Common Q1 MTD Filing Mistakes and How to Fix Them

With Q1 MTD filing deadline just weeks away, we're seeing the same costly mistakes crop up year after year. Wrong quarter dates, missed allowances, and incorrect expense codes are turning straightforward quarterly updates into compliance headaches.

Here's your fix-it guide for the most common Q1 filing errors - plus step-by-step solutions to get your submission back on track before the 7 August deadline.

The Most Common Q1 Filing Mistakes

After helping thousands of sole traders and landlords through their first MTD quarters, these five errors account for roughly 80% of all Q1 filing problems:

• Wrong quarter date ranges (especially April start dates)
• Missing or incorrectly applied annual allowances
• Expense categorisation errors that trigger HMRC queries
• Failed amendment attempts after initial submission
• Mixed income types filed under wrong business streams

Let's fix each one.

Mistake 1: Wrong Quarter Date Ranges

The Problem

Q1 runs from 6 April 2026 to 5 July 2026. Not 1 April. Not 30 June. Those five days at each end catch people out constantly.

We see submissions covering:

• 1 April to 30 June (calendar quarter)
• 6 April to 30 June (mixing tax year start with calendar end)
• 1 April to 5 July (mixing calendar start with tax quarter end)

All wrong. All potential penalties.

The Fix

Check your software's date range before every submission:

1. Q1 2026-27: 6 April 2026 to 5 July 2026
2. Q2 2026-27: 6 July 2026 to 5 October 2026
3. Q3 2026-27: 6 October 2026 to 5 January 2027
4. Q4 2026-27: 6 January 2027 to 5 April 2027

Most MTD software auto-fills these dates, but double-check. If you've already submitted with wrong dates, you'll need to amend your submission. Our amendment guide walks through the process.

Mistake 2: Missing or Incorrectly Applied Allowances

The Problem

Annual allowances like the Trading Allowance (£1,000) or Property Allowance (£1,000 for landlords) get applied incorrectly across quarters.

Common errors include:

• Claiming the full £1,000 Trading Allowance in Q1 when you should spread it
• Forgetting to claim allowances entirely
• Double-claiming allowances across multiple income streams
• Mixing Trading Allowance with actual expense claims

The Fix

Annual allowances work per tax year, not per quarter. Here's how to handle them:

For Trading Allowance:

• If your total annual gross income is under £1,000, claim the allowance instead of expenses
• If over £1,000, claim actual expenses - you can't mix both
• Apply the allowance in your final declaration, not quarterly updates

For Property Allowance:

• Same rules - under £1,000 annual rental income means you can use the allowance
• Over £1,000 means actual expenses only
• One allowance per person, not per property

If you've already claimed allowances incorrectly, amend your submission before the deadline. The penalties for incorrect allowance claims can be significant.

Mistake 3: Incorrect Expense Categorisation

The Problem

HMRC's expense codes are specific. Get them wrong and you'll trigger queries or investigations.

We see these categorisation errors repeatedly:

• Personal expenses claimed as business costs
• Capital expenses (like equipment purchases) claimed as revenue expenses
• Mixed personal/business expenses without proper apportionment
• Travel expenses without adequate records
• Home office costs calculated incorrectly

The Fix

Review every expense against HMRC's allowable expenses list. Our complete guide covers what you can and can't claim.

Key fixes for common errors:

Personal vs Business Split: If an expense covers both personal and business use, only claim the business portion. Keep records showing your calculation.

Capital vs Revenue: Equipment over £500 usually counts as capital expenditure. You can't claim the full cost immediately - it needs depreciating over time or claiming through Annual Investment Allowance.

Travel Expenses: You can claim business travel, but not travel to your normal place of work. Keep a mileage log and receipts for all claims.

Home Office Costs: Use HMRC's simplified expenses (£10-26 per month based on hours worked) or calculate actual costs with detailed records. Don't guess.

Mistake 4: Failed Amendment Attempts

The Problem

You've spotted an error after submitting your Q1 update. You try to amend it. The amendment fails or gets rejected.

Common amendment failures:

• Trying to amend through the wrong software
• Amendment deadlines missed
• Incorrect amendment codes used
• Multiple amendments creating conflicts

The Fix

HMRC's amendment process is strict but straightforward if you follow the right steps:

1. Use the same software: Amend through the same MTD software you used for the original submission
2. Check the deadline: You have until the later of one year from the filing deadline or one year from when you filed
3. Make all changes at once: Don't submit multiple amendments for the same quarter
4. Keep records: Document what changed and why

Our amendment guide walks through the technical steps, but the key is acting quickly and getting it right first time.

Mistake 5: Mixed Income Types Filed Incorrectly

The Problem

Many people have multiple income streams - freelance work plus rental property, for example. Filing these under the wrong business categories creates compliance issues.

Common mixing errors:

• Rental income filed under self-employment
• Multiple self-employed businesses lumped together
• Partnership income filed as sole trader income
• Employment income mixed with self-employment

The Fix

Each income type needs separate treatment in your MTD submissions:

Self-Employment Income: Filed under your business name/description. Include all trading income and allowable business expenses.

Property Income: Filed separately under property business. Include rental income and property-specific expenses only.

Multiple Businesses: Each separate business needs its own section. Don't combine a consultancy business with a retail business, even if you run both.

If you have multiple income sources, read our detailed guide on handling mixed income streams properly.

Emergency Fixes for Last-Minute Problems

With weeks left until the 7 August deadline, here's your emergency checklist if you've spotted any of these mistakes:

If You Haven't Filed Yet

1. Check your quarter dates are correct (6 April to 5 July 2026)
2. Review all expense categorisations against HMRC's allowable lists
3. Verify you're not double-claiming allowances
4. Separate different income types properly
5. Submit with time to spare for any last-minute issues

If You've Already Filed with Errors

1. Calculate the extent of the error - is it worth amending?
2. If yes, start the amendment process immediately
3. Use the same software you filed through originally
4. Keep detailed records of what you're changing and why
5. Consider professional help for complex amendments

Preventing These Mistakes in Future Quarters

Learn from Q1 to make Q2 smoother:

Set up proper systems now: Our 75-day setup guide helps you build processes that prevent these errors.

Use pre-submission checks: Our final checks list catches most errors before they become problems.

Keep better records: Review what HMRC actually needs from your record-keeping.

Understand the bigger picture: Read up on quarterly updates vs final declarations so you know what goes where.

Getting Back on Track

These Q1 mistakes are fixable, but they require quick action. Whether you're amending an existing submission or filing for the first time, focus on accuracy over speed.

The patterns we see suggest most errors stem from rushing or misunderstanding how MTD quarterly updates work. Take time to get it right - the penalties for repeated mistakes can be substantial.