Amending a CT600 or Accounts Filed Elsewhere

TinyTax can amend a CT600 that was originally filed outside TinyTax — including returns filed via HMRC's online service (now closed) or another software provider.

Amending Companies House accounts that were filed externally is handled separately via Companies House WebFiling.

Amending a CT600 Filed Outside TinyTax

TinyTax has a dedicated flow for this. Once your company is set up on TinyTax, the relevant accounting period will appear on your dashboard. The CT600 for that period will show as "Filed Elsewhere" — you can then amend it directly:

1. On your dashboard, find the accounting period you want to amend
2. Click the filing options dropdown (the arrow next to the period row)
3. Select Amend CT600
4. Enter your corrected figures and file — TinyTax submits the amendment to HMRC electronically

HMRC processes this as a formal amendment to the original return. The 12-month amendment deadline still applies (12 months from your original filing deadline, not the period end).

If you're new to TinyTax

If you haven't registered yet:

1. Create an account at https://tinytax.co.uk
2. Enter your company registration number — TinyTax fetches your company details automatically
3. Your filing periods will appear on your dashboard once the company is set up
4. Follow the steps above to amend your CT600

Note on HMRC's CATO service: HMRC's free Company Accounts and Tax Online (CATO) service closed permanently on 31 March 2026. If you previously filed through that service, TinyTax (or another commercial software provider) is now the route for all CT600 submissions and amendments.

Amending Accounts Filed Outside TinyTax (Companies House)

TinyTax can only amend Companies House accounts that were originally filed through TinyTax. If your original accounts were filed via HMRC's CATO service, Companies House WebFiling, or another platform, the amendment must go through Companies House WebFiling:

1. Log in to Companies House WebFiling at ewf.companieshouse.gov.uk
2. Select File revised accounts for the relevant period
3. Complete and submit the amended accounts

Amendment Deadline

You must submit your CT600 amendment within 12 months of the original filing deadline (not the period end date). For example, if your CT600 deadline was 31 October 2025, the amendment deadline is 31 October 2026.

Using TinyTax for Future Filings

Once you've set up your company on TinyTax, all future filings — and amendments of those filings — can be handled directly from your dashboard.

Common Questions

HMRC's online service has closed — can I still amend my CT600?

Yes. TinyTax (and other commercial software providers) are now the standard route for all CT600 submissions and amendments. HMRC's CATO closure does not affect your ability to amend — the 12-month deadline still applies.

Can I amend a CT600 in TinyTax if the original was filed via HMRC's service?

Yes — use the Amend CT600 option from the filing dropdown on your dashboard. This is specifically designed for returns filed outside TinyTax.

What if I want to switch to TinyTax from my next accounting period?

You can add your company to TinyTax at any time. Your dashboard will show all open and upcoming periods automatically.