that you forgot a legitimate expense, you can submit an amendment and HMRC will adjust your tax calculation — and refund any difference if it changes what you're owed.
Your Refund Is Ready When You Are
The Self Assessment system is open from April. Your CIS refund — money you've already earned, tax that's already been withheld from your invoices — can be back in your account by early summer if you act on it promptly. Waiting until January doesn't earn you anything. It just delays the return, increases the chance of missing something, and adds unnecessary pressure to a process that, done properly, isn't complicated.
Set a reminder for April. Pull your CIS statements together. Go through your receipts while they're still fresh. The work is the same whether you do it in April or January — the difference is just how long you wait for the outcome.
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Find Out What HMRC Owes You — Before January.
Use our free CIS refund calculator to check your position now, and get your invoicing in order so your records are ready when the filing window opens.
This article is for informational purposes only and does not constitute formal financial, accounting, or legal advice. Self Assessment deadlines, penalty thresholds, and HMRC processing times are subject to change. Always consult a qualified accountant before submitting your return. Aligned to 2026/27 HMRC guidelines.