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

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.