Why NI and CIS are completely separate calculations
The most expensive misunderstanding in CIS is treating deductions as an all-in advance against your entire tax and National Insurance bill. They are not. CIS deductions are advance payments against income tax only. National Insurance sits entirely outside the CIS mechanism: it is calculated on your Self Assessment return, and the bill is due on 31 January regardless of how much CIS has been deducted during the year.
This catches subcontractors out every January. A year of 20% CIS deductions may have broadly covered the income tax due, leaving little or nothing to pay on that front. But the Class 4 NIC bill, which can easily reach £1,500 to £2,000 on a typical sole-trader profit, arrives on the same date and has not been collected at source at any point. If you have not put money aside for it, it is a shock.
This guide explains exactly which NI classes apply to CIS subcontractors, the rates and thresholds for 2026/27, and worked examples for both the typical sole-trader and the limited-company director structure. The figures below are locked 2026/27 rates drawn from the Trade Tax Specialists house positions (FA 2026-verified).
NI classes: a summary for CIS workers
Different NI classes apply depending on how you work. The table below sets out every class relevant to a CIS subcontractor or director for 2026/27.
| NI class | Who pays | Rate 2026/27 | Threshold | How it is paid |
|---|---|---|---|---|
| Class 2 (sole trader) | Self-employed sole-trader subcontractor | £0 (treated as paid above £7,105) | Small profits threshold: £7,105 | Self Assessment; voluntary below threshold |
| Class 4 (sole trader) | Self-employed sole-trader subcontractor | 6% on £12,570 to £50,270; 2% above £50,270 | Lower profits limit: £12,570 | Self Assessment alongside income tax |
| Employee Class 1 | Any employee (including a director drawing salary above £12,570) | 8% on £12,570 to £50,270; 2% above | Primary threshold: £12,570 | PAYE, deducted at source |
| Employer NIC | Limited company (on director or employee salary) | 15% above £5,000 secondary threshold | Secondary threshold: £5,000 | PAYE; paid by the company |
| No NI via CIS | GPS sole trader on CIS income | Not applicable to NI | NI still due via Self Assessment as normal | Self Assessment; GPS changes nothing |
For the vast majority of CIS subcontractors who work as sole traders, the relevant classes are Class 2 (effectively zero cost in 2026/27 for most) and Class 4. Class 4 is where the meaningful NI bill sits.
Class 2 NIC for sole-trader subcontractors
Class 2 NIC was reformed in 2024. For 2026/27 the position is straightforward: if your taxable profit is above the small profits threshold of £7,105, Class 2 NIC is £0. HMRC treats the liability as paid, and your National Insurance record (State Pension entitlement, contributions towards certain benefits) is credited as if you had paid. You do not write a cheque.
If your profits fall below £7,105, Class 2 is not automatically credited. You can pay it voluntarily at a flat weekly rate to keep your NI record intact. This matters most to subcontractors who have had a short year, a period of illness, or are close to the State Pension qualifying years they need. For most CIS subcontractors, profits are well above £7,105 and Class 2 simply does not generate a bill.
Class 2 is collected through the Self Assessment return alongside Class 4 and income tax. There is no separate mechanism and no separate deadline. It appears on the same January payment summary.
Class 4 NIC: the main NI cost for sole traders
Class 4 NIC is the substantive NI charge for a self-employed sole trader. For 2026/27 the rates are:
- 6% on taxable profits between £12,570 and £50,270
- 2% on taxable profits above £50,270
Class 4 is calculated on taxable profit (income after allowable expenses), not on gross CIS income. CIS deductions do not reduce the profit figure that Class 4 is applied to; they are advance income tax payments, separate from the NI calculation entirely. If you are unsure which expenses are allowable, our guide to allowable expenses for CIS subcontractors covers the full list.
One rate that is now firmly out of date and must not be used is the old 9% Class 4 rate that applied before 2024. The current main rate is 6%. Any calculation or online tool quoting 9% is using stale figures.
Worked example: sole-trader subcontractor, £42,000 profit
Take a CIS sole trader with taxable profit of £42,000 for 2026/27, after allowable expenses have been deducted. This is inside the basic-rate band and below the upper profits limit, so the calculation is clean.
- Class 2 NIC: £0. Profit of £42,000 is above the £7,105 small profits threshold, so Class 2 is treated as paid.
- Class 4 NIC: 6% on (£42,000 minus £12,570) = 6% on £29,430 = £1,765.80. No 2% upper-rate charge because £42,000 is below £50,270.
- Income tax: 20% on (£42,000 minus £12,570) = 20% on £29,430 = £5,886.00. Basic rate only; no higher-rate tax.
- Total Self Assessment liability: £1,765.80 + £5,886.00 = £7,651.80.
Now consider the CIS deduction picture. If this subcontractor was registered and had 20% deducted on their labour income throughout the year, the deductions collected by contractors will appear as a credit on the Self Assessment calculation and reduce the amount left to pay. If the deductions exceed the income tax due, there may be a refund. But regardless of the deduction credit, the £1,765.80 Class 4 NIC is owed in full: CIS deductions have not touched it. That is the amount many subcontractors fail to budget for. Our CIS Self Assessment calculator lets you model this for your own figures.
Worked example: limited-company director, salary £12,570
A common approach for a limited-company CIS subcontractor is to draw a director's salary equal to the personal allowance of £12,570, paying no income tax on the salary and extracting further profit as dividends. The NI picture for this structure in 2026/27 is as follows.
- Employer NIC (paid by the company): 15% on salary above the £5,000 secondary threshold. On a salary of £12,570, that is 15% on (£12,570 minus £5,000) = 15% on £7,570 = £1,135.50. This is a company cost, deductible against corporation tax.
- Employee NIC (director personally): £0. Employee Class 1 NIC applies to salary above the primary threshold of £12,570. A salary set at exactly £12,570 sits at the threshold, so no employee NIC is triggered.
- Income tax on salary: £0. The salary equals the personal allowance, so no income tax is due on it.
The total personal NI cost on the salary is nil. The company bears £1,135.50 in employer NIC. Dividends drawn from the company are taxed at dividend rates (10.75% basic, 35.75% higher, 39.35% additional for 2026/27) but are not subject to NIC.
This structure is common in construction precisely because it manages the NI cost. The employer NIC rate of 15% on salary above £5,000 (which rose from 13.8% above £9,100 in April 2025 and carries into 2026/27) is a genuine company cost and needs to be factored into the payroll calculation. Do not use the old 13.8% rate or the old £9,100 threshold: both are out of date.
Gross Payment Status and NI: no exemption
A sole trader with Gross Payment Status is paid with no CIS deduction taken at source. This is often misread as affecting the NI position. It does not. GPS changes the income tax advance mechanism only. The GPS holder's Class 4 NIC is calculated on the same profit figure, at the same rates, via the same Self Assessment return, and falls due on the same 31 January deadline. GPS simply means the whole of the income tax bill, and the whole of the NI bill, arrives together in January with no advance collections to offset against it.
A GPS sole trader therefore needs to budget for both the income tax and the NI bill from cash kept in the business throughout the year. The absence of monthly deductions makes this more comfortable during the year (the cash stays in the business) but requires discipline at year end. Our guide to CIS deduction rates explained sets out the full mechanics of how the 0%, 20% and 30% rates work and what each means for year-end cash flow.
MTD ITSA and NI from April 2026
Making Tax Digital for Income Tax (MTD ITSA) came into force for the first group of sole traders and partnerships from April 2026. The threshold is gross income over £50,000. From April 2027 it drops to £30,000.
The NI calculation does not change under MTD. What changes is the reporting process: those in scope must submit quarterly digital updates through MTD-compatible software, with a final declaration at the end of the year replacing the traditional SA return. Quarterly updates report both income and, where applicable, NI information through the same submission.
The CIS-specific angle here is easy to miss. The £50,000 MTD threshold is tested on gross income before CIS deductions, not on net cash received after deductions. A subcontractor who receives £48,000 into their bank after 20% CIS has been withheld on £60,000 of gross labour income is tested on the £60,000 figure and is in scope from April 2026. The bank balance suggests they are below the threshold; the gross figure puts them firmly above it. If you are unsure whether you are in scope, the gross receipts figure on your CIS payment and deduction statements is the number to use, not bank receipts.
HMRC has confirmed a year-1 penalty grace for 2026/27: no penalty points will be issued for late quarterly updates in the first year. That grace covers quarterly update points only. Late annual returns and late payment penalties still apply in full.
The common mistake: budgeting as if CIS covers everything
The pattern that leads to January problems is straightforward. A subcontractor sees 20% deducted from every payment throughout the year and assumes this covers their whole tax liability. When they or their accountant prepare the Self Assessment return, the CIS deduction credit wipes out most or all of the income tax due. There may even be a refund on the income tax side. But the Class 4 NIC, typically £1,500 to £2,000 or more depending on profit, has not been touched by the CIS deductions. It is payable in full on 31 January.
The fix is simple. Set aside a proportion of profit specifically for the NI bill throughout the year. A rough guide is to reserve 6% of the slice of taxable profit that sits above the £12,570 lower profits limit, on top of whatever income tax provision you are making. For the worked example above (£42,000 profit, so £29,430 above the limit) that is the £1,765.80 Class 4 figure. Keeping it separate from operating cash means there is no surprise in January.
If your previous Self Assessment return resulted in a refund on the income tax side but still left you with an unexpected NI bill, this is almost certainly why. Our CIS refund service handles the reclaim, but the ongoing NI position is a separate and permanent feature of the sole-trader structure. Getting both right is what a CIS-specialist accountant manages as a matter of routine.
Putting it together
The NI position for a CIS subcontractor in 2026/27 comes down to a few clear rules. Sole traders pay Class 2 at £0 if profits are above £7,105, and Class 4 at 6% on profits between £12,570 and £50,270, then 2% above. Both are due on 31 January via Self Assessment. CIS deductions collected during the year do not offset either NI class. Limited-company directors drawing a salary above the £5,000 secondary threshold face employer NIC at 15% as a company cost, but can keep personal NI to zero by setting salary at or below the primary threshold of £12,570. GPS changes nothing about the NI calculation. MTD ITSA from April 2026 changes the reporting process but not the underlying liability.
If you want to model your own figures before the January deadline, use our CIS Self Assessment calculator. If you need help making sure the full picture (CIS deductions, income tax, NI and any refund due) is handled correctly, the right starting point is a conversation with a CIS-specialist accountant.