Corporation Tax: small profits relief: company with associated companies
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
Where in any accounting period a company has one or more associated companies, the upper and lower limits are divided by the total number of companies associated, namely one plus the number of companies associated with the subject company. Thus if the company has three associated companies and its accounting period is for the twelve months to 31 March 2011, the lower and upper limits are £75,000 (£300,000 / 4) and £375,000 (£1,500,000 / 4) respectively. Where the accounting period is less than twelve months, these figures are proportionately reduced.