Why associated companies change the CT600 answer

Marginal Relief is not only a rate lookup. Since the reintroduction of the small profits rate from 1 April 2023, the CT600 computation needs to decide whether profits fall below the lower limit, above the upper limit, or in the Marginal Relief band.

For a standalone 12-month period, the familiar limits are GBP 50,000 and GBP 250,000. HMRC guidance says those limits are proportionately reduced for short accounting periods and by the number of associated companies. That is where many small-company CT600 reviews become fragile.

The core rate workflow

For non-ring-fence Corporation Tax profits from 1 April 2023, the working pattern is usually:

Profits at or below the lower limit may qualify for the 19% small profits rate.
Profits above the upper limit are normally charged at the 25% main rate.
Profits between the limits may get Marginal Relief, reducing the main-rate charge.

HMRC's rates and allowances page shows the lower limit, upper limit, and standard fraction. The CT600 file should not only contain the final tax figure. It should also show the thresholds used and why they were appropriate for that company and period.

What counts for the associated-company review

Associated-company work is a control question before it is a tax calculation. The practice needs to identify companies under common control and confirm whether the association affects the period being filed. One extra associated company can halve the effective limits. Several associated companies can push an apparently modest profit figure into the main-rate band.

The review should record:

Companies under common ownership or control during the accounting period.
Whether each company was associated for all or only part of the period.
Any short accounting period apportionment.
Augmented profits or distributions considered in the Marginal Relief test.
The final lower and upper limits used in the computation.

Short periods and straddling 1 April

Short accounting periods need proportionate thresholds. Periods that straddle a rate-change date can need time-apportioned calculations. HMRC's Company Taxation Manual gives examples where the accounting period is split into notional periods and the relevant limits are adjusted before comparing profits to the band.

This is one of the reasons Robocount treats the CT600 as a computation workflow rather than a single-page form. The date range, associated-company count, profit apportionment, and rate basis should be visible before the return is approved.

Common traps in practice files

Using GBP 50,000 and GBP 250,000 without reducing them for associated companies.
Forgetting that a short accounting period reduces the limits before the rate decision.
Mixing taxable total profits and augmented profits without documenting which figure drives the relief formula.
Assuming last year's associated-company position still applies without asking the client about new group or common-control companies.
Letting the rate calculation sit in an unreviewed spreadsheet outside the CT600 approval file.

Review checklist before filing

Confirm the accounting period start and end dates.
Confirm the company is not excluded from Marginal Relief, such as being a close investment-holding company.
Record the associated-company count for the relevant period.
Calculate the adjusted lower and upper limits.
Check the CT computation agrees to the CT600 tax charge.
Keep manager approval evidence for any manual adjustment or override.

How Robocount supports the workflow

Robocount keeps the Marginal Relief review close to the CT600 computation. The aim is to make rate-sensitive cases easier to inspect before submission, especially for owner-managed companies with more than one entity.

Captures accounting-period dates and rate-year context.
Surfaces associated-company and threshold assumptions for review.
Connects the rate calculation to the main CT600 liability workflow.
Preserves approval evidence for practices managing multiple small-company clients.
Supports API-first review flows where tax agents or AI systems need structured calculation states.

FAQ

Does every small company get the 19% Corporation Tax rate?

No. The small profits rate depends on the company's profit level and other factors. Associated companies and short periods can reduce the limits used in the rate decision.

Where does Marginal Relief appear in the CT600?

Marginal Relief is part of the Corporation Tax computation feeding the CT600 liability. The return file should show the rate basis and relief calculation, not only the final amount payable.

Why do associated companies matter for an owner-managed business?

Many owner-managed groups are informal: one director owns a trading company, a property company, and a dormant or side-project company. The associated-company question decides whether thresholds need to be divided, so it should be asked before the CT600 is approved.

Useful HMRC references

This guide is general product and filing workflow information, not tax advice. Check the current HMRC guidance and the company's facts before filing.

CT600 associated companies and Marginal Relief without spreadsheet guesswork.