UK Self Assessment From Scratch — Part 2: UTR and Government Gateway Setup

Quick answer

To get yourself set up for Self Assessment, you need three things, in this order:

1. Decide which registration route fits you (self-employed sole trader / partner / other-income only / former filer reactivating).
2. Register on gov.uk — you'll receive your UTR by post in ~10 working days.
3. Activate your Government Gateway account using a separate code that also arrives by post.

After that you're set — same login is used for filing your return, paying tax, viewing past returns, requesting refunds, and changing your address.

This is Part 2 of 8 in our Self-Assessment From Scratch series. See Part 1: Do you need to file? if you haven't confirmed you actually need to register.

What you'll need to hand before you start

Have these ready:

• National Insurance number (format AB 12 34 56 C — find it on a P60, payslip, or in your HMRC app).
• Passport or driving licence for identity verification.
• A current UK address that HMRC can post to (paper letters are part of the process — you can't do this fully online).
• A working email address.
• A UK phone number for two-factor authentication later.
• If self-employed: the date you started self-employment, the nature of your business, and (optionally) a business name if you trade under one.

Step 1 — Pick the right registration route

There are three main routes:

Route A — Self-employed (sole trader)

Use this if you've started earning self-employed income (freelance, gig work, side hustle over £1k turnover, sole-trader business). Go to:

gov.uk/log-in-file-self-assessment-tax-return/register-if-youre-self-employed

This route registers you for both Self Assessment and as a sole trader for National Insurance purposes simultaneously.

Route B — Not self-employed, but other untaxed income

Use this if you need to file Self Assessment for reasons other than self-employment — rental income, side dividends, foreign income, HICBC, savings interest over PSA, etc. Go to:

gov.uk/register-for-self-assessment → choose "I'm not self-employed".

This uses the SA1 form. You'll need to specify the reason for registering (drop-down list).

Route C — Partner in a partnership

Use this if you've joined a business partnership. The partnership itself also needs to register (form SA400, done by the nominated partner). Individual partners use SA401. Both partnership and individual UTRs are needed.

Route D — Former filer, reactivating

If you've previously had Self Assessment and HMRC removed you (because you didn't need it), you can ask to be put back on the SA register via the same routes — HMRC will pick up your existing UTR rather than issuing a new one.

Step 2 — Complete the online registration

The form is short (5–10 minutes if you have your details to hand):

1. Verify your identity via Government Gateway. If you don't already have a Gateway account, you'll create one in this step — passport or driving licence is the easiest identity option.
2. Enter your personal details — name, address, NI number, date of birth.
3. State the reason for registering (self-employed, rental income, HICBC, etc.).
4. For self-employed: start date of business, nature of business activity (a SIC code may be suggested), trading name if any.
5. Submit.

You'll see a confirmation screen with a reference number — save this. HMRC sends an immediate email confirming receipt.

Step 3 — Wait for your UTR by post

HMRC processes new registrations in 5–10 working days. They send your UTR number to your registered address by post — on an HMRC-branded letter titled "Your Unique Taxpayer Reference".

The letter contains:

• Your UTR (10 digits, formatted as 1234567890).
• Confirmation of the tax year for which you're now required to file.
• A reminder of the upcoming filing deadlines.

Save this letter. You'll quote the UTR on every future HMRC communication.

Step 4 — Activate your Government Gateway account

A second letter arrives separately, typically a few days after the UTR letter. It contains an activation code for your Government Gateway online tax account.

1. Go to gov.uk/log-in-file-self-assessment-tax-return and click "Sign in".
2. Sign in with the Government Gateway user ID you created at registration.
3. You'll be prompted for the activation code from the letter.
4. Enter it within 28 days of issue — after that, you'll have to request a new one (delays things by another 1–2 weeks).

Once activated, you'll see your Self Assessment dashboard.

Step 5 — Set up two-factor authentication

HMRC requires two-factor for all tax accounts:

• By default: SMS to a registered UK mobile number.
• Alternative: the HMRC app (preferred — app-based codes are faster than SMS and don't require signal).
• Backup codes: print a set when prompted, store somewhere safe.

The HMRC app also lets you view your UTR, NI record, tax code, employer details, State Pension forecast and Self Assessment status — useful even if you don't file from the app itself.

Common problems

"I never received my UTR letter"

If 15 working days have passed since registration:

• Check the address you registered with — HMRC posts to the address on file at the time of registration.
• Sign in to your Government Gateway and check whether the UTR is visible in your account (sometimes it's there before the letter arrives).
• If still missing, call HMRC Self Assessment helpline (0300 200 3310) with your NI number to request a reissue.

"The activation code expired"

Within Government Gateway, request a new activation code — it'll be reposted. Typical wait: 7 working days.

"I have multiple Government Gateway accounts"

Quite common — many people created one for Personal Tax Account, then another by accident. HMRC strongly recommends merging or just standardising on one. The "Manage your accounts" link in Government Gateway lets you check what's linked.

"I forgot my Gateway user ID"

The "Forgotten your user ID" link emails it to the address registered. The recovery process needs your NI number.

"I'm abroad and don't have a UK address"

You can still register but you'll need to update your address with HMRC before each major communication. Some non-UK addresses cause significant delays (postal time). HMRC has a "non-resident" Self Assessment process — speak to them before registering if you're moving abroad.

What you'll be able to do once set up

Logged into Government Gateway / HMRC app, you can:

• File your annual Self Assessment return (online form, save and resume).
• View previous returns and statements.
• Pay tax owed (direct debit, debit card, Faster Payment).
• Track payments on account, refunds and balances.
• Request Time to Pay arrangements.
• Change address, phone number, marriage status.
• Update your business details (start/stop trading).
• View your tax code and PAYE employment data feed.
• Download your Self Assessment SA302 (tax calculation summary — sometimes needed for mortgage applications).
• Authorise an accountant to act on your behalf (a separate code-exchange process).

When to register an accountant (and how)

If you decide to use an accountant for your Self Assessment, they need to be authorised to access your tax record:

1. Your accountant submits a "Agent Authorisation" request to HMRC against your UTR.
2. HMRC posts you a code (3–7 days).
3. You give the code to your accountant, who enters it on HMRC's agent portal.
4. They can now see your record, file your return, and correspond with HMRC on your behalf.

You retain full personal access throughout. You can revoke an agent at any time via gov.uk.

What's next in the series

• Part 3 — Declaring income (employed, self-employed, dividends, foreign, crypto)
• Part 4 — Allowable expenses if you're self-employed
• Part 5 — Capital gains in your Self Assessment
• Part 6 — Payments on account explained
• Part 7 — Making Tax Digital — what's changing
• Part 8 — After you file: refunds, audits, amendments

Once registered with your UTR and active Government Gateway, you're set for life — the rest of the series is about what you actually put on the form.

Sources

• HMRC: Register for Self Assessment
• HMRC: Self Assessment tax returns
• HMRC: HMRC app
• HMRC: Get help finding your UTR
• gov.uk: HMRC services: sign in or register