The CIS Monthly Return — A Contractor's Guide to Filing CIS300 on Time (2026)

What the CIS300 Monthly Return Is

Every contractor in the Construction Industry Scheme must send HMRC a return for each tax month declaring all payments made to subcontractors in that month and the CIS deductions taken from them. This is the CIS300. It is a legal obligation for contractors — not subcontractors — and it is separate from your own Self Assessment or company tax.

On the return you list each subcontractor you paid, the total amount paid (excluding VAT and excluding the cost of materials), and the deduction you made. The deduction rate depends on the subcontractor's verification status:

• 0% — gross payment status: the subcontractor is paid in full with no deduction, because HMRC has granted them gross status.
• 20% — registered and verified: the standard rate for a subcontractor who is registered under CIS and verified against HMRC's records.
• 30% — unverified or unregistered: the higher rate that applies when you have not been able to verify the subcontractor, or they are not registered under CIS.

The deduction is only ever applied to the labour element. Materials, plant hire the subcontractor pays for, fuel for plant, and VAT are all excluded from the figure you deduct from. Getting this split right on the return is what stops you over-deducting from your subbies.

The Deadlines That Actually Matter

The CIS calendar does not follow calendar months. The CIS tax month runs from the 6th of one month to the 5th of the next, mirroring the PAYE tax month. Everything keys off that. Here is the timetable for each tax month — miss none of these dates.

So for the tax month running 6 April to 5 May: the return and the subcontractor statements are due by 19 May, and the money you deducted must reach HMRC by 22 May if you pay electronically (or 19 May if you still pay by post). If the 22nd falls on a weekend or bank holiday, the cleared payment must reach HMRC by the last working day before it.

The Late-Filing Penalty Ladder

This is where contractors get hurt. The penalties for a late CIS300 are fixed and cumulative — they stack on top of each other as the return stays outstanding. A few months of forgetting can run well past £1,000 for a single return.

Read the ladder carefully: the £100 hits the day after the 19th. After two months a further £200 is added. At six months and again at twelve months you pay the greater of £300 or 5% of the deductions shown on that return — so on a return with large deductions, the 5% figure can dwarf the fixed amounts. A return filed a year late therefore typically costs at least £1,200, and considerably more if your deductions were high. Each monthly return that is late attracts its own ladder, so falling behind on several months multiplies the damage.

Nil Returns — You Still Have to Tell HMRC

If you made no payments to subcontractors in a tax month, you do not simply skip the return. HMRC still expects to hear from you, and there are two ways to deal with it.

• File a nil return: submit the CIS300 declaring that no payments were made that month. This keeps your record clean and confirms there is nothing outstanding.
• Tell HMRC you will not be filing for a period: if you expect no subcontractor payments for several months (for example over a quiet winter), you can notify HMRC using the "no longer making payments" / inactivity option so they do not expect a return for up to six months.

HMRC has, in some cases, stopped automatically penalising a missed nil return, but do not rely on that as a strategy. The safe approach is always to notify — either file the nil return or set the period to inactive — so HMRC's system is not expecting a return it never receives. If their system expects a return and none arrives, the penalty ladder can still be triggered, and unwinding an incorrectly issued penalty costs you more time than the two minutes it takes to file nil.

How to File the Return

There are three practical routes to submitting your CIS300, and most contractors settle on one and stick with it.

• HMRC's CIS online service: log in through your Government Gateway and file directly. This is free and fine for contractors with a handful of subcontractors.
• Commercial CIS / payroll software: software that handles verification, calculates deductions, produces subcontractor statements and submits the return for you. Worth it once you pay more than a few subbies or want the deduction maths done automatically.
• Through your accountant or bookkeeper: many contractors hand the whole CIS process to their accountant, who files on their behalf as an agent. You still need to get them the payment figures before the 19th — agents cannot file what they have not been given.

Whichever route you use, the return includes a declaration. By submitting it you are confirming that the employment status of each subcontractor has been considered (that they are genuinely self-employed and not really an employee), and that every subcontractor on the return has been verified, or that you were not required to verify them. Do not tick that declaration lightly — it is the basis on which HMRC may later challenge whether someone should have been on your payroll instead.

Worked Example: One Contractor's Monthly Return

Take a contractor for the tax month ending 5 May who paid three subcontractors. The deduction always applies to the labour element only — materials are stripped out first.

• Subbie A — gross status (0%): paid £4,000 labour. Deduction: £0. Paid in full.
• Subbie B — registered, verified (20%): paid £3,000, of which £600 is materials. Deduct 20% of the £2,400 labour = £480. They receive £2,520.
• Subbie C — unverified (30%): paid £1,500 labour, no materials. Deduct 30% = £450. They receive £1,050.

The CIS300 for this month shows total payments and the deductions made from B and C. Total CIS deducted = £480 + £450 = £930. That £930 is what the contractor pays over to HMRC, due by 22 May if paying electronically. Each of the three subbies must also receive a payment and deduction statement by 19 May showing what they were paid and what was withheld, so they can claim the deductions back through their own tax return.

Correcting Errors and Amending a Return

Mistakes happen — a wrong figure, a subcontractor left off, the wrong deduction rate applied. If you spot an error after filing, you amend the return rather than ignoring it. You can amend a previously submitted CIS300 through the HMRC CIS online service or your software, or by contacting the CIS helpline for older months.

Common corrections include adding a subcontractor you missed, removing one entered in error, or fixing a deduction where you applied 30% to someone who was actually verified at 20%. Correct it as soon as you notice — an over-deduction means a subcontractor has had too much withheld, and under-deducting leaves you owing HMRC. Amending promptly also helps if a penalty is ever queried, because it shows you took reasonable care.

Record-Keeping

You must keep your CIS records for at least 3 years after the end of the tax year they relate to. HMRC can ask to see them, and if you cannot produce them you risk a penalty of up to £3,000. Keep, for every month:

• The gross amount paid to each subcontractor, excluding VAT
• The cost of any materials deducted before working out the CIS deduction
• The amount of CIS deducted from each payment
• The verification number where the subcontractor was paid at the higher 30% rate
• Copies of the payment and deduction statements you issued

Good records are not just about compliance. They are what let you prove the labour/materials split, defend your verification decisions, and reconcile the deductions you paid over against the returns you filed.

Getting Verification and the Return Right Protects Your Subbies

Over-deducting is one of the most common ways contractors damage relationships with good subcontractors. If you fail to verify someone and default to the 30% unverified rate when they are actually registered and should be at 20%, you have withheld an extra 10% of their labour value for no reason. They eventually reclaim it, but only after their own tax return is processed — which could be many months away. That is real cash flow taken out of their pocket because your verification or your return was wrong.

The fix is to verify before you pay, apply the correct rate, strip materials out before deducting, and report it accurately on the CIS300. Do that and your subbies get paid the right amount, your records reconcile, and you avoid the awkward conversations and amendments that follow an over-deduction.

Appealing a Late-Filing Penalty

If HMRC issues a penalty you believe is wrong or unfair, you can appeal — usually within 30 days of the penalty notice. The grounds that succeed are a genuine error in the penalty itself, or a reasonable excuse for filing late.

A reasonable excuse is something unexpected and outside your control that stopped you filing on time — serious illness, a bereavement around the deadline, a fire or flood, an HMRC online service failure, or unexpected postal delays. It is not a reasonable excuse that you found the system confusing, were too busy, relied on someone else who let you down, or simply forgot. If you have a genuine excuse, file the outstanding return as soon as you can and submit the appeal explaining what happened and when. HMRC also operates penalty caps in some circumstances, so it is worth checking whether a string of fixed penalties on a single late return has been correctly limited.

Quick Reference: CIS Monthly Return Essentials