What happens after registering for Self Assessment: 15 days

Your first January bill is one and a half times the tax you actually owe. On £28,000 of profit that means £6,017.70 leaving your account on 31 January 2027, not the £4,011.80 the year’s tax comes to. Nobody mentions that on the registration screen. So this is what happens after registering for Self Assessment, in the order it happens, with dates, and with the arithmetic for that first bill further down.

I re-checked every figure here against GOV.UK on 11 September 2026. The one that surprised me is the UTR timing. GOV.UK now says around 15 days, and a lot of older guides still say 10 working days.

The UTR letter: around 15 days, not 10

Nothing is emailed. Your Unique Taxpayer Reference comes by post to the address you gave when you registered, and GOV.UK’s “Find your UTR number” page puts it like this: “You’ll usually get your UTR by post around 15 days after you register. It takes longer if you live overseas.” It’s a 10-digit number and you’ll need it for every return, every payment and every call to HMRC from now on.

Once it exists, the same number also shows in your personal tax account and in the HMRC app, and it’s printed on later HMRC letters. So the letter matters less than people think, as long as you can sign in. If the letter hasn’t turned up after three weeks, use GOV.UK’s “Check when you can expect a reply from HMRC” tool, which has a Self Assessment option covering UTRs, before you ring the Self Assessment helpline on 0300 200 3310 (Monday to Friday, 8am to 6pm). Don’t register a second time. Two registrations mean two records to untangle. There’s more on the number itself in our UTR number guide.

One catch worth knowing. If you’d already registered for Self Assessment for another reason (rental income, say) and have now started trading, GOV.UK says you need to register again as a sole trader so that Class 2 National Insurance is set up on your record. Same UTR, but a second registration through the self-employed route.

Government Gateway, One Login and the activation code

There are two ways into HMRC online services now: a Government Gateway user ID (up to 12 characters) with a password, or GOV.UK One Login with an email address and password. GOV.UK’s advice is to keep using whichever sign-in details you already have. If you created new ones during registration, those are the ones to use in January. Write them down somewhere that isn’t the same phone you’ll use for the security code.

Now the activation code question, because the answer changed quietly. The public GOV.UK registration pages no longer mention a code at all. The detail lives in HMRC’s own Self Assessment Manual (SAM126001, updated 21 August 2026), which says that once you’ve registered for the Self Assessment online service you get an activation code, that “this Code is sent through the post within 7 days of enrolment and should arrive within 10 days (21 days if it is being sent abroad)”, and that the service “must be activated within 28 days of the date shown on the Activation Code letter”. HMRC’s Self Assessment contact page still lists “activation codes” as a topic its digital assistant handles. So yes, codes still exist in 2026.

Whether you get one depends on your account. When the UTR letter arrives, sign in and look for Self Assessment on your account page. If it’s already there, you’re done and no code is coming. If it isn’t, add the service using your UTR, and expect the letter. Miss the 28-day window and you request another one, which costs you a fortnight you may not have in January.

Download the HMRC app while you’re at it. GOV.UK’s app page (updated 14 August 2026) lists your UTR, how much Self Assessment you owe and making a Self Assessment payment among the things it does. You sign in with the same details, then set a 6-digit PIN, fingerprint or face unlock so you’re not typing a password every time.

Registered on 25 September 2026: the dated timeline

Say you registered on Friday 25 September 2026, online, through the self-employed route, for income you started earning in the 2025/26 tax year. Here is what should land and when, with the job for each week. Dates in the “what arrives” column come from GOV.UK and the HMRC manual; the “what you do” column is mine.

Two things the table can’t show. First, HMRC’s “around 15 days” is a typical figure, not a promise, and late September is a busy registration period, so treat 10 October as the earliest you should start wondering, not the day you start ringing. Second, the 5 October deadline is about telling HMRC you exist. It has nothing to do with when the return or the money is due. Both of those are 31 January 2027 regardless. Our first Self Assessment walkthrough covers the return itself, box by box.

Your first January bill on £28,000 profit

This is the part that catches people. Take a sole trader whose 2025/26 profit (income minus allowable expenses, 6 April 2025 to 5 April 2026) was £28,000, with no employment income, no student loan and an address in England, Wales or Northern Ireland. The rates are the 2025/26 ones from GOV.UK, and they’re unchanged for 2026/27: Personal Allowance £12,570, basic rate 20%, Class 4 National Insurance 6% on profits between £12,570 and £50,270. Our free UK tax calculator runs on the same rates if you’d rather not do it by hand.

Why the extra £2,005.90? Payments on account kick in when last year’s bill was £1,000 or more and less than 80% of it was collected at source, which describes almost every full-time sole trader. GOV.UK’s rule: each payment is half of the tax you owed last year, including Class 4, due 31 January and 31 July. In year one you pay the whole of 2025/26 and half of 2026/27 on the same day. That’s £6,017.70, or 21.5% of the £28,000. From the second January onwards it settles down: if profit stays at £28,000, January 2028 is a balancing payment of nil plus one payment on account of £2,005.90.

If you expect 2026/27 profit to be lower, you can ask HMRC to reduce the payments on account. Reduce them too far and interest runs on the shortfall, so guess carefully. The mechanics, and what happens if you had a PAYE job alongside, are in payments on account explained. One more thing on the £28,000 profile: it’s under the £30,000 Making Tax Digital line that starts in April 2027 and the £20,000 line from April 2028, so this first return is the ordinary online one, not quarterly updates.

The Side Hustle Tax Starter course takes a new registrant from this point through to the submitted return, with the pot percentages worked out for your own numbers.

After registering late: what happens if you missed 5 October

Register after Monday 5 October 2026 and two things change. GOV.UK’s deadlines page says HMRC “will send you a letter or email with a different deadline to send your tax return by – this will be 3 months from the date on the letter or email”. So the filing date can move later than 31 January. The payment date doesn’t. Tax for 2025/26 is still due by 11:59pm on 31 January 2027, and interest runs from then on anything unpaid.

The second change is that GOV.UK says “you could get a penalty” for telling HMRC late. In practice that penalty is worked out on tax still unpaid at 31 January, so paying on time is what protects you even if the return itself is on a later date. The detail is in our guide to the penalty for registering self-employed late. Filing late once the return is due costs £100 straight away, then £10 a day after three months, up to £900.

Questions new registrants ask

How long does it take to get a UTR number after registering?

GOV.UK says you’ll usually get it by post around 15 days after you register, and longer if you live overseas. It also appears in your personal tax account and the HMRC app once issued. After three weeks with nothing, use the “Check when you can expect a reply from HMRC” tool, then the helpline on 0300 200 3310.

Do I still need an activation code for Self Assessment in 2026?

Only if Self Assessment isn’t already showing on your online account when you sign in with your UTR in hand. If you have to add the service, HMRC’s Self Assessment Manual says the code is posted within 7 days, should arrive within 10 (21 abroad) and must be used within 28 days of the date on the letter.

When is my first tax return due after registering for Self Assessment?

If you registered by 5 October 2026 for the 2025/26 tax year, the online return is due by 11:59pm on 31 January 2027 (paper by 31 October 2026), with payment due 31 January 2027. Register after 5 October and HMRC’s letter or email gives you 3 months from its date to file, but the payment date stays at 31 January 2027.

What to do, in order

• Put 25% of every client payment into a separate pot from today. On £28,000 the first January needs £6,017.70, and that’s before the July instalment.
• Diary 10 October as the earliest UTR date and 16 October as the day you check the reply-time tool if nothing has arrived.
• When the letter lands, sign in the same day, confirm Self Assessment shows, and add it if it doesn’t so any activation code is in your hands by late October.
• Install the HMRC app and confirm your UTR appears there, so losing the letter costs you nothing.
• Draft the return in November using the payment methods and lead times to pick how you’ll pay: Faster Payments or card if it’s the last weekend, Bacs by 26 January otherwise.
• File and pay before 31 January 2027, then set a reminder for 31 July 2027 and the second £2,005.90.

Sources

• GOV.UK: Register for Self Assessment
• GOV.UK: Become a sole trader: Register as a sole trader
• GOV.UK: Find your UTR number
• GOV.UK: HMRC online services: sign in or set up an account
• GOV.UK: HMRC Self Assessment Manual SAM126001, filing returns online
• GOV.UK: Self Assessment: general enquiries
• GOV.UK: Check when you can expect a reply from HMRC
• GOV.UK: Download the HMRC app
• GOV.UK: Self Assessment tax returns: Deadlines
• GOV.UK: Self Assessment tax returns: Penalties
• GOV.UK: Understand your Self Assessment tax bill: Payments on account
• GOV.UK: Pay your Self Assessment tax bill
• GOV.UK: Pay your Self Assessment tax bill: Through your tax code
• GOV.UK: Income Tax rates and allowances for current and past years
• GOV.UK: Rates and allowances: National Insurance contributions

Figures checked against GOV.UK and the HMRC Self Assessment Manual on 11 September 2026. This is general information, not financial or tax advice.

Read next

• Self Assessment Deadlines: Which Return Are You Filing?
• Reducing Your Payments on Account: When and How to Apply