How TinyTax Connects to HMRC and Companies House

@AdamsWinter connects directly to HMRC and Companies House using their official software submission channels. This guide explains how that works and what CATO (Corporation Tax Online) is — and is not — in that process.

How @AdamsWinter Submits to HMRC

When you click Submit Corporation Tax, @AdamsWinter sends your CT600 directly to HMRC's secure XML Transaction Engine — the same channel used by all authorised accounting software. This is a direct machine-to-machine connection that happens in the background.

Your Government Gateway credentials (UTR, user ID, and password) authenticate the submission. HMRC accepts it and returns a confirmation reference immediately.

What Is CATO?

CATO stands for Corporation Tax Online — it's the HMRC online portal where company directors can log in and view their corporation tax records, check submission history, and see payment references.

Think of CATO as a viewer, not a submission channel. After @AdamsWinter files your CT600 via the Transaction Engine, HMRC processes it and — after a few days — it appears in CATO for you to view.

Does @AdamsWinter Need Access to CATO?

No. @AdamsWinter does not access CATO as part of its service. Its connection to HMRC is entirely through the Transaction Engine, which is unaffected by any changes to the CATO portal.

If HMRC changes or retires the CATO portal, your ability to file through @AdamsWinter is not affected.

Where Does @AdamsWinter Get Prior Year Data?

@AdamsWinter automatically fetches your prior year accounting figures — used to pre-fill the Prior Year column on the balance sheet — from the Companies House public register. This is the same publicly available information you can find at find-and-update.company-information.service.gov.uk.

No special access is required. If your previous accounts are on the public register, @AdamsWinter can retrieve them automatically.

Summary