Self-Assessment Deadline 2025: Everything You Need to Know

The January self-assessment deadline is one of those dates that creeps up fast. If you’re a sole trader, company director, landlord, or anyone with untaxed income, you need to file your 2023–24 tax return by 31 January 2025 — and pay any tax you owe on that same day. Missing it costs money from the moment it’s late.

The Critical Dates

Understanding the full self-assessment calendar is more useful than just knowing the January deadline:

• 5 April 2024 — End of the 2023–24 tax year. This is the period your return covers.
• 5 October 2024 — Deadline to register for self-assessment if you’re filing for the first time.
• 31 October 2024 — Deadline for paper tax returns.
• 31 January 2025 — Online return filing deadline, payment of 2023–24 tax due, and first payment on account for 2024–25.
• 31 July 2025 — Second payment on account for 2024–25.

The 31 January date bundles three financial obligations at once. Many people are caught off guard by the payment on account system, which requires prepaying an estimated portion of the following year’s tax bill alongside settling last year’s balance.

What to Gather Before You File

Rushing a self-assessment return is a reliable way to make mistakes. Aim to have everything gathered by mid-December.

From your employer or pension provider:

• P60 — Total pay and tax deducted for the year ending 5 April 2024. Must be provided by employers by 31 May 2024.
• P45 — If you left a job during 2023–24, your P45 covers pay and tax up to your leaving date.
• P11D — Reports expenses or benefits in kind (company car, private medical insurance, interest-free loans over £10,000). Due from employers by 6 July 2024.

If you’re self-employed:

• All business income received between 6 April 2023 and 5 April 2024.
• All allowable business expenses broken down by category.
• Your profit-and-loss summary from accounting software.
• Mileage records if you claim HMRC-approved rates (45p per mile for the first 10,000 miles, 25p thereafter).

Other income sources:

• Bank interest (HMRC receives data feeds from banks, but you still declare it).
• Dividend income (the annual dividend allowance is £1,000 for 2023–24).
• Rental income and allowable expenses if you’re a landlord.
• Capital gains from property, share disposals, or other assets.
• Child Benefit: if you or your partner earned over £50,000, the High Income Child Benefit Tax Charge may apply.

Penalties for Late Filing and Payment

HMRC’s penalty structure escalates the longer you delay. There is no grace period.

Late filing:

• Immediate (1 day late): £100 fixed penalty, regardless of whether you owe any tax.
• 3 months late: £10 per day for up to 90 days (maximum £900 additional).
• 6 months late: An additional 5% of the tax due, or £300, whichever is greater.
• 12 months late: A further 5% of the tax due, or £300, whichever is greater.

Late payment:

• Interest accrues from 1 February 2025 on unpaid tax at the HMRC late payment rate (7.75% per annum as of January 2026).
• A 5% surcharge applies if tax remains unpaid 30 days after the deadline.

If you genuinely cannot pay on time, contact HMRC before the deadline to arrange a Time to Pay agreement. Interest still accrues, but surcharges can be avoided.

Common Mistakes to Avoid

Not declaring all income. HMRC receives data feeds from employers, pension providers, banks, and platforms like Airbnb and Etsy. Undeclared income is a red flag that triggers enquiries.

Forgetting payments on account. If your 2023–24 tax bill exceeds £1,000 and less than 80% was collected via PAYE, HMRC will require advance payments toward 2024–25. Many people budget for last year’s bill and are blindsided by an additional 50% due on the same day.

Basis period transitional rules. For 2023–24, transitional rules applied to sole traders and partnerships as HMRC moved to a tax-year basis. If your accounting year doesn’t end on 5 April, you may have overlap profits and transitional adjustments to handle.

A Practical Preparation Checklist

1. Log into HMRC online and confirm your Unique Taxpayer Reference (UTR) is working.
2. Gather all P60s, P45s, and P11Ds from the 2023–24 tax year.
3. Run a year-end report from your accounting software covering 6 April 2023 to 5 April 2024.
4. Reconcile bank statements to business records for the full year.
5. Check your payments on account balance in your HMRC account.
6. Set a bank transfer ready to pay by 31 January — HMRC’s Faster Payments system allows same-day payment, but don’t leave it to 11pm.

If you’re working with an accountant, aim to have everything submitted to them by mid-December. January is the busiest period for every practice in the country.

Making It Easier Next Year

The most reliable way to reduce self-assessment stress is to treat tax as an ongoing process:

• Set aside 20–25% of every invoice into a separate tax savings account.
• Reconcile your bookkeeping monthly, not in January.
• Keep all receipts — digital photographs are accepted by HMRC.
• Use Making Tax Digital-compatible software if your qualifying income is over £50,000 — mandatory from 6 April 2026 for the 2026/27 tax year onwards.

Self-assessment is manageable with preparation. The penalties are avoidable. If the process still feels overwhelming, working with an accountant costs far less than the stress and financial risk of doing it badly alone.

If you’d like help with your 2023–24 return or want a system in place for the year ahead, get in touch with the Page Ivy team.