How Do You File a Tax Return as a Locum Dentist in the UK? A Step-by-Step Guide

What This Guide Covers

Filing a tax return as a locum dentist in the UK differs from filing as a salaried associate or a practice owner. Your income arrives from several practices, your expenses follow your own kit and travel, and you are responsible for your own National Insurance and payments on account. This guide walks you through the process step by step, with the figures for the current 2026/27 tax year. If you also do associate work, the mechanics overlap closely, and our dental associate Self Assessment guide covers that angle in full.

It is written for locum dentists who are self-employed sole traders, which is how most locums operate. If you provide your services through a personal service company, the off-payroll (IR35) regime can apply and the engaging practice usually decides your status; for that route see our locum dentist tax service page. To weigh how your fee structure shapes your take-home, read locum dentist day rate vs percentage.

Step 1: Register for Self Assessment

If you have not filed a Self Assessment return before, you must register with HMRC. Do this as soon as you start locum work, not after the tax year ends. Registration gives you a Unique Taxpayer Reference (UTR) and activates your online tax account.

To register as self-employed, go to gov.uk and complete the online form. HMRC posts your UTR within about 10 working days, after which you can activate your Government Gateway account and access HMRC online services.

Deadline note: you must register by 5 October following the end of the tax year in which you started self-employment. If you began locum work during 2025/26 (6 April 2025 to 5 April 2026), your registration deadline is 5 October 2026. Miss it and you risk a penalty, so register early.

If you previously filed a return as an employed dentist, you may already have a UTR, so check your HMRC online account and simply add a self-employment source to your existing return rather than registering afresh.

Step 2: Record All Your Locum Income Accurately

Your locum income is every fee you receive from practices for covering sessions, whether the work is NHS or private. Record the gross amount before any deductions for lab fees, materials or surgery costs. Do not net expenses off at the income recording stage, because HMRC wants to see turnover and expenses separately.

Example. If you locum at three practices in a month, add the gross fees from all three together as that month's turnover and claim your expenses separately. Never enter only the net figure.

Keep a running monthly total, using accounting software such as FreeAgent, Xero or QuickBooks, or a well-structured spreadsheet. HMRC expects you to keep records for at least 5 years after the 31 January filing deadline.

NHS versus private income

Distinguish NHS from private income in your records. Any NHS pension contributions are based on your NHS pensionable earnings only, and where you apportion mixed-use costs such as travel, the split affects the deductible proportion. Keep NHS payment schedules and remittance advices, and issue and keep an invoice for each private engagement. Tidy records make the return quick and defend your figures if HMRC ever asks.

Step 3: Claim Your Allowable Expenses Correctly

As a self-employed locum you can deduct a wide range of costs. The governing test is that an expense must be incurred wholly and exclusively for your trade. HMRC applies this strictly, so keep evidence for everything.

Common allowable expenses for locum dentists

• Travel and mileage: claim 55p per mile for the first 10,000 business miles in the tax year (from 6 April 2026, up from 45p) and 25p per mile after that. As a locum with no single fixed practice, each engagement is normally a temporary workplace, so the journeys count, but the trip from home to your first practice can be ordinary commuting if that practice has become a permanent base. Keep a mileage log.
• Professional indemnity: your defence organisation subscription (Dental Protection, MDU or MDDUS) in full.
• GDC and professional subscriptions: your GDC annual retention fee, plus approved bodies on HMRC's List 3 such as the BDA and specialty societies. A GDC restoration fee is not allowable.
• CPD and training: fees, travel and materials for CPD that maintains or updates your existing skills. Training that creates a genuinely new skill can be capital rather than revenue.
• Equipment and instruments: loupes, headlights, handpieces and other clinical kit, usually via capital allowances. The Annual Investment Allowance gives 100% relief in the year of purchase on most qualifying equipment, so a typical purchase is relieved up front.
• Other insurance: public liability and income protection relating to your work.
• Accountancy fees: preparing your return and advising on your tax.
• Use of home as office: for admin done at home, claim either HMRC's simplified flat rate or a fair proportion of household running costs by rooms used and hours worked.

Expenses you cannot claim

You cannot claim everyday clothing, even if you only wear it for work, although protective scrubs or tunics specific to clinical use are an exception. You cannot claim fines or penalties, the GDC restoration fee, or the cost of entertaining clients. For a fuller, locum-specific breakdown with worked examples, see what expenses a locum dentist can claim.

Step 4: Check Your Self-Employed Status

As a sole trader you are not inside the off-payroll (IR35) rules, which apply only to work through a limited company. HMRC can still challenge whether you are genuinely self-employed, and status turns on the substance of the arrangement, not the contract label: control over how and when you work, your right to send a substitute, mutuality of obligation, financial risk, and how far you are integrated into the practice.

Most locums working session by session, carrying their own indemnity and equipment and choosing which bookings to accept, are comfortably self-employed. The risk arises where a practice treats a locum like an employee in substance, so keep your contracts and a record of your working arrangements to defend your position. If you operate through a personal service company on NHS engagements, the off-payroll rules mean the medium or large client, not you, determines your IR35 status and issues a Status Determination Statement; our locum dentist tax service covers that route.

Step 5: Work Out Your Income Tax and National Insurance

Your income less your allowable expenses gives your taxable profit, which drives both your income tax and your Class 4 National Insurance for the year.

Income tax (2026/27, England, Wales and Northern Ireland). The personal allowance is £12,570. You pay 20% on profit between £12,571 and £50,270, 40% between £50,271 and £125,140, and 45% above £125,140. The personal allowance tapers away once your income exceeds £100,000.

Class 4 National Insurance (2026/27). You pay 6% on profits between £12,570 and £50,270, then 2% on profits above £50,270. There is no longer any Class 2 charge to budget for. The Class 2 liability was removed from 6 April 2024, and if your profits are at or above the small-profits threshold you are treated as having paid it, so your State Pension record is protected without a weekly payment. If your profits are very low and fall below that threshold, you can still pay Class 2 voluntarily to keep your record complete.

Worked example. If your taxable profit after expenses is £63,500, the first £12,570 is covered by the personal allowance, you pay 20% on the £37,700 in the basic-rate band and 40% on the slice above £50,270, and Class 4 National Insurance adds 6% on profit up to £50,270 plus 2% on the profit above it. The two together are your liability for the year, settled through Self Assessment.

Step 6: Plan for Payments on Account

Self-employed locums usually pay tax in advance through payments on account. Once your Self Assessment bill exceeds £1,000 (and under 80% was collected at source), HMRC asks for two interim payments towards the following year, each 50% of your prior year's liability.

• The first payment on account is due by 31 January, alongside any balancing payment for the year just ended.
• The second payment on account is due by 31 July.
• A balancing payment (or refund) then trues up the difference on the next 31 January.

This often catches locums in their first profitable year, because the January bill bundles the tax for the year just filed with the first instalment towards the next year, so you can face around 150% of one year's tax in a single January. Set money aside from each payment so the bill is funded when it falls due. If your profits have genuinely dropped you can apply to reduce your payments on account, but cutting them too far attracts interest.

Step 7: Get Ready for Making Tax Digital

Making Tax Digital for Income Tax is now live. From 6 April 2026 it is mandatory for self-employed people and landlords whose qualifying income (gross trading plus property income) is more than £50,000. The threshold then drops to £30,000 from 6 April 2027 and £20,000 from 6 April 2028. Most full-time locum dentists earn well above £50,000, so they are caught from the start, in April 2026.

Qualifying income is gross income before expenses, tested on your most recent Self Assessment return, so the return you file now determines whether you are mandated for the year ahead. HMRC writes to confirm if their records show you over the threshold, but the responsibility to check and sign up on time is yours.

In practice, being in Making Tax Digital means keeping digital records and submitting quarterly updates through compatible software, with a final declaration after the tax year. If you are still on paper or a basic spreadsheet, move to compatible software now so the change is routine. Limited companies are not in scope, because the regime applies to income tax, not corporation tax.

Step 8: File Your Return and Pay on Time

The Self Assessment cycle runs to fixed dates. The online filing deadline and the payment deadline both fall on 31 January following the end of the tax year; the paper deadline is the earlier 31 October. Almost all locums file online: log into your HMRC account, open Self Assessment, complete the self-employment pages (the short SA103S or full SA103F depending on turnover), add any other income such as interest or dividends, then submit and pay.

Late filing penalties:

• £100 automatic penalty if your return is up to 3 months late.
• Daily penalties of £10 per day, up to 90 days, after 3 months.
• 5% of the tax due (minimum £300) after 6 months.
• A further 5% after 12 months.

Interest is charged on tax paid late, and it accrues daily, so even a short delay adds up. If cash flow is tight, arrange a Time to Pay plan with HMRC rather than simply missing the deadline.

Step 9: Keep Your Records for Future Years

HMRC can open an enquiry into your return for a period after the filing deadline, and longer where it suspects error. Keep all records for at least 5 years after the 31 January deadline: invoices, receipts, mileage logs, bank statements and any pension contribution certificates. Back up software data or spreadsheets each year. Good records also make Making Tax Digital straightforward, because the quarterly updates simply draw on what you are already capturing.

Common Mistakes Locum Dentists Make on Their Tax Return

• Registering too late. Register as soon as you start, never past the 5 October deadline.
• Claiming mileage without a log. HMRC wants each journey recorded with date, destination, purpose and miles. A spreadsheet or app is enough.
• Confusing gross and net income. Record gross income and claim expenses separately; do not net off lab fees or materials at the income stage.
• Budgeting for a Class 2 charge that no longer exists. Class 2 has not been payable since 6 April 2024 for profits above the small-profits threshold.
• Forgetting payments on account. The first profitable January bundles the balancing payment with the next year's first instalment, so plan for it.
• Leaving Making Tax Digital too late. If your qualifying income tops £50,000 you are mandated from April 2026, so move to compatible software now.

When to Use a Dental-Specialist Accountant

Filing your own return is realistic if your affairs are simple. But locums often juggle several income streams, mixed NHS and private work, mileage across practices, capital allowances, payments on account and Making Tax Digital, which is where specialist help earns its keep. A dental-specialist accountant can make sure every allowable expense is claimed and evidenced, confirm your self-employed status is robust, project your payments on account so January does not catch you out, set up compatible software for Making Tax Digital, and represent you if HMRC opens an enquiry. Accountancy fees are themselves allowable, and the advice often pays for itself in tax saved and penalties avoided.

Summary: The Self Assessment Cycle for a Locum Dentist

• 5 October: register for Self Assessment if you started locum work in the tax year just ended.
• 31 October: paper filing deadline (most locums file online instead).
• 31 January: online filing deadline, balancing payment, and first payment on account.
• 31 July: second payment on account.
• 6 April 2026 onward: Making Tax Digital for Income Tax is mandatory if your qualifying income is over £50,000 (then £30,000 from April 2027 and £20,000 from April 2028).

If any part of your locum tax position is unclear, speak to a dental-specialist accountant who works with locum dentists. The cost of getting it wrong, in penalties, interest and enquiries, far outweighs the cost of getting it right first time.