Self-Assessment Made Simple: A Guide for Salisbury Sole Traders

For a lot of sole traders, Self Assessment is the most stressful date in the calendar. It doesn’t need to be. The actual return is more straightforward than people imagine. What creates the stress is leaving it to the last minute, not knowing what to include, and worrying about getting it wrong.

This guide walks through what Self Assessment actually involves for a sole trader in Salisbury, the deadlines that matter, the common mistakes that catch people out, and when it’s worth handing the whole thing to an accountant.

What Self Assessment Actually Is

Self Assessment is the system HMRC uses to collect income tax from people whose tax isn’t automatically taken off their pay. If you’re a sole trader, a freelancer, a landlord, or a company director, you’ll usually need to file one.

You’ll typically need to register for Self Assessment if you:

• Earned more than £1,000 from self-employment in a tax year
• Are a partner in a business partnership
• Have rental income above £1,000
• Earn additional untaxed income (some dividends, savings interest, foreign income)
• Need to claim certain tax reliefs

If this is your first year, you must register with HMRC by 5 October following the end of the tax year you started trading. HMRC will then send you a Unique Taxpayer Reference (UTR) which you’ll need to file.

The Deadlines That Matter

The UK tax year runs from 6 April to 5 April. For the 2025-26 tax year (which ended 5 April 2026), the dates to know are:

The big one is 31 January. Miss it and there’s an automatic £100 penalty, even if you owe no tax. Penalties escalate quickly from there.

Heads up: From April 2026, sole traders earning above £50,000 are subject to Making Tax Digital for Income Tax, which means quarterly digital reporting in addition to your annual return. We’ve covered MTD in a separate post, so if it applies to you, read that first.

What You Need to Include

For most sole traders in Salisbury, a Self Assessment return covers:

• Self-employment income: total turnover from the business
• Allowable business expenses: costs you can deduct from your income (more on this below)
• Other income: employment income (if you also have a job), dividends, interest, rental income, etc.
• Tax already paid: through PAYE, on dividends, etc.
• Pension contributions: for tax relief
• Gift Aid donations: also tax-relieved

You don’t send receipts or invoices to HMRC, but you must keep them for at least 5 years after the 31 January deadline. HMRC can ask to see them in a compliance check.

What You Can Claim as Business Expenses

Allowable expenses are costs that are “wholly and exclusively” for your business. The most common ones for a Salisbury-based sole trader:

• Office costs: stationery, phone bills, printer ink, business software
• Travel costs: mileage for business journeys (currently 55p/mile for the first 10,000 miles), train tickets, parking, accommodation
• Premises costs: if you rent an office or workspace; or use of home as office if you work from home
• Stock and materials: anything you buy to sell or use in delivering your service
• Staff costs: wages, subcontractor costs, employer NIC
• Professional fees: accountant fees, legal fees, business insurance
• Marketing: website costs, advertising, business cards
• Bank charges: on a business account

There are two key things people miss:

1. Use of home as office: If you work from home, you can claim a proportion of household bills (heating, electricity, broadband, council tax) based on the rooms used and the time spent. HMRC also offers a simplified flat rate (£10-£26 per month depending on hours worked) if you don’t want to do the calculation.
2. Capital allowances: If you use the accruals basis (i.e. account for transactions on the dates of invoices, rather than on the cash basis (i.e. account for transactions when actually paid and received), bigger items such as a laptop, a van or equipment usually go through capital allowances rather than being claimed as a one-off expense. You can often claim the full cost in the year you buy it under the Annual Investment Allowance.

The Mistakes That Catch People Out

In our experience working with sole traders in Salisbury and across Wiltshire, the same handful of mistakes come up every year:

• Forgetting payments on account: If your tax bill is over £1,000, HMRC will ask you to pay 50% of next year’s bill in advance, on top of this year’s bill. Your January payment can be much bigger than you expect.
• Mixing personal and business expenses: Buying business supplies on a personal card (or vice versa) makes it harder to track and easier to miss claims. Open a separate business account; even sole traders benefit.
• Underclaiming: Many sole traders are so worried about overclaiming that they leave money on the table. Mileage, use of home, software subscriptions and professional memberships all add up.
• Leaving it until January: The return itself isn’t bad. Doing it in a panic with the wrong paperwork is.
• Missing the registration deadline: If you started trading in 2025-26 and haven’t registered yet, you’ve got until 5 October 2026.

When to File It Yourself vs. Using an Accountant

If your situation is simple, with one source of self-employment income, no big assets and no rental property, filing your own return is perfectly doable. HMRC’s online system walks you through it, and there’s no shame in keeping costs down when the return is straightforward.

It tends to be worth getting an accountant involved when:

• You’re not sure what you can claim (and you suspect you’re underclaiming)
• You have multiple income sources, such as employed plus self-employed plus rental
• Your turnover is growing and you’re starting to wonder about VAT or going limited
• You’ve had a year with anything unusual, such as selling business assets, taking on staff, or big equipment purchases
• The stress isn’t worth the saving

For most of the sole traders we work with, a Self Assessment is a small piece of a bigger picture, alongside bookkeeping, VAT, and tax planning. Having the same person who knows your business handle it usually catches things a one-off filing misses.

Getting Help With Your Salisbury Self Assessment

If you’d like help with your Self Assessment, whether that’s a one-off return or ongoing support, we’d be happy to talk it through.

Caboodle Business Support is a Salisbury-based accountancy and business support firm. Our enthusiastic team brings decades of combined experience helping sole traders and small businesses right across Salisbury and Wiltshire, with Self Assessment, bookkeeping, VAT and the full range of small business accountancy services, all in plain English.

If you’re not sure whether you should stay a sole trader or go limited, our Sole Trader or Limited Company? post covers that question. To see how we work with our clients, have a look at our process, or get in touch and we’ll set up a chat.