How to Fill Out and Submit HMRC Form SA1: Deadlines and Penalties

HMRC Form SA1 is how you register for Self Assessment when you owe tax on income that hasn’t already been taxed at source and you are not self-employed. You submit it online or by post, and you must do so by 5 October following the end of the tax year in which the income arose. Once HMRC processes it, you receive a Unique Taxpayer Reference (UTR) and can file your return.

When You Need to Submit Form SA1

SA1 is the right form when your tax obligation comes from passive or investment income rather than a trade or profession. The common triggers are:

UK rental income from residential or commercial property.
Capital gains on a second home, shares, or other assets where Capital Gains Tax is due.
Foreign income, including overseas pensions, dividends, and rental profits.
Savings interest above your Personal Savings Allowance. Basic-rate taxpayers get £1,000, higher-rate taxpayers £500, and additional-rate taxpayers nothing.¹
Yearly income from a trust, settlement, or the estate of someone who has died.²
The High Income Child Benefit Charge. If your adjusted net income is over £60,000 you owe a charge that claws back Child Benefit at 1% for every £200 above the threshold, reaching 100% at £80,000.³

If you are a sole trader or in a business partnership, SA1 is the wrong form. Self-employed people register through Form CWF1, which also opens their National Insurance record.⁴ And if you have filed Self Assessment before but didn’t file last year, check HMRC’s online registration service first: you may need to reactivate an existing record rather than submit a new SA1.⁵

What to Have Ready Before You Start

The online form cannot be saved partway through, so gather everything first. You need your full name as it appears on official records, your date of birth, a postal address (UK or overseas), a daytime phone number, and your National Insurance number. If you don’t have a National Insurance number, the form asks you to explain why.⁴

You will also select the reason for registering (rental income, capital gains, foreign income, trust income, or another category) and give the date the income started. That date matters. HMRC uses it to work out which tax year you first need to file for, so a wrong date can leave a gap in your record or trigger an unnecessary late-filing penalty. Use the date you first received the income or the date you sold the asset, not the date you decided to register.²

How to Submit Form SA1

Online

Go to the GOV.UK page “Register for Self Assessment if you are not self-employed” and click “Start now.” Sign in with your existing Government Gateway credentials, create new sign-in details during the process, or use your email address to receive a one-time confirmation code.² You complete the form on screen in one sitting.

By Post

Download the printable SA1 from the same GOV.UK page, complete it on your computer, and print it. HMRC’s postal address is on the form. Send it by tracked delivery so you have proof of postage if a dispute about timing ever comes up. Postal applications take longer because HMRC keys the data in by hand.

The 5 October Deadline

You must notify HMRC that you need to file a Self Assessment return by 5 October following the end of the tax year. The UK tax year runs from 6 April to 5 April, so untaxed income received during 2025–26 means your SA1 must reach HMRC by 5 October 2026.⁶

Register after 5 October and HMRC will write to you with a different filing deadline, typically three months from the date of the letter. Either way, the tax itself is still due by 31 January, and a late payment carries its own penalty separate from any late-notification charge.⁶

Penalties for Registering Late

HMRC’s late-notification penalty is a percentage of the tax you would otherwise have paid on time. The band depends on whether the failure was deliberate, whether any deliberate failure was concealed, and whether you came forward on your own (unprompted) or HMRC contacted you first (prompted):⁷

Non-deliberate, unprompted (within 12 months of tax being due): 0% to 30%.
Non-deliberate, prompted: 10% to 30%.
Deliberate but not concealed, unprompted: 20% to 70%.
Deliberate but not concealed, prompted: 35% to 70%.
Deliberate and concealed, unprompted: 30% to 100%.
Deliberate and concealed, prompted: 50% to 100%.

Someone who simply didn’t know they needed to register, comes forward voluntarily, and cooperates fully has the best chance of a 0% penalty.

HMRC will consider a reasonable excuse when something genuinely prevented you from registering on time. Accepted reasons include serious illness, the death of a close relative near the deadline, a hospital stay, a fire or flood that destroyed your records, a failure in HMRC’s own online services, or reliance on someone else who let you down. Not having the money, finding the online system difficult, or not receiving a reminder from HMRC will not get a penalty waived.⁸

After HMRC Processes Your Registration

HMRC issues a Unique Taxpayer Reference: a 10-digit number you use on every return and in every piece of correspondence about your Self Assessment. If you registered online, expect it by post within about 15 working days; postal applications take longer, and overseas addresses longer still.⁹ If your UTR hasn’t arrived by then, check current HMRC processing times on GOV.UK before calling.⁵

Once you have your UTR, the filing and payment dates for each tax year are:

31 October: paper returns.
31 January: online returns, and payment of the tax owed. The 2025–26 return and payment are due by 31 January 2027.
31 July: second payment on account, where HMRC has split your estimated bill for the following year into two advance instalments.⁶

Keep the UTR somewhere safe. It appears on your return, payment reminders, and HMRC letters, and if you lose it you can recover it through your Personal Tax Account on GOV.UK or by calling HMRC.⁹

1
GOV.UK. Tax on Savings Interest: How Much Tax You Pay
2
GOV.UK. Register for Self Assessment if You Are Not Self-Employed
3
GOV.UK. High Income Child Benefit Charge
4
ICAEW. How to Register Clients for Self Assessment or as Self-Employed
5
GOV.UK. Check How to Register for Self Assessment
6
GOV.UK. Self Assessment Tax Returns: Deadlines
7
GOV.UK. Compliance Checks – Penalties for Failure to Notify – CC/FS11
8
GOV.UK. Disagree With a Tax Decision or Penalty: Reasonable Excuses
9
GOV.UK. Find Your UTR Number