Written by the TaxTune Editorial Team · Reviewed by our qualified accountants · Updated for 2026
The quick answer The costliest company accounts mistakes are missing the Companies House filing deadline, which brings escalating penalties, muddling the corporation tax and accounts figures, mishandling the director loan account, and filing accounts that do not agree with the CT600. Each is avoidable with proper preparation.

The mistakes that cost the most

  1. Filing late at Companies House. Penalties start at £150 and rise to £1,500 for a private company, and double if you are late two years running.
  2. Missing the two separate deadlines. Accounts go to Companies House, the CT600 and accounts to HMRC, and the corporation tax is due earlier still.
  3. Accounts that do not agree with the CT600. The tax return, computations and accounts must all reconcile.
  4. Mishandling the director loan account. Overdrawn balances not disclosed or repaid correctly cause tax charges and questions.
  5. DIY errors. Company accounts must follow accounting standards, which is where filing software built for accountants matters.

Company accounts due soon?

The deadlines are firm and the penalties escalate. TaxTune prepares your accounts and CT600 together, makes sure everything reconciles, and files on time with both Companies House and HMRC.

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Frequently asked questions

What is the penalty for filing company accounts late?

For a private company, penalties start at £150 for being up to a month late and rise to £1,500 for more than 6 months. They double if you file late two years running.

What are the deadlines for company accounts?

Accounts go to Companies House, usually 9 months after the year end, and to HMRC with the CT600 within 12 months. The corporation tax payment is due earlier, at 9 months and 1 day.

Do my accounts and CT600 have to match?

Yes. Your statutory accounts, tax computations and CT600 must reconcile. Figures that do not agree can lead to a rejected return or HMRC questions.

Can I prepare company accounts myself?

You can, but they must follow accounting standards and usually need commercial software. Many directors use an accountant to avoid errors, penalties and rejected filings.

What happens if I disclose the director loan account wrongly?

An overdrawn loan not disclosed or repaid correctly can trigger tax charges and HMRC questions, so it must be tracked and reported accurately in the accounts.