What do I need to do with HMRC if I switch from sole trader to a limited company mid-year?

Switching mid-year is very manageable as long as you treat it as a clean break with HMRC rather than a continuation. First, incorporate the company at Companies House, then register it for Corporation Tax within three months of starting to trade — the company gets its own Corporation Tax UTR, which is entirely separate from your personal one. Separately, tell HMRC your sole-trader self-employment has ended (you can do this through your Personal Tax Account or on your final Self Assessment return, which will still be needed for the sole-trader period up to incorporation). Your personal UTR continues for Self Assessment — it does not get replaced by the company's UTR. If you will take a salary, register a new PAYE scheme for the company before the first payday. Finally, formally transfer the trade across: assets and goodwill move from you to the company, and incorporation relief under section 162 TCGA 1992 may let you hold over capital gains on goodwill provided the conditions are met — this is where an accountant earns their fee. Pick one fixed switch-over date, close out the sole-trader books on it, and keep separate bank accounts from day one to keep everything tidy.