UK Self-Assessment Tax Return 2025: Complete Filing Guide

UK Self-Assessment tax returns reconcile income tax obligations for self-employed individuals, landlords, high earners, and anyone with untaxed income. The 2024/25 tax year (6 April 2024 – 5 April 2025) must be filed by 31 January 2026 (online) or 31 October 2025 (paper). This guide covers filing requirements, deadlines, deductions, penalties, and step-by-step submission procedures.

Who Must File Self-Assessment?

Self-Employed: Sole traders, freelancers, partners earning above £1,000.
Company Directors: Even if PAYE applied.
High Earners: Income above £100,000 (loses personal allowance).
Child Benefit High Income Charge: Individual or partner income above £65,000.
Landlords: Rental income exceeding £2,500 (or £1,000 if no property allowance used).
Capital Gains: Disposals above annual exempt amount (£3,000 in 2024/25).
Foreign Income: Untaxed overseas earnings, dividends, or pensions.
Multiple Incomes: More than one employer simultaneously.

Key Deadlines for 2024/25 Tax Year

5 October 2025: Register for Self-Assessment if first-time filer (receive Unique Taxpayer Reference).
31 October 2025: Paper return deadline (processed by HMRC).
30 December 2025: Online filing deadline to have tax calculated for payment on account.
31 January 2026: Online filing deadline + payment of balancing charge + first payment on account for 2025/26.
31 July 2026: Second payment on account for 2025/26.

Income Types and Reporting

Employment and PAYE (SA102)

Report salary, bonuses, benefits-in-kind (P11D).
Claim employment expenses not reimbursed by employer (professional fees, uniforms, travel).

Self-Employment (SA103)

Report gross business income and allowable expenses.
Simplified expenses (flat rates for vehicles, home office) or actual costs.
Annual Investment Allowance (AIA): 100% deduction on equipment up to £1,000,000.
Claim losses against other income or carry forward.

Property Income (SA105)

UK Property: Rental income minus expenses (mortgage interest restricted to 20% tax credit).
Furnished Holiday Lets: Special reliefs; treated like trade income.
Property Allowance: £1,000 tax-free; elect actual expenses if higher.

Dividends and Interest (SA100)

Dividends: £500 allowance tax-free (2025/26); excess taxed at 8.75% (basic), 33.75% (higher), 39.35% (additional).
Interest: £1,000 allowance (basic rate) / £500 (higher rate); taxed at marginal rates above.

Capital Gains (SA108)

Annual Exempt Amount: £3,000 (2024/25).
Rates: 10%/20% (assets), 18%/24% (residential property).
Report all disposals; deduct purchase costs, improvement costs, fees.

Allowable Deductions and Reliefs

Self-Employment Expenses

Home Office: £6/week simplified or proportion of rent/mortgage interest, utilities.
Vehicle Costs: 45p/mile (first 10,000 miles), 25p thereafter (simplified) or actual costs with business use %.
Professional Fees: Accountancy, legal, subscriptions, professional indemnity insurance.
Marketing: Advertising, website, branding.
Equipment: Computers, software, tools (AIA or capital allowances).

Pension Contributions

Deduct personal pension contributions (net amount × 1.25 for gross relief).
Annual allowance: £60,000 (tapered for high earners above £260,000 income).

Gift Aid Donations

Extend basic-rate band by gross donation amount.
Higher/additional-rate taxpayers claim extra 20-25% relief.

Payment on Account System

If tax bill exceeds £1,000 and less than 80% tax collected at source, HMRC requires:

First Payment on Account: 31 January 2026 (50% of prior year's liability).
Second Payment on Account: 31 July 2026 (remaining 50%).
Balancing Payment: 31 January 2026 (difference between actual 2024/25 liability and payments on account).

Example

2024/25 tax bill: £8,000

31 Jan 2026: Pay £4,000 (first POA for 2025/26) + balancing charge for 2024/25.
31 Jul 2026: Pay £4,000 (second POA for 2025/26).
Total 2026 Payments: £12,000 (£8,000 for 2024/25 + £4,000 advance for 2025/26).

Filing Methods

1. Online (HMRC Website)

Sign in with Government Gateway account.
Navigate to Self-Assessment section.
Complete return sections (employment, self-employment, property, etc.).
Review tax calculation; submit electronically.
Instant confirmation and calculation.

2. Commercial Software

Use approved accounting software (QuickBooks, Xero, FreeAgent).
Import data from bank feeds, invoices.
Submit directly to HMRC via MTD (Making Tax Digital) bridging software.

3. Tax Agent

Hire accountant; grant agent authorization via HMRC online.
Agent files on your behalf; extended deadline often applies.

Penalties for Late Filing and Payment

Filing Penalties

1 Day Late: £100 fixed penalty.
3 Months Late: £10/day (max 90 days = £900).
6 Months Late: £300 or 5% of tax owed (whichever higher).
12 Months Late: Additional £300 or 5% (total up to £1,600 + tax).

Payment Penalties

30 Days Late: 5% of unpaid tax.
6 Months Late: Additional 5%.
12 Months Late: Additional 5%.
Interest: Charged daily from due date (currently ~7-8% annually).

Common Mistakes to Avoid

Missing Deadlines: Set calendar reminders; late penalties escalate quickly.
Incorrect UTR: Verify Unique Taxpayer Reference before filing.
Not Claiming Expenses: Itemize all allowable costs to reduce liability.
Forgetting Student Loans: Repayments calculated automatically if Plan details provided.
Ignoring Tax Code Changes: PAYE adjustments affect self-assessment; review P60, P45 forms.
Not Keeping Records: Retain invoices, receipts, bank statements for 5+ years (6 if self-employed).

Refunds and Overpayments

If payments on account exceed final liability:

HMRC refunds excess automatically within 5-10 days of filing.
Request refund via online account or contact HMRC.
Reduce future payments on account via online adjustment (if income expected to drop).

Amending a Return

Amend online within 12 months of filing deadline (by 31 January 2027 for 2024/25).
After 12 months, write to HMRC with corrections; subject to review.
HMRC can amend returns within 4 years (errors, omissions); respond promptly to queries.

Next Steps: Register for Self-Assessment (if new), gather P60, P11D, expense receipts, and property/investment statements. Use our UK tax calculator to estimate your 2024/25 liability and download a filing checklist for 31 January 2026 submission.

UK Self-Assessment Tax Return 2025: Deadlines, Penalties, and Filing Guide