CIS | Contractors

CIS300 Monthly Contractor Returns 2026 - Deadlines, Penalties, How to File

19 May 2026 10 min read Talha Alvi

If you are a contractor in the construction industry and you pay subcontractors, the Construction Industry Scheme places a serious monthly obligation on you. Every single month, by the 19th, you must file a CIS300 return to HMRC declaring exactly how much you paid each subcontractor and exactly how much CIS tax you deducted from them.

This is not the same return your subcontractors file. It is a separate, contractor-specific obligation. Get it wrong, miss the deadline, or skip a month and HMRC's penalty system kicks in fast. £100 from day one, escalating sharply from there.

This guide explains who needs to file a CIS300, when the deadline falls, exactly what goes on the return, the penalty regime, and how to keep your monthly contractor filings clean.

Who Needs to File CIS300 Monthly Returns?

You need to file CIS300 monthly returns if you are a contractor under the Construction Industry Scheme. HMRC defines a CIS contractor in two ways:

Mainstream contractors: businesses whose main activity is construction work and who pay subcontractors to carry it out. Building firms, civil engineering companies, electrical contractors, plumbing firms, and similar trades.
Deemed contractors: businesses outside the construction industry whose average annual spend on construction exceeds £3 million across any rolling three-year period. Property developers, large retailers refitting stores, and large landlords with significant construction programmes can fall into this category.

If you fit either definition, you are a contractor and you must operate the scheme. That includes registering with HMRC as a contractor, verifying every subcontractor you pay, deducting CIS tax at the correct rate, paying that tax to HMRC monthly, issuing payment and deduction statements to subcontractors, and filing the CIS300 monthly return.

The Deadline: 19th of Every Month

The CIS300 return for each tax month (which runs from the 6th of one month to the 5th of the next) must be filed by the 19th of the same month. So the return covering 6 April to 5 May must be filed by 19 May. The return covering 6 May to 5 June must be filed by 19 June. And so on, every single month, indefinitely.

The CIS tax you have deducted from subcontractors must also be paid to HMRC by the 22nd of the same month if paying electronically (or the 19th if paying by post, but virtually no one does this any more).

You must file a CIS300 every month, even if you have not paid any subcontractors that month. This is called a nil return and HMRC will penalise you if you skip it. If you genuinely will not be paying any subcontractors for a while, you can tell HMRC you have temporarily stopped using CIS to pause the monthly filing obligation.

What Goes on the CIS300 Return

For each subcontractor you have paid during the tax month, the CIS300 must show:

The subcontractor's name and Unique Taxpayer Reference (UTR).
The verification number HMRC issued when you verified them (more on verification below).
The total payment you made (gross amount, excluding VAT).
The cost of materials within that payment that are not subject to CIS deduction.
The amount of CIS tax you deducted (which depends on the subcontractor's verification status: 0 per cent for gross payment status, 20 per cent for standard verification, 30 per cent for unverified subcontractors).
The net amount you actually paid the subcontractor.

You also need to make a declaration that you have considered the employment status of every subcontractor on the return and are satisfied they are genuinely self-employed (not disguised employees). This is the bit most contractors gloss over and HMRC pays close attention to it.

How to File Your CIS300

You file the CIS300 online through HMRC's CIS service. Most contractors do this in one of three ways:

Option 1: HMRC's CIS Online Service

Free to use. You log into your HMRC online account, navigate to the CIS service, and enter each subcontractor's details manually. Fine for small contractors paying one or two subcontractors a month. Tedious and error-prone for anyone paying more than that.

Option 2: Commercial CIS Software

Most modern bookkeeping software (Xero, QuickBooks, Sage, and CIS-specific packages) handles CIS300 filing directly. You enter subcontractor payments through normal bookkeeping, the software calculates the CIS deductions and tax to deduct, and you file the return at the click of a button. Much cleaner for any contractor paying more than a handful of subcontractors monthly.

Option 3: Through Your Accountant

Most CIS contractors hand the monthly return to their accountant. You send your subcontractor payment records each month (usually pulled directly from your accounting software), the accountant verifies subcontractors as needed, calculates the deductions, files the return, and issues the payment and deduction statements. This is what we do for most of our Harrow and Stanmore CIS contractor clients.

The Penalty Regime: Why You Cannot Afford to Miss the Deadline

HMRC's CIS penalty regime is one of the strictest in the tax system. Penalties stack fast:

1 day late: £100 fixed penalty.
2 months late: further £200 penalty (so £300 total).
6 months late: further penalty of the higher of £300 or 5 per cent of the CIS deductions on the return.
12 months late: further penalty of the higher of £300 or 5 per cent of the CIS deductions on the return (and substantially more if HMRC considers the failure deliberate).

Critically, these penalties apply per return. If you miss three months of returns and file all three after the six-month point, you are looking at penalties of £700 or more per return. Three months of missed filings can easily produce a £2,000-plus penalty bill before HMRC has even looked at whether the figures on the returns are right.

You can appeal CIS penalties where you have a reasonable excuse (genuine system failures, serious illness, bereavement). HMRC accepts reasonable excuse appeals more often than people think, but you need to engage early and properly. We routinely help contractors appeal penalties for missed CIS300 returns and have a good success rate where the circumstances genuinely justify it.

Payment and Deduction Statements: The Other Monthly Obligation

Alongside the CIS300 return, you must issue a payment and deduction statement to every subcontractor you have paid during the tax month, by the 19th of the same month (the same deadline as the return).

The statement shows the subcontractor exactly how much they were paid gross, how much was deducted under CIS, how much they received net, and includes your contractor name, address and UTR. Your subcontractor then uses these statements to claim their CIS deductions back through their own Self-Assessment return (which is what triggers their refund, often substantial).

Most bookkeeping software produces these statements automatically as a by-product of filing the CIS300, and emails them to subcontractors directly. If you are filing manually through HMRC's online service, you have to produce the statements yourself.

Real-Life Example

Harrow building contractor: £4,400 of avoidable penalties

Client M ran a building firm in Harrow specialising in residential extensions and loft conversions. He typically engaged five or six subcontractors at any one time, ranging from a regular bricklayer he used most weeks to electrical and plumbing subcontractors on specific projects.

When he came to us, he had not filed a CIS300 return for the previous seven months. His previous accountant had left mid-year, his bookkeeper had assumed someone else was handling it, and Client M himself thought everything was up to date. HMRC had been writing to him about missed returns and the letters had been buried in the pile on his kitchen table.

By the time he came to us, the seven missed returns had accumulated penalties of around £4,400 (£100 per return as standard plus additional penalties on the older ones at the two-month and six-month stages). On top of that, the unpaid CIS tax he had deducted from subcontractors was sitting at around £18,000 with interest accruing.

We reconstructed all seven months of subcontractor payments from his bank statements and Xero records, filed the seven outstanding CIS300 returns within ten days, paid over the £18,000 of CIS tax he had deducted, and submitted a reasonable excuse appeal for the penalties citing the accountant transition and the genuine confusion about responsibility. HMRC accepted the appeal in part and reduced the penalties to around £1,400. We then put a proper monthly process in place so it cannot happen again.

How to Keep Your CIS300 Clean Every Month

A few simple disciplines protect against the chaos that builds up when CIS filings slip:

One person responsible. Either you, your bookkeeper, or your accountant. Not "whoever gets to it first". CIS300 needs an owner.
Diarised every month. The 19th is the deadline, but a sensible business files by the 15th to allow for last-minute issues. Put it in the calendar as a recurring event.
Verify subcontractors before you pay them, not after. Unverified subcontractors must be deducted at 30 per cent (not 20 per cent), which they will not thank you for. Verification takes minutes and avoids a lot of difficult conversations later.
Use bookkeeping software that handles CIS. Xero, QuickBooks and Sage all do this well. You enter the payment, the software calculates the deduction, and the monthly return falls out automatically.
Send payment and deduction statements straight after filing. Subcontractors will ask for them anyway and waiting just creates noise.
If you fall behind, act fast. Penalties escalate at the 2-month and 6-month points. Filing two months late is much cheaper than filing seven months late.

Frequently Asked Questions

Do I need to file a CIS300 even if I have not paid any subcontractors this month?

Yes. You file what is called a nil return. If you skip it entirely, HMRC will treat you as a late filer and apply the £100 penalty. If you genuinely will not be paying subcontractors for an extended period, you can tell HMRC you have temporarily stopped using CIS, which pauses the filing obligation until you reactivate.

What is the difference between a mainstream and a deemed contractor?

A mainstream contractor's main business is construction work itself (builders, plumbers, electricians, civil engineers). A deemed contractor is a business outside construction (property developers, large retailers, large landlords) whose average construction spend exceeds £3 million across any rolling three-year period. Both must operate the scheme identically once they meet the definition.

Can I file my CIS300 late if I have a good reason?

You can appeal CIS penalties on reasonable excuse grounds. HMRC accepts genuine system failures, serious illness, bereavement and similar circumstances. Forgetfulness, being busy, or your accountant changing are usually not accepted on their own. The appeal needs to be specific and supported by evidence where possible.

What rate do I deduct from an unverified subcontractor?

30 per cent, not 20 per cent. This is a punitive rate designed to encourage subcontractors to register properly for CIS. Once you verify them through HMRC's online service and they come back as standard registered, you can drop to 20 per cent. If they have gross payment status, you do not deduct anything at all.

Does the £3 million deemed contractor threshold include VAT?

No. The £3 million test is on construction spend excluding VAT, averaged across any rolling three-year period. So a non-construction business spending £9 million on a single year of construction work would be caught, even if it spent nothing in the previous two years.

Need help with your CIS300 monthly returns?

At Your Tax Help Accountants in Stanmore, we handle CIS300 returns for contractors across Harrow, Wembley, Edgware and London. Fixed monthly fees, every return on time, no surprises.

📅 Book Free 15-Min Call 📞 07478 645331

Or email info@yourtaxhelp.co.uk | yourtaxhelp.co.uk

General guidance only. Not personal tax advice. Contact us for advice specific to your situation. All figures are for the 2026/27 tax year unless otherwise stated.