Missed the 5 October Self Assessment Deadline? Register Late + Spreadsheet Setup (UK)

This article is general information about registration and record-keeping, not personalised tax advice. Rules change. Confirm everything on GOV.UK or with a qualified adviser before you act.

Who this is for

This is for people in the UK who may need Self Assessment for a tax year that has already ended (6 April to 5 April) and who have either:

• never sent a tax return before, or
• registered before but did not need to send a return for the year before that

Typical first-timers include PAYE employees with a new side hustle, freelancers, and sole traders. Other reasons exist too (property income, certain gains, and more). Do not guess from a blog — use HMRC’s checker.

GOV.UK currently says you must send a return if, in the last tax year, you were self-employed as a sole trader and earned more than £1,000 (before reliefs), among other triggers. Re-check that page for the live list — thresholds and reasons can change.

Savings interest alone is a separate rabbit hole. If that is your only question, use GOV.UK’s checker rather than treating this side-hustle checklist as the whole answer.

The 5 October rule (and what “late” means)

The registration date is not the same as the filing date.

On GOV.UK, the recurring pattern is: you must tell HMRC by 5 October after the end of the tax year in which you had the untaxed income / Self Assessment trigger, if you have not sent a return before (or you need to re-engage after a year with no return). Always re-check the live register and deadlines pages for the dates that apply to your tax year.

If you already missed 5 October

You can still register. GOV.UK’s practical message is to register as soon as possible.

What can change when you register late (confirm on GOV.UK — do not treat blog summaries as final):

• Failure to notify: if you register after 5 October and you do not pay all of the tax you owe by 31 January, you may get a “failure to notify” penalty. GOV.UK links that penalty to tax still left to pay and says you receive it within 12 months after HMRC receives your return — see Penalties.
• Filing date: if you register after 5 October, GOV.UK says HMRC will send a letter or email with a different deadline to send your return (often described as 3 months from the date on that letter or email).
• Payment date: you must still pay any tax due by 31 January (11:59pm on the deadline GOV.UK shows for that year) or you can get a late-payment penalty. Late registration does not move the usual payment deadline.

Registering promptly still matters because you need a Unique Taxpayer Reference (UTR) before you can file online.

Checklist: register late, get your UTR, gather records

Work through this in order even if 5 October has passed.

1. Confirm you need a return

Use who must send a tax return and HMRC’s checker. Keep a note of the date you checked.

2. Register (or reactivate) on GOV.UK today

Start at Check how to register for Self Assessment. Sole traders usually register as self-employed through that journey. If you registered before but skipped a year, GOV.UK notes you may need to reactivate inside the service.

Do not wait for a perfect spreadsheet before you register.

3. Watch for your UTR

HMRC issues a Unique Taxpayer Reference. GOV.UK’s Find your UTR number page currently says you will usually get your UTR by post around 15 days after you register (longer if you live overseas). Timing can vary by how you registered and what HMRC shows in your Personal Tax Account or the HMRC app — check GOV.UK’s live guidance on what to expect after registration rather than relying on a fixed wait.

4. Gather records for the tax year you are reporting

You will need figures for income and allowable expenses (and proof). Digital records are fine if they are accurate. See business records if you are self-employed.

5. Set up a spreadsheet the same week you register

A blank brain dump in Notes will not survive January. Use columns for date, source, income/expense, category, amount (GBP), and a tax-year tag (for example 2025/26).

6. Diary January — and any later filing date HMRC gives you

Online filing and payment of tax owed usually fall on 31 January after the tax year. If you registered late, also diary any different return deadline in HMRC’s letter or email. Re-check deadlines before you rely on any date.

What to track from day one (then read the full SA103 guide)

Minimum useful log for a side hustle:

1. Date
2. Source / client / platform
3. Type — income or expense
4. Category
5. Amount (GBP)
6. Tax year — e.g. 2025/26
7. Notes / invoice ref

Log weekly. Reconcile to bank, Stripe, or Etsy payouts monthly.

For a fuller walkthrough of SA103-style bookkeeping, columns, and common expense categories, read UK Self Assessment for Side Hustles: What to Track in a Spreadsheet (SA103).

That guide is about what to track. This page is about registering (including late) and starting the sheet before January.

How UkMoneySheets fits

If you want a ready-made Google Sheets layout (not Excel) for side-hustle income and Self Assessment prep, the filing-year sheet is the UK Side Hustle Tax Spreadsheet on UkMoneySheets on Etsy.

A Side Hustle Bookkeeping Bundle is a separate product if you want more than one year or extra tabs. It is not this link.

Delivery is typically a PDF with a Sheets link — then File → Make a copy into your Drive.

A template organises numbers. It does not register you with HMRC, file your return, or replace GOV.UK.

FAQ

Is 5 October the same as the tax return deadline?

No. 5 October is when many first-timers must tell HMRC they need a return. Online filing is usually 31 January on the current GOV.UK deadlines page — always re-check for your tax year.

I missed 5 October — am I banned from registering?

No. Register as soon as you can on GOV.UK. Late notification can mean a penalty in some cases, especially if tax is still unpaid at 31 January. See penalties.

What if I only started my side hustle recently?

If your income in the tax year that has ended triggers a return under GOV.UK’s rules, the registration date still matters. Check who must send a tax return for your facts.

Is this tax advice?

No. Checklist and education only.

Soft next step

1. Open register for Self Assessment today if the checker says you need a return — even if 5 October has passed
2. Start (or copy) an income and expense sheet for the tax year you are reporting
3. Read the full tracking guide: UK Self Assessment for Side Hustles (SA103)
4. If you want a UK Google Sheets template, grab a side-hustle / SA tracker from UkMoneySheets

Missed October? Register now. January is for filing with clean numbers — and for paying on time if you owe tax.