If you became self-employed, started renting out a property, or earned untaxed income for the first time between 6 April 2025 and 5 April 2026, HMRC needs to know by 5 October 2026 — confirmed on GOV.UK. The same deadline applies if you registered before but didn't need to send a return for 2024-25 and now do.
Miss it and the return deadline doesn't go away — HMRC will just write to you with a new one, usually three months from the date of that letter — but you still have to pay whatever you owe by 31 January 2027 regardless of when you were told to file. Register late and don't clear the bill by then, and you're in "failure to notify" penalty territory, calculated as a percentage of the tax still outstanding. It's not a fixed fine like the £100 late-filing penalty; it scales with what you owe, which makes ignoring it considerably more expensive than a five-minute form.
None of that is complicated on its own. What actually trips people up in their first year of trading is not the registration screen — it's realising in January that ten months of invoices, mileage and expenses were never written down anywhere, and now someone has to reconstruct a year from bank statements and memory.
Here's how to use the next two weeks properly.
What the 5 October deadline actually covers
This deadline is specifically about telling HMRC you exist as a taxpayer, not about filing anything yet. If any of these applied to you between 6 April 2025 and 5 April 2026, and you've never sent a Self Assessment return before, you need to register:
- You started working for yourself as a sole trader.
- You began earning rental income as a landlord.
- You had other untaxed income — dividends beyond your allowance, savings interest, foreign income — that needs declaring.
The actual return isn't due until 31 January 2027 (online) or 31 October 2026 (paper). Registration just gets you a Unique Taxpayer Reference and, if you're a sole trader, sorts out Class 2 National Insurance in the same process. Do it now on GOV.UK's registration checker and the UTR will typically be through well before the real work — the return — starts in earnest.
If you're not sure whether you actually need to register at all, HMRC's check if you need to send a tax return tool answers that in a couple of minutes. Worth doing before, not after, you register — reactivating or closing a Self Assessment record you didn't need is its own small headache.
Step 1: Register today, not the week of the 5th
Registration itself is quick, but HMRC's system has a habit of taking a few days to post back your UTR and activation code by letter, and you need those before you can file anything online. Registering on 3 October instead of 22 September buys you nothing and risks the letter arriving after the deadline has technically passed, even if HMRC treats the online submission date as the one that counts. Do it this week and it's off your list.
Step 2: Work out what "the year" actually contains
Once you're registered, the harder question is what you're going to put on the return. If you've been trading or letting since last spring without a proper system, the first job is establishing what a full year of income and expenses actually looks like — not guessing at it in January.
Pull together everything you have: bank statements for the business or rental account, any invoices you've raised, receipts for expenses, and — if you're a sole trader who drives for work — a rough log of business mileage, since HMRC's mileage rates apply from the very first business mile, not from when you got around to tracking it.
Step 3: Get outstanding invoices and receipts into software now
This is the step people skip in year one, and it's the one that costs the most time later. A pile of supplier receipts and client invoices sitting in an inbox or a shoebox is not a set of accounting records — it's raw material for a very long January evening.
If you're behind on entering months of paperwork, an AI invoice importer reads supplier, date, amount and VAT off a photo or PDF and posts it into Xero or QuickBooks, or produces a Sage 50-ready CSV, so a backlog becomes a reviewed ledger in an afternoon rather than a weekend. You still check what it's extracted — automation should remove the typing, not the judgement call about what's genuinely a business expense.
Step 4: Set up going forward, not just backward
Registering is also the natural moment to fix how you'll capture things from here, since everything from 6 April 2026 onward is next year's return, and you don't want to be having this same conversation with yourself in September 2027. Pick one place to send invoices and receipts as they arrive — a forwarding email address, a folder, whatever you'll actually use — and keep a simple mileage log from your very next business journey. A habit started in October is worth more than a resolution made in January.
The quick checks, for free
If you just need a number rather than a system — what your mileage claim is worth, what VAT applies to a figure, what interest you can charge on an overdue client invoice — our free mileage, VAT, CIS and late-payment calculators answer that in seconds. No sign-up, no email capture.
What "done" looks like by 5 October
- You're registered for Self Assessment, or you've confirmed with HMRC's checker that you don't need to be.
- You know roughly what income and expenses the 2025-26 return needs to cover.
- Outstanding invoices and receipts from this year are on their way into software, not sitting in a pile.
- You have a way to capture the next twelve months as you go, not in a rush next January.
If self-employment is new to you this year, the free plan on the AI Invoice Importer handles 15 documents a month with no card required — a reasonable way to find out whether it beats typing before you commit to a full year of it.