Running more than one company is common. What catches people out is that the corporation tax rules don't look at each company in isolation — they look at the group as a whole.

If your companies are "associated," your thresholds for the 19% and 25% corporation tax rates get divided between them. Have three associated companies, and a threshold that was £50,000 as a standalone business drops to £16,667. A profit level that comfortably sat at the small profits rate can suddenly fall inside the marginal relief band, where the effective rate is 26.5%.

It's not just about direct ownership either. The rules can bring in companies controlled by your spouse, children or business partners, and even a company you thought was dormant can still count if it's had any financial activity at all.

This guide explains how the associated company rules work, who counts as associated, and the situations that most often catch owner-managers out — from the old company kept for a brand name, to spouses running separate businesses that share staff or premises. There's a practical checklist at the end to help you review your position.

If you run more than one company, or have family members with companies of their own, it's worth checking where you stand before your next year-end. We're happy to talk it through.