5.1 PAYE Income Tax & Tax Codes

5.1 PAYE Income Tax & Tax Codes

How this is assessed: the AAT scope of content for Tax Processes for Businesses (FA2025) explicitly excludes the calculation of Income Tax, National Insurance contributions and student loan repayments from the assessment — you are given those figures and asked to work with them. You still need to understand how a tax code drives the deduction, because the assessment asks you to identify taxable gross pay, reconcile gross to net, and explain the effect of a wrong code. Treat the worked calculations in this section as understanding-builders, not as items you will be asked to reproduce from scratch.

1. Statutory Framework and Principles of PAYE

Pay As You Earn (PAYE) is the statutory system operated by UK employers to deduct Income Tax and Class 1 Primary National Insurance Contributions (NICs) from employees' gross pay at the point of payment. Governed primarily by the Income Tax (Earnings and Pensions) Act 2003 (ITEPA 2003) and the PAYE Regulations 2003 (SI 2003/2682), PAYE ensures that tax liabilities are collected progressively throughout the tax year (6 April to 5 April) rather than in a single end-of-year lump sum.

Under PAYE, employers act as non-salaried tax collectors for HM Revenue & Customs (HMRC). Every time payroll is processed, the employer calculates taxable gross pay, applies the employee's designated tax code, deducts the appropriate tax and NICs, and reports the figures to HMRC via Real Time Information (RTI) using a Full Payment Submission (FPS) on or before the pay date. The tax deducted must be remitted to HMRC by the 22nd day of the following tax month (or 19th if paying by cheque).

$\text{Taxable Gross Pay} = \text{Gross Contractual Remuneration} - \text{Allowable Tax-Exempt Deductions}$

---

2. Structure and Anatomy of a Tax Code

A tax code is an alphanumeric identifier issued by HMRC to inform employers how much tax-free income (Personal Allowance) an employee is entitled to in a given tax year, or whether additional tax must be collected.

The Numerical Element

The numerical portion of a standard tax code reflects the annual tax-free Personal Allowance to which the employee is entitled, divided by 10 and rounded down. For the 2025/26 tax year, the standard UK Personal Allowance is £12,570.

$\text{Tax Code Number} = \frac{\text{Annual Personal Allowance}}{10} = \frac{£12,570}{10} = \mathbf{1257}$

If an employee has adjustments to their Personal Allowance—such as professional subscription allowances, flat-rate job expenses, or deductions for untaxed benefits in kind—HMRC adjusts the total figure before dividing by 10:

• Base Personal Allowance: £12,570
• Add: Allowable Flat-Rate Professional Expenses (+£300)
• Less: Untaxed Benefit in Kind (Company Car) (-£1,870)
• Net Allowance: £11,000
• Resulting Tax Code: 1100L (£11,000 ÷ 10)

---

3. Tax Code Suffixes and Prefixes

Letters in a tax code provide specific operational instructions to payroll software.

Mechanics of Prefix 'K' Codes

When an employee receives substantial untaxed income or non-cash benefits in kind (e.g. company car, private medical insurance) that exceed their annual Personal Allowance, HMRC issues a K code (e.g., K450).

Unlike suffix codes which grant tax-free pay, the number in a K code represents additional untaxed income to be added to gross pay for tax calculation purposes:

$\text{Additional Taxable Pay} = (\text{K Code Number} \times 10) + 9$

Critical Statutory Limit (The 50% Overriding Limit): To prevent hardship, the maximum PAYE Income Tax that can be deducted from an employee's pay packet in any single pay period under a K code is capped at 50% of their actual cash gross pay.

---

4. Emergency Tax Codes

An Emergency Tax Code is issued when an employer does not have sufficient tax details for a new employee (e.g. the employee has not provided a Form P45 or completed a Starter Checklist).

For 2025/26, the standard emergency tax code is 1257L W1, 1257L M1, or 1257L X.

• W1 (Week 1): Used for weekly paid employees.
• M1 (Month 1): Used for monthly paid employees.
• X: Used for non-standard pay frequencies.

Operational Characteristic: Non-Cumulative Basis

An emergency tax code operates strictly on a non-cumulative (Week 1 / Month 1) basis. This means the payroll calculation considers only the gross pay earned and the tax-free allowance available in that specific pay period (e.g. £12,570 ÷ 12 = £1,047.50 per month).

It completely ignores prior earnings and tax paid in earlier pay periods during the tax year. Unused tax-free allowances from earlier months are lost for that period and cannot be carried forward until HMRC issues a cumulative tax code.

---

5. Cumulative vs. Non-Cumulative Calculation Methods

The Cumulative Method (Standard PAYE Operation)

The primary method of PAYE operation in the UK is cumulative. Under this method, every pay period recalculates tax due based on total year-to-date (YTD) figures:

1. Calculate Total YTD Taxable Gross Pay (Gross pay to date minus exempt pension).
2. Calculate Total YTD Tax-Free Allowance ($\text{Monthly Allowance} \times \text{Current Month Number}$).
3. Calculate Total YTD Taxable Pay ($\text{YTD Taxable Gross} - \text{YTD Allowance}$).
4. Apply Annual Tax Bands Pro-Rata (e.g. Basic Rate band of £37,700 ÷ 12 × Month Number).
5. Deduct YTD Tax Previously Deducted in prior months to arrive at the Net Tax Due in Current Month.

Cumulative vs. Non-Cumulative Comparison

---

6. Derivation of Taxable Gross Pay

Not all cash payments received by an employee are subject to PAYE Income Tax. Payroll software must calculate Taxable Gross Pay before applying tax bands.

$\text{Taxable Gross Pay} = \text{Gross Salaries/Wages/Overtime/Bonuses} - \text{Allowable Tax-Exempt Pension Contributions}$

Tax-Exempt Pension Arrangements

1. Net Pay Arrangement: Pension contributions are deducted from gross pay before PAYE Income Tax is calculated. The employee receives immediate full tax relief at their marginal rate. (Note: NICs are still charged on gross pay before pension deduction!).
2. Salary Sacrifice: The employee contractual salary is reduced in exchange for an employer pension contribution. This reduces both PAYE Income Tax and Class 1 NICs.
3. Relief at Source (Contrast): Pension contributions are deducted after PAYE tax. The employer deducts net 80%, and the pension provider reclaims 20% basic rate tax from HMRC. (Do NOT deduct Relief at Source pensions when calculating PAYE gross!).

---

7. Worked Calculation Example: Cumulative PAYE

Scenario

An employee has tax code 1257L (Cumulative) for 2025/26.

• Month 1 (April): Gross Taxable Pay = £4,000.
• Month 2 (May): Gross Taxable Pay = £4,000.
• Monthly Tax-Free Allowance: £12,570 ÷ 12 = £1,047.50.
• Monthly Basic Rate Band (20%): £37,700 ÷ 12 = £3,141.67.

Month 1 Calculation

• YTD Gross Pay: £4,000.00
• YTD Tax-Free Allowance (1 month): £1,047.50
• YTD Taxable Pay: £4,000.00 - £1,047.50 = £2,952.50
• Tax Due: £2,952.50 × 20% = £590.50
• Tax Deducted in Month 1: £590.50

Month 2 Calculation (Cumulative)

• YTD Gross Pay (2 months): £4,000 + £4,000 = £8,000.00
• YTD Tax-Free Allowance (2 months): £1,047.50 × 2 = £2,095.00
• YTD Taxable Pay: £8,000.00 - £2,095.00 = £5,905.00
• YTD Basic Rate Band (2 months): £3,141.67 × 2 = £6,283.34 (Taxable pay £5,905 is fully within Basic Rate band)
• Total YTD Tax Liability: £5,905.00 × 20% = £1,181.00
• Less Tax Already Paid in Month 1: £590.50
• Tax Deducted in Month 2: £1,181.00 - £590.50 = £590.50

---

8. Exam Traps & Common Pitfalls

Exam Trap 1: Misunderstanding Prefix 'K' Codes Under a K code (e.g. K450), candidates often subtract £4,500 from gross pay. This is wrong! A K code means untaxed benefits exceed allowances, so £4,500 must be ADDED to gross earnings to calculate taxable pay.

Exam Trap 2: Deducting Pension Contributions for National Insurance Pension contributions made under a Net Pay Arrangement reduce PAYE Income Taxable Pay, but they DO NOT reduce Class 1 NIC taxable gross pay (unless operated under a formal Salary Sacrifice agreement).

Exam Trap 3: Emergency Code Allowance Accumulation On a Month 1 emergency code (1257L M1), an employee starting in Month 4 receives only 1 month of Personal Allowance (£1,047.50) for that pay period, NOT 4 months of accumulated allowance (£4,190.00).