Somewhere in your broker's app there may be a sale from last winter: a fund trimmed to pay for a boiler, or a tech holding cashed in after a good run. If that sale happened outside an ISA and left you with a profit, it belongs to the 2025/26 tax year, and HMRC's cut-off for telling it you need a tax return falls on Monday 5 October 2026. The date tends to catch out people who have never filed a return in their lives, because nothing in the day-to-day experience of owning shares prompts the question. Your salary has already been taxed through PAYE, the dividends arrive in your account looking like finished business, and the whole picture feels settled until a gain or a dividend total crosses a line you did not know was there.
What the 5 October date does and does not mean
Registering for Self Assessment and filing a tax return are two separate steps, and Monday's deadline only concerns the first.
Under GOV.UK's rules, you must tell HMRC by 5 October 2026 if you need to complete a return for 2025/26, the year that ran from 6 April 2025 to 5 April 2026. That applies if you have either never sent a return before or registered earlier but did not need to send one for 2024/25. Registering is how you tell them. The return itself is due later: 31 October 2026 for a paper version, or 11:59pm on 31 January 2027 online, which is also the date any tax you owe must reach HMRC. If you register after 5 October, HMRC writes to you with a different filing deadline, set three months from the date of its letter or email, but the 31 January payment date does not move. Late registrants who have not paid everything by then can also receive a "failure to notify" penalty, calculated on whatever is still unpaid. That penalty is issued within 12 months of HMRC receiving your return, so it can land long after the original lapse has been forgotten. Missing Monday is therefore not a catastrophe if you pay on time, though it leaves you in a worse position than registering this weekend. And since the paper deadline is three months earlier than the online one, the online return is the better choice for nearly everyone.
Which investment activity can trigger a return
Shares held inside a stocks and shares ISA sit outside this question: no Capital Gains Tax on disposals and no tax on dividends. GOV.UK also lists UK government gilts and Qualifying Corporate Bonds among the assets that do not attract Capital Gains Tax when sold. The exposure sits in a general investment account, or wherever you hold shares, unit trusts and certain bonds directly, and it comes in three forms.
- Gains above £3,000. The annual exempt amount is £3,000 for both 2025/26 and 2026/27, and gains above it are taxed at 18% within your basic-rate band and 24% above it.
- Dividends above £500. The dividend allowance is £500, and dividend payments that exceed both your unused Personal Allowance and that allowance need to be reported to HMRC.
- If you are already registered for Self Assessment, total sale proceeds of more than £50,000 in the year have to go on the return even when the gain itself is under £3,000, so a large sale of a holding that barely moved still counts.
A worked example shows how much the rate depends on everything except the shares. Take a basic-rate taxpayer who sold shares from a general investment account in February for a £10,000 gain, with no other disposals that year. After the £3,000 exempt amount, £7,000 is taxable, and if that amount added to your taxable income still falls inside the £37,700 basic-rate band, the bill is £1,260 at 18%. Someone with the identical gain whose income already fills that band pays 24% on the same £7,000, which is £1,680. The £420 difference comes entirely from where the rest of the income sits, not from anything done with the shares.
Self Assessment or the real-time service: two routes for one gain
HMRC lets you report a gain on shares, funds or most other assets that are not UK residential property either in a Self Assessment return or through its "real time" Capital Gains Tax service. The service accepts gains on assets sold in 2025/26 and 2026/27, it is only for UK residents reporting on their own behalf, and it wants a copy of your calculations attached to the report. For a 2025/26 gain you report by 31 December 2026 and pay by 31 January 2027, with HMRC sending a payment reference beginning with an X once the report is in. There is a catch for anyone already registered for Self Assessment: you must also include the sale on your return, so the service does not replace it.
Someone with a single sale and nothing else to declare may find the real-time route lighter. If you are not sure which route applies to you, register on Monday and sort it out afterwards. Registering is not free of consequences, because once HMRC expects a 2025/26 return, filing after 31 January 2027 starts with a £100 penalty, followed after three months by £10 a day up to a maximum of £900. If it later turns out you did not need to file, GOV.UK has a process for telling HMRC you no longer need to send a return.
Working out the gain without guessing
Many people who build a holding through monthly purchases assume the gain is measured against the most recent buy. It is not. For shares of the same type in the same company bought at different times, HMRC uses the average cost, and GOV.UK's own example makes the arithmetic clear: 100 shares at 80p and then 300 at £1.20 give 400 shares costing £440, an average of £1.10 each, so selling 150 of them carries a cost of £165 for tax purposes. Dealing charges such as stockbroker fees, and the Stamp Duty Reserve Tax you paid when buying, can be deducted from the gain.
Buy the same shares back within 30 days of selling and special rules replace the simple average for working out the cost, which is why a quick sell-and-rebuy to bank a loss rarely works as hoped. A Bed and ISA transfer starts with a sale in the general investment account, and that sale is a disposal like any other, so it belongs in the calculation if the shares had risen in value. A platform's sales history, where it provides one, is a starting point rather than a finished calculation, and whether its gain figures follow HMRC's matching rules is something you need to check yourself, especially if you hold the same company in more than one account.
Dividends are smaller sums and easier to forget
The £500 dividend allowance sounds generous until you hold a handful of income-paying shares outside an ISA. The rates that apply to the 2025/26 return are 8.75% for basic-rate taxpayers, 33.75% for higher-rate and 39.35% for additional-rate. From 6 April 2026 the first two have risen to 10.75% and 35.75%, with the additional rate unchanged at 39.35%, so the 2025/26 return uses the old, lower rates while dividends received since April are taxed at the new ones.
Suppose £1,500 of dividends arrived outside an ISA and earnings have already used up your Personal Allowance. After the £500 allowance, £1,000 is taxable, which costs a basic-rate taxpayer £87.50 for 2025/26 and £337.50 for a higher-rate taxpayer. The same dividends in 2026/27 would cost £107.50 and £357.50 respectively. The sums are small, but they are exactly the kind that grow quietly as a portfolio does. The same £1,500 paid inside an ISA would have cost nothing, because dividends from shares held in an ISA are not taxed. If your dividends sit around the allowance every year, moving income-paying holdings into an ISA is a better long-term answer than filing a return annually to account for them.
Before Monday, and for the months after
- Pull the 2025/26 sales history from every platform you used outside an ISA, including the old account you rarely open.
- Total the sale proceeds and the gains, then compare them with the £3,000 and £50,000 figures above.
- Add up the dividends you received outside ISAs and set them against the £500 allowance.
- If any answer is yes or unclear, register on GOV.UK by Monday.
The dates worth putting in a calendar after that are 31 October 2026 for a paper return, 31 December 2026 for a real-time Capital Gains Tax report, and 31 January 2027 for the online return and the payment.
One last detail for anyone whose year produced a loss rather than a gain: you can claim an allowable loss up to four years after the end of the tax year in which you made the disposal, so Monday is not a deadline for that. You claim it on a tax return, or by writing to HMRC if you have never made a gain and are not registered for Self Assessment. Losses are set against gains from the same year first, and any left over can be carried forward, which is worth remembering the next time a sale goes the wrong way.
Dates, rates and allowances in this article come from GOV.UK guidance checked on 4 October 2026. It is general information, not personal tax advice.