What accounting and finance services does Flintham Mackenzie offer?

We offer a full range of business and personal accountancy services, bookkeeping, Corporation Tax planning, VAT, payroll, audit & assurance, business start-up support, and cloud accounting on Xero, QuickBooks and Sage. On the personal side, we also help with self-assessment, estate planning, and retirement strategies.

Where is your London office located?

Our head office is on Chiswick High Road (277-279 Chiswick High Road, Chiswick, London, W4 4PU), with a second office in Southgate, North London (286c Chase Road, N14 6HF). We've also got offices in Geneva, Dubai and New Delhi for clients with international operations.

Which industries do you specialise in?

We've built up strong sector expertise in construction, retail, manufacturing, medical & pharmacy, investment & financial services, hospitality, and e-commerce so the advice you get is geared to the real challenges your industry faces, not generic guidance.

How do I get started with Flintham Mackenzie?

It's straightforward. Book a free discovery call so we can get to know your business and what you need. From there, we put together a tailored proposal with clear, upfront pricing (no hidden costs), then get you set up on our cloud accounting tools.

Do you support businesses outside the UK?

Yes. As well as our Chiswick and Southgate offices, we support international clients through Geneva, Dubai and New Delhi, helping with cross-border tax planning, compliance and financial management.

Self-Assessment Deadline 2026: Key Dates and Penalties You Can't Miss

By Admin • 23 October 2026

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Self-Assessment Deadline 2026: Key Dates and Penalties

Self-Assessment deadlines can look different for sole traders, company directors and other individual taxpayers, but the main HMRC deadlines for personal tax returns apply to everyone who has to file one. If you are self-employed, knowing these dates is the best way to avoid penalties and interest charges.

With the 2026 filing season just around the corner, here is a summary of the key payment rules, deadlines and penalties so you are not caught off guard at tax time. FMLLP offers accounting and tax assistance to help you and your business pay and get paid as easily as possible.

What Is the Self-Assessment Deadline for 2026?

The 2026 Self-Assessment tax return covers the 2025-26 tax year, which runs from 6 April 2025 to 5 April 2026. You must report all relevant income and claim any reliefs on your return. The key dates are:

5 October 2026: deadline to register for Self-Assessment if you are new to it
31 October 2026: deadline for paper tax returns
31 January 2027: deadline for online tax returns, and for paying any tax owed for 2025-26
31 July 2027: second payment on account for 2026-27, if you make payments on account

Get prepared early and submit your return as soon as possible to avoid missed deadlines, late penalties and having to pay at short notice.

Who Has to File a Self-Assessment Tax Return?

You might need to complete a Self-Assessment tax return if you:

are self-employed
are a landlord or receive rental income
owe Income Tax or Capital Gains Tax that is not collected through PAYE
have other income that is not taxed through PAYE

Company directors and people with other sources of income should also check their filing requirements. If HMRC sends you a notice to file a return, you must do so unless HMRC formally tells you the requirement has been cancelled.

If you are new to Self-Assessment and need to file a return for the 2025-26 tax year, you must normally register with HMRC by 5 October 2026. Registering late can cause problems, especially if you have tax to pay.

Self-Assessment Late Filing Penalties

HMRC charges penalties if you file your return after the deadline:

One day late: an initial 100 penalty, even if you owe no tax or have paid your tax on time
Three months late: daily penalties of 10 for up to 90 days, a maximum of 900
Six months late: a further penalty of 5% of the tax due or 300, whichever is higher
Twelve months late: another 5% of the tax due or 300, whichever is higher

These charges add up quickly, so preparing early pays off. If you missed a deadline for a reasonable excuse, you can appeal against the penalties, but HMRC will assess your reasons.

What Happens If I Pay My Tax Late?

Filing your return is separate from paying your tax bill. Filing on time does not protect you from penalties if you do not pay the tax by the deadline.

HMRC can charge late payment penalties of 5% of the unpaid tax at 30 days, six months and twelve months after the payment deadline. Interest also accrues on the outstanding amount.

If you cannot pay in full, contact HMRC immediately to explain your situation and ask about a Time to Pay arrangement. Remember that a payment plan does not remove interest, and penalties may still apply.

Getting Ready for the 2026 Self-Assessment Deadline

Preparing your return early makes everything easier. Start by collecting all your income records, business costs, rental income, pension contributions and any other relevant documents.

If you are self-employed, make sure your bookkeeping is up to date and that your expenses are backed up by the right supporting evidence. Landlords should organise their rental income and identify expenses that are deductible against property income. It is also worth looking back at your previous tax returns and payments on account.

Flintham Mackenzie (FMLLP) provides accounting and Tax Services to help you organise your records, prepare your tax documents and meet your obligations. Find out more about our services at FMLLP.

Double-check your deadlines, clear your desk and build time into your schedule to review your return. Do the groundwork now, and your return can be filed without added stress or expense.