Setting up your business for MTD ITSA as a business owner

Last updated: April 23, 2026

Connect your HMRC Making Tax Digital for Income Tax Self Assessment (MTD ITSA) data to Briefcase One to sync your obligations and file your submissions.

You must be signed up for MTD ITSA with HMRC before connecting Briefcase.

Step 1 — Sign yourself up with HMRC

Before signing up, make sure you are already registered for Self Assessment and have submitted at least one tax return. Then use the GOV.UK sign-up journey:

You'll sign in with your personal Government Gateway user ID, confirm your identity, and provide your business name, the nature of your business, start date and accounting period. You can sign up for sole trader income, property income, or both.

If you'd prefer your accountant or bookkeeper to handle this, ask them to sign you up using their Agent Services Account instead.

Step 2 — Create your business in Briefcase

In Briefcase, go to Clients → Add Client → Briefcase One.
Fill in the Client Information form with your own details — legal name, NI number, email, and VAT number if you're VAT-registered — then click Continue.

Step 3 — Connect to HMRC

On the Add Business step, click Connect to HMRC.

You'll be redirected to HMRC. Sign in with your personal Government Gateway user ID and password — the one linked to your NI number.
Grant the requested permissions on the consent screen.
You'll be redirected back to Briefcase, and your businesses, accounting periods and accounting method will sync automatically.

If HMRC is unavailable or your sign-up hasn't been activated yet, use Add manually. You can reconnect later without losing your setup.

Troubleshooting

If you see "client or agent not authorised", your MTD ITSA sign-up isn't fully in place yet. Check that:

Sign-up (Step 1) is complete and at least 72 hours have passed — HMRC takes a little while to activate the record.
You signed in at HMRC with the Government Gateway account linked to your NI number (your personal tax account), not a business tax account.
If an accountant is signing you up on your behalf, they've completed sign-up in their Agent Services Account first.

Once these are in order, return to Briefcase and click Connect to HMRC again to retry.