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To enter the number of associated companies:
1. Scroll to the Tax Computation section of your CT600 form
2. Find the Specialist adjustments heading and click Show ▾ to expand it (or click Show all at the top of the Tax Computation section to open every chevron at once)
3. Find the Associated companies row (Box 326)
4. Enter the number of *other* associated companies — not including the company you are currently filing for
For example, if you have one associated company, enter 1. This halves the marginal relief thresholds to £25,000 and £125,000.
Note for advanced filing type: If your company uses the advanced or mixed income filing type, the field is Box 625 — "Number of 51% group companies" — rather than Box 326. You will find it in the same Specialist adjustments section.
More detail on how associated companies affect your tax calculation: https://tinytax.co.uk/support/associated-companies-and-marginal-relief