Tax

Self Assessment Tax Return UK — Step-by-Step Guide

How to complete your Self Assessment tax return. Who needs to file, deadlines, what you can claim, and avoiding common mistakes.

By PocketWise · · 5 min read

Tax information is based on HMRC rules for the 2026/27 tax year. Tax rules can change — always verify current rates at GOV.UK. This is not tax advice. Consider consulting a qualified tax adviser for your personal situation.

Self Assessment can seem daunting, but it’s manageable step by step. Here’s how to get it right.

Do You Need to File?

You Must File If You’re…

Situation File?
Self-employed (over £1,000 income) Yes
Company director Yes
Partner in a partnership Yes
Income over £150,000 Yes
Rental income Yes
Foreign income Yes
Capital gains over allowance Yes
Receiving Child Benefit, income over £60,000 Yes
Untaxed savings interest/dividends Yes

You Don’t Usually Need to File If…

Situation File?
Only PAYE employment No
State Pension only Usually no
Tax fully deducted at source No

Not Sure?

Action Details
Check gov.uk Tool to check
HMRC will tell you If they think you need to
When in doubt Register anyway

Key Dates

Tax Year 2025-26

Date Event
6 April 2025 Tax year starts
5 April 2026 Tax year ends
5 October 2026 Register for Self Assessment (if new)
31 October 2026 Paper return deadline
31 January 2027 Online return deadline
31 January 2027 Pay tax owed
31 July 2027 Second payment on account (if applicable)

Payments on Account

What Advance payments toward next year’s tax
When If tax bill over £1,000
Amount 50% of previous year’s bill
Due 31 January and 31 July
First year No payments on account

Before You Start

Register (If New)

Step Action
1 Register at gov.uk
2 Receive UTR (Unique Taxpayer Reference)
3 Wait for activation code
4 Set up Government Gateway
Timeline Allow 2-3 weeks

Gather Documents

Document For
P60 Employment income
P45 If left job during year
P11D Benefits in kind
Bank statements Interest earned
Dividend vouchers Share dividends
Rental accounts Property income
Business records Self-employment
Pension contributions Tax relief claims
Gift Aid receipts Charity donations

Completing the Return

The Sections

Section Who Completes
Personal details Everyone
Employment If employed
Self-employment If self-employed
Partnership If in partnership
UK property If rental income
Foreign income If overseas income
Capital gains If sold assets
Additional info Various

Employment Section

Enter Details
Employer name From P60
Pay Total before tax
Tax deducted Amount already paid
Benefits From P11D

Self-Employment Section

Calculate Details
Total income All business earnings
Allowable expenses See below
Profit Income minus expenses
This is what’s taxed

Expenses You Can Claim

Self-Employed Expenses

Category Examples
Office costs Stationery, phone, software
Travel Business mileage, public transport
Clothing Uniforms, protective equipment
Staff costs Wages, subcontractors
Stock Goods for resale
Premises Rent, utilities, repairs
Advertising Marketing, website
Professional fees Accountant, legal
Insurance Business insurance
Training Courses for current trade

Working From Home

Method How It Works
Simplified £6/week flat rate
Actual costs Proportion of bills
Calculation Hours worked × rate or % of home

Vehicle Expenses

Method Rate
Simplified (mileage) 45p/mile first 10,000, then 25p
Actual costs Fuel, insurance, repairs × business %

Expenses You Can’t Claim

Not Allowable Why
Personal expenses Not business
Entertainment HMRC rule
Fines Not allowed by law
Your own wages That’s the profit
Living costs Personal

Property Income

What to Include

Income Include
Rent received All rent
Service charges If you keep them
Insurance payouts For lost rent

Property Expenses

Deductible Examples
Letting agent fees Management costs
Repairs Maintenance, fixing
Insurance Landlord insurance
Ground rent Leasehold costs
Council Tax If you pay it
Utilities If you pay them
Mortgage interest 20% tax credit only

Mortgage Interest (Restricted)

Current Rule Details
No deduction From rental income
Instead 20% tax credit
Higher rate taxpayers Pay more tax than before

Common Mistakes to Avoid

Filing Errors

Mistake Consequence
Missing deadline £100+ penalties
Wrong UTR Return rejected
Forgetting income Penalties, interest
Over-claiming expenses Investigation risk
No records Can’t prove claims

Income Often Forgotten

Income Source Remember
Bank interest Even if small
Dividends Outside ISA
Occasional freelance Over £1,000
Rental rooms Over Rent-a-Room limit
Crypto gains Capital gains section

Paying Your Tax Bill

How to Pay

Method Details
Direct Debit Set up via HMRC
Online banking Use HMRC account details
BACS Bank transfer
Credit card Via third party (fee)
Budget payment plan Monthly payments

Can’t Pay?

Option Details
Time to Pay HMRC payment plan
Call HMRC Before deadline
Be honest About circumstances
Set up online For debts under £30,000

After Filing

Keep Records

How Long 5 years after 31 January deadline
What All receipts, bank statements, invoices
Why HMRC can investigate
How Digital or paper

If You Made a Mistake

Timeframe Action
Before deadline Amend and resubmit
After deadline Contact HMRC
Within 12 months Amend online
After 12 months Write to HMRC

Summary: Self Assessment Checklist

Before Starting

Task Done
Registered for Self Assessment ☐
Have UTR number ☐
Government Gateway access ☐
All documents gathered ☐

Income to Report

Source Amount
Employment £
Self-employment £
Rental £
Dividends £
Interest £
Other £

Expenses Claimed

Category Amount
Business expenses £
Property expenses £
Pension contributions £
Gift Aid £

Key Deadlines

Date What
5 October Register if new
31 January File online + pay tax
31 July Second payment on account

After Filing

Task Done
Saved confirmation ☐
Paid tax owed ☐
Set aside for next year ☐
Records stored safely ☐

Self Assessment is straightforward once you’ve done it once. Keep good records throughout the year, gather everything before you start, and don’t leave it until the last minute. Most people can complete it themselves using HMRC’s online system.

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Sources

  1. HMRC — Self Assessment tax returns