New Side Income This Year? Why 5 October Matters for Richmond Taxpayers

A quiet bit of extra income rarely announces itself as taxable. The date that decides whether it becomes a problem, rather than a five minute job, is 5 October.

You picked up a little work on the side this year, or let a room, or a run of sales quietly turned into something regular. None of it felt like starting a business. The money arrived, the days were full, and tax was the last thing on your mind. Then someone mentions Self Assessment, and you are not sure whether it applies to you, or by when. The reassuring part is that the rules are clearer than they feel, and the one date worth carrying in your head falls in early October, while the year is still fresh.

What you will learn

Do I need to file a Self Assessment tax return this year?

You do if you had income that tax was not already taken from, above the small allowances that exist for it. The most common triggers are self-employment income over £1,000, rental profit, dividends above the £500 dividend allowance, a capital gain, or the High Income Child Benefit Charge. If any of these applied for the 2025 to 2026 tax year, you must tell HMRC by 5 October 2026, and then file, usually online, by 31 January 2027.

At a glance

• Income that has not been taxed at source can create a duty to file, even when the amounts feel minor.
• The trading allowance and the property allowance are £1,000 each, measured on income before expenses, not on profit.
• You must register with HMRC by 5 October following the tax year, so by 5 October 2026 for the 2025 to 2026 year.
• File online and pay any tax by 31 January 2027; a paper return is due earlier, by 31 October 2026.
• A fixed £100 penalty applies to a late return even when no tax is due, so timing matters more than the size of the bill.

What actually counts as income HMRC wants to see

The clearest test is not how much you earned, but whether the tax has already been taken off before the money reached you. Your salary has; the tax comes out through PAYE and there is nothing more to do. Side income usually has not, and that is what puts it on HMRC’s radar.

Self-employment is the most common trigger. Earn more than £1,000 from working for yourself, before taking off any expenses, and a return is expected. Rental profit is the next most common locally, which is no surprise where a spare room or an inherited flat can quietly become a source of rent. Beyond those, you will generally need to file if you owe Capital Gains Tax after selling a second property or a meaningful holding of shares, if dividends take you past the £500 allowance, or if the High Income Child Benefit Charge applies and is not being collected through your tax code.

None of these is exotic. What they share is that the tax was not deducted at source, and that single feature is the one worth carrying in your head.

The £1,000 allowances that catch people

The two £1,000 allowances, one for trading and one for property, do a lot of quiet work. They keep the smallest amounts out of the system, and they lull people into assuming the threshold is higher than it is. A thousand pounds is not much across a full year.

The point that surprises people is that the allowance is measured against income, not profit. It is the money coming in before costs, not what is left after them. A small venture that feels barely worthwhile once you have paid for materials can still sit well above the line that matters to HMRC.

A designer in Twickenham took on a handful of freelance projects alongside her employed job, thinking of it as pocket money. By the spring it added up to a little over four thousand pounds. She had assumed that because tax came off her salary, and because the side work felt minor, there was nothing to do. There was. The work was over the trading allowance, and it needed reporting. Handled in good time, it was an hour of admin. Left until January, it would have been a scramble.

What 5 October is, and what it is not

It helps to separate two dates that often get blurred. The 5 October deadline is about telling HMRC that you need to be in the system for the year just ended. The filing itself comes later. A paper return is due by 31 October, and an online return by the following 31 January, which is also when any tax must be paid.

For the 2025 to 2026 tax year, that means registering by 5 October 2026 and filing online by 31 January 2027. Most people file online, so in practice the two dates that matter are early October to register and the end of January to file and pay.

The people who get caught out are almost never trying to hide anything. They simply did not realise a spare room or a few invoices had tipped them into Self Assessment. Ten minutes in October to check, and register if needed, saves a difficult evening at the end of January.

Donovan Crutchfield, ACA, Area Managing Partner, Xeinadin Richmond

What is worth doing now

The useful work at this stage is not the return itself. It is making the return easy to complete when the time comes. That means knowing which of your income sources are untaxed, and starting a simple record of them rather than trying to reconstruct the year from a bank statement in January.

For most people with a bit of side income the list is short. Keep a note of what came in and when, hold on to invoices or booking records, and set aside the paperwork for anything you sold that might create a gain. If you are genuinely unsure whether your situation crosses the line, that uncertainty is itself worth resolving early. It is far easier to check now, register if needed, and move on, than to discover in the new year that a deadline passed months ago.

If you want to go further, our guide Do I Need to File a UK Tax Return This Year? walks through the triggers in more detail, and How to Work Out What You Actually Owe HMRC covers turning those figures into the bill itself. If a property sale is part of the picture, our case study on a missed 60-day CGT deadline on a Richmond buy-to-let shows how quickly that particular deadline can bite.

Key takeaways

• If income reached you without tax deducted, assume HMRC will want to know, and check rather than hope.
• The £1,000 allowances are measured on income before expenses, so they are crossed sooner than people expect.
• Register by 5 October 2026, file and pay by 31 January 2027, and a late return costs a fixed £100 even with no tax due.

If any of this feels familiar, and you are not sure whether this is a year you need to file, a short conversation can usually settle it quickly. Our tax planning team at Xeinadin Richmond would rather help you check in the autumn than untangle a penalty in February.

This article is general information reflecting the tax rules and figures in force at the time of writing. Those rules and figures can change, including at a Budget, and any figures should be treated as indicative. It is not advice for your particular circumstances. For guidance tailored to you, please speak to us directly.