How to Fill in the SA103F Form: Complete Guide for UK Sole Traders

The SA103F is the full version of the self-employment supplement that accompanies your Self Assessment tax return (SA100). If your annual turnover from self-employment exceeds the VAT registration threshold, or if your business is more complex, HMRC requires you to complete the SA103F rather than the shorter SA103S version. For the 2025/26 tax year, the threshold that determines which form you use is a turnover above 85,000 pounds.\n\nAs a sole trader tradesperson, you will need to file the SA103F if your business has grown past the threshold, if you have multiple income streams from self-employment, or if you need to claim certain reliefs such as overlap relief or capital allowances on significant equipment purchases. Even if your turnover dips below the threshold during the year, you may still need to use the full form if HMRC has previously directed you to do so.\n\nThe SA103F covers your business income, allowable expenses, capital allowances, and calculates your taxable profit or loss. It feeds directly into the main SA100 return to determine your overall income tax and National Insurance liability. Understanding each section of this form is essential because errors can trigger HMRC enquiries, delays in processing your return, or incorrect tax calculations that cost you money.\n\nYou must submit the SA103F alongside your main Self Assessment return. The deadline for online filing is 31 January following the end of the tax year. For the 2025/26 tax year, this means filing by 31 January 2027. Paper returns have an earlier deadline of 31 October 2026.

The income section of the SA103F captures your total business turnover and any other business income. Box 1 asks for your total turnover, which is the total sales and fees earned by your business during the tax year, before deducting any expenses. For tradespeople, this includes all invoiced work, cash payments, and any materials you charged to customers on top of your labour.\n\nBox 2 covers any other business income not included in your turnover figure. This might include bank interest on your business account, grants received, or income from hiring out your tools or equipment. It is important not to double-count income that already appears in Box 1. If you received the Small Business Rate Relief or any government grants, check whether they count as taxable income.\n\nBoxes 3 to 6 handle situations involving trading and professional income adjustments, including goods or services for personal use and balancing charges on disposed assets. If you took materials from a job for personal use, you need to add the market value back as income in these boxes. Balancing charges occur when you sell equipment for more than its written-down value in your capital allowances pool.\n\nGetting the income section right is crucial because HMRC cross-references your declared turnover against bank records, CIS statements, and data from your customers. Underreporting income is one of the most common triggers for an HMRC enquiry. TradeTally automatically totals your invoices for the tax year and separates business income by type, making this section straightforward to complete.

The expenses section is where most sole traders reclaim a significant portion of their tax liability. Boxes 7 through 30 break your business costs into specific categories that HMRC uses to verify the nature of your spending. For tradespeople, the key boxes are cost of goods sold (materials purchased for jobs), wages and subcontractor payments, vehicle costs or mileage, tools and equipment, telephone and internet, and professional subscriptions.\n\nBox 10 covers the cost of goods bought for resale or goods used. For a plumber, this would include copper pipe, fittings, boilers, and bathroom suites purchased for customer installations. An electrician would include cabling, switches, consumer units, and testing equipment consumables. Only include materials you actually used or sold during the tax year, not stock sitting in your van at year end.\n\nBox 17 is for travel costs including vehicle expenses. You have two options here: claim the actual costs of running your vehicle (fuel, insurance, servicing, road tax) proportioned for business use, or use the simplified mileage allowance of 45p per mile for the first 10,000 business miles and 25p thereafter. Once you choose a method for a particular vehicle, you must stick with it for as long as you use that vehicle in your business.\n\nBoxes 19 to 24 cover premises costs, repairs, general administrative expenses, phone costs, and other allowable business expenses. If you work from home, you can claim a proportion of household costs (heat, light, broadband, council tax) based on the rooms and time used for business, or use the simplified flat rate of between 10 and 26 pounds per month depending on hours worked from home. Keep detailed records of every claim, as HMRC can request receipts and documentation for any expense line.\n\nTradeTally categorises your expenses automatically as you photograph receipts and log costs throughout the year. When it is time to fill in the SA103F, each box total is pre-calculated from your expense records, saving hours of manual sorting.

Capital allowances let you deduct the cost of business assets from your profits before calculating tax. The Annual Investment Allowance (AIA) allows you to claim 100 percent of the cost of qualifying plant and machinery up to one million pounds per year, which covers virtually any tool, vehicle, or equipment a tradesperson might purchase.\n\nBox 31 is for the Annual Investment Allowance claim itself. If you bought a new van for 25,000 pounds, a pipe-freezing kit for 800 pounds, and a laser level for 300 pounds, you would claim the full 26,100 pounds in this box, assuming you use them wholly for business. Where an asset has mixed personal and business use, you can only claim the business proportion.\n\nBoxes 32 to 38 deal with other types of capital allowances including the writing-down allowance for assets not covered by AIA, small pools allowance, and balancing allowances or charges when you dispose of business assets. The writing-down allowance is 18 percent for the main rate pool and 6 percent for the special rate pool per year. Most tradespeople will claim through AIA rather than writing-down allowances, as the million-pound limit is more than sufficient.\n\nThe total capital allowances figure from this section reduces your taxable profit. Getting this right can make a substantial difference to your tax bill. For example, a builder who buys a new van at 30,000 pounds can save 6,000 pounds in income tax if they are a higher-rate taxpayer, plus savings on National Insurance. TradeTally tracks asset purchases separately from day-to-day expenses and calculates capital allowances automatically for your SA103F.

The profit calculation section brings together your income, expenses, and capital allowances to arrive at the taxable profit figure that will appear on your main SA100 return. Box 42 is your net profit or loss, calculated by taking your total income and subtracting total expenses and capital allowances.\n\nBoxes 43 to 52 handle various adjustments including disallowable expenses, balancing charges added back, goods for personal use, and any overlap relief you are claiming. Overlap relief is relevant if you changed your accounting date in a previous year and had profits taxed twice during the overlap period. You can claim this relief when you change your accounting date again or when you cease trading.\n\nThe basis period rules determine which accounting period your profits fall into for tax purposes. For most sole traders with a 5 April year end, this is straightforward since the accounting year aligns with the tax year. Following the basis period reform that took full effect from April 2024, all sole traders are now taxed on profits arising in the tax year itself, regardless of their accounting date. This simplification means your SA103F figures should match your 6 April to 5 April accounting period.\n\nBox 53 shows your final adjusted profit, which flows onto the main SA100 return. This is the figure on which your income tax and Class 4 National Insurance will be calculated. If you made a loss, boxes 54 and 55 allow you to decide how to use it, whether by setting it against other income in the same year, carrying it back to the previous year, or carrying it forward against future profits from the same trade.

The most common mistake on the SA103F is mixing up turnover and profit. Your turnover in Box 1 is the total amount invoiced before any expenses are deducted. Some tradespeople accidentally enter their profit figure here, which creates an inconsistency that HMRC will flag. Always use the gross invoiced amount including VAT recharges if applicable.\n\nAnother frequent error is claiming capital items as revenue expenses. If you bought a piece of equipment costing more than a few hundred pounds that will last several years, it should go through capital allowances in Box 31, not through the general expenses boxes. While the end result on your tax bill may be similar thanks to the AIA, putting items in the wrong boxes can trigger HMRC queries.\n\nFailing to separate business and personal use is a risk area for tradespeople who use their vehicle and phone for both purposes. If your van is also your family car at weekends, you cannot claim 100 percent of the running costs as a business expense. HMRC expects a reasonable apportionment, and claiming 100 percent for a mixed-use asset is a red flag during compliance checks.\n\nFinally, not reconciling your records before filing causes errors. Your declared turnover should match the total of your issued invoices, which should broadly match money received into your bank account, allowing for timing differences on outstanding invoices. TradeTally provides a reconciliation report that highlights discrepancies between invoices, payments received, and declared income, helping you file a clean and accurate SA103F.