How to File Your Self Assessment Tax Return: Step-by-Step Guide

Filing your Self Assessment tax return online is the standard way to report income to HMRC. You can file through HMRC's Government Gateway or using commercial software that submits directly via the HMRC API. The deadline for 2024/25 online returns is 31 January 2026.

This step-by-step guide explains how to file your Self Assessment, what information you need, and how to submit your return successfully.

Before You File: What You Need

Gather these items before starting your Self Assessment:

Essential Documents

| Document | Purpose | |----------|---------| | Unique Taxpayer Reference (UTR) | 10-digit number identifying you to HMRC | | National Insurance number | Required for all tax returns | | Government Gateway login | Username and password for HMRC online services | | P60 (if employed) | Shows employment income and tax paid | | P45 (if you left a job) | Shows earnings and tax from previous employer | | P11D (if applicable) | Shows taxable benefits from employment |

Income Records

Depending on your income sources, you'll need:

Self-Employment (SA103)

• Total business income received during the tax year
• Bank statements showing business transactions
• Invoices issued and payments received
• Records of cash payments

Property Income (SA105)

• Rental income received from each property
• Letting agent statements
• Tenant payment records

Other Income

• Bank and building society interest statements
• Dividend vouchers from investments
• Foreign income records
• Pension statements

Expense Records

• Receipts for business expenses
• Mileage logs for business travel
• Home office calculations
• Professional subscription invoices
• Equipment and software purchases

Keep records for 6 years after the tax year they relate to—HMRC can enquire within this period.

Option 1: File via HMRC Government Gateway

HMRC's free online service is available at gov.uk/self-assessment-tax-returns.

Step 1: Sign In to Government Gateway

1. Go to gov.uk/self-assessment-tax-returns
2. Click "Sign in" and enter your Government Gateway User ID and password
3. If you don't have an account, select "Create sign in details"
4. Complete identity verification if prompted

First time? You'll need your UTR and National Insurance number to register. HMRC posts an activation code within 7 days.

Step 2: Start Your Return

1. Select "Complete your tax return"
2. Choose the tax year (2024/25 for the 31 January 2026 deadline)
3. Click "Start" to begin the SA100 form

Step 3: Complete Personal Details

HMRC pre-fills some information. Check and update:

• Your name and address
• Date of birth
• National Insurance number
• Marital status (affects certain reliefs)

Step 4: Select Your Income Types

Tell HMRC which income types you have:

• [ ] Self-employment income
• [ ] Partnership income
• [ ] Property income
• [ ] Employment income (if not fully taxed through PAYE)
• [ ] Foreign income
• [ ] Capital gains
• [ ] Other income

Each selection adds the relevant supplementary pages to your return.

Step 5: Complete Income Sections

Self-Employment (SA103)

If you're a sole trader, enter:

| Field | What to Enter | |-------|---------------| | Business name | Your trading name | | Business description | What your business does | | Business start date | When you started trading | | Turnover | Total business income (gross) | | Allowable expenses | Business costs by category | | Net profit | Turnover minus expenses |

You can enter expenses as one total or itemised by category (office costs, travel, professional fees, etc.).

Property Income (SA105)

If you're a landlord, enter:

| Field | What to Enter | |-------|---------------| | Total rental income | Rent received during the tax year | | Property expenses | Allowable costs (repairs, insurance, agent fees) | | Mortgage interest | For 20% tax credit calculation | | Net rental profit | Income minus expenses |

Learn more: Landlord Tax Guide

Other Income Sections

Complete any other relevant sections:

• Employment income not taxed through PAYE
• Bank interest above your Personal Savings Allowance
• Dividends above your Dividend Allowance
• Foreign income

Step 6: Claim Reliefs and Deductions

Don't forget to claim:

• Pension contributions — personal pension payments
• Gift Aid donations — charitable giving
• Marriage Allowance — transfer unused Personal Allowance to spouse
• Trading losses — offset against other income

Step 7: Review Your Tax Calculation

HMRC automatically calculates your tax liability. Review:

• Total income from all sources
• Total allowable expenses and reliefs
• Income Tax due (at 20%, 40%, or 45%)
• National Insurance due (Class 2 and Class 4)
• Any tax already paid through PAYE
• Final amount to pay (or refund due)

If the calculation looks wrong, go back and check your entries.

Step 8: Submit Your Return

1. Read the declaration carefully
2. Tick to confirm information is correct and complete
3. Click "Submit"
4. Save or print your submission confirmation
5. Note the submission reference number

You've filed your Self Assessment!

Option 2: File via Commercial Software

Commercial software like TaxFolio offers advantages over HMRC's basic service:

How Software Filing Works

1. Connect your bank — transactions import automatically via Open Banking
2. Categorise income and expenses — AI sorts transactions for you
3. Review your figures — check totals match your records
4. Submit to HMRC — software sends data via the official HMRC API
5. Receive confirmation — stored digitally in your account

Benefits of Using Tax Software

| Feature | HMRC Gateway | Commercial Software | |---------|-------------|---------------------| | Bank connection | No | Yes (automatic import) | | Transaction categorisation | Manual | Automatic (AI-powered) | | Real-time tax estimates | No | Yes | | Submission time | 30-60 minutes | 15-20 minutes | | Record storage | Manual download | Automatic for 6+ years | | MTD-ready | Basic | Full compliance |

Filing with TaxFolio

TaxFolio simplifies Self Assessment filing:

1. Connect your bank accounts via TrueLayer (FCA-authorised Open Banking)
2. Review AI-categorised transactions — most are sorted automatically
3. Add any missing income or expenses not in your bank feed
4. Check your tax calculation — see exactly what you owe
5. Submit with one click — direct to HMRC via the API
6. Store your records — digital archive for 6+ years

The entire process takes about 15 minutes with organised records.

After Filing: What Happens Next

Confirmation

After submitting, you receive:

• HMRC acknowledgement — confirms receipt of your return
• Submission reference — unique identifier for your return
• Tax calculation — final amount to pay or refund due

Payment

If you owe tax, pay by 31 January to avoid late payment penalties.

Payment methods:

• Online banking — fastest option
• Debit card — via HMRC's payment portal
• Direct Debit — set up for future payments
• BACS/CHAPS — bank transfer

Payment reference: Use your UTR followed by "K" (e.g., 1234567890K).

Learn more: How to Pay Your Tax Bill

Payments on Account

If your tax bill exceeds £1,000 (and less than 80% was collected through PAYE), you must make advance payments for next year:

| Payment | Deadline | Amount | |---------|----------|--------| | First payment on account | 31 January | 50% of current year's bill | | Second payment on account | 31 July | 50% of current year's bill |

Example: Your 2024/25 tax bill is £3,000. On 31 January 2026, you pay £3,000 (the bill) plus £1,500 (first payment on account for 2025/26). On 31 July 2026, you pay another £1,500.

You can reduce payments on account if you expect to earn less.

Common Filing Mistakes to Avoid

1. Wrong Tax Year

The 2024/25 tax year runs from 6 April 2024 to 5 April 2025. Income received on 6 April 2025 belongs to 2025/26, not 2024/25.

2. Missing Income

Report all income, even if tax was already deducted:

• Bank interest (even small amounts)
• Dividends from investments
• Casual or freelance earnings
• Foreign income

HMRC receives data from banks and employers—unreported income is likely to be discovered.

3. Overclaiming Expenses

Only claim expenses "wholly and exclusively" for business. Common errors:

• Claiming 100% of phone bills (claim business proportion only)
• Personal travel as business mileage
• Clothing (unless protective or uniform)
• Client entertaining (not allowable)

4. Missing Deadlines

File by 31 January to avoid:

• £100 automatic penalty (even if no tax owed)
• Daily £10 penalties after 3 months (up to £900)
• Further penalties at 6 and 12 months

5. Not Keeping Records

HMRC requires records for 6 years. Without records, you cannot:

• Defend expense claims if HMRC enquires
• Prove income figures are accurate
• Claim reliefs you're entitled to

Filing Deadlines

| Return Type | Deadline | |-------------|----------| | Paper return | 31 October | | Online return | 31 January | | Tax payment | 31 January | | Second payment on account | 31 July |

For the 2024/25 tax year:

• 31 January 2026 — online return deadline and payment deadline
• 31 July 2026 — second payment on account

Making Tax Digital: Future Changes

From April 2026 (income over £50,000) or April 2027 (income over £30,000), filing changes under Making Tax Digital:

| Current System | MTD System | |----------------|------------| | Annual return only | Quarterly updates + final declaration | | File by 31 January | Submit quarterly + 31 January | | Software optional | HMRC-recognised software required |

TaxFolio is HMRC-recognised for both Self Assessment and MTD, making the transition seamless.

When to Use an Accountant

You can file your own Self Assessment for straightforward tax affairs. Consider professional help if:

• You have multiple income sources
• You've made capital gains
• You have foreign income
• You've started or sold a business
• You're unsure about complex reliefs
• HMRC is investigating previous returns

For simple self-employment or rental income, tax software handles everything most people need.

File Your Self Assessment with TaxFolio

TaxFolio makes Self Assessment filing simple:

• Open Banking connection — transactions import automatically from all UK banks
• AI categorisation — expenses sorted into correct tax categories
• Real-time calculations — see your tax liability as you go
• One-click HMRC submission — files via the official API
• 6-year record storage — meets HMRC requirements
• MTD-ready — seamless transition to quarterly reporting

From £69.99/year with no monthly fees.

Start your free 30-day trial and file your Self Assessment in 15 minutes.