Setting up your client for MTD ITSA

Last updated: April 28, 2026

Connect a client's HMRC Making Tax Digital for Income Tax Self Assessment (MTD ITSA) data to Briefcase One to sync their obligations and file their submissions.

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

Step 1 — Sign your client up with HMRC

Before you can sign up any clients, you need an Agent Services Account (ASA) and authorisation in place for each client you act for:

  1. Create an agent services account (GOV.UK) — you only do this once for your practice.

  2. Use your ASA to put client authorisation in place:

    • For existing clients, copy across your existing Self Assessment authorisations into the ASA.

    • For new clients, request authorisation through the ASA.

  3. Sign the client up for MTD ITSA on GOV.UK: Sign up your client for Making Tax Digital for Income Tax. You'll need the client's NI number, UTR and business details.

Step 2 — Fill in client details in Briefcase

  1. Go to Clients → Add Client → Briefcase One.

  2. Fill in the Client Information form (legal name, NI number, email, and VAT number if the client is VAT-registered), then click Continue.

Step 3 — Connect to HMRC

On the Add Business step, click Connect to HMRC.

  1. You'll be redirected to HMRC. Sign in with your own Government Gateway user ID and password:

    • Agents — use your Agent Services Account (ASA) user ID.

    • Business owners signing themselves up — use your personal Government Gateway ID linked to your NI number.

    If you're an agent, HMRC may ask you to verify your identity before allowing you to sign in. Have a document such as your P60, passport or UK driving licence to hand. This is an HMRC requirement and is usually only needed the first time you sign in.

  2. Grant the requested permissions on the consent screen.

  3. You'll be redirected back to Briefcase, and the client's businesses, accounting periods and accounting method will sync automatically.

If HMRC is unavailable or the client isn't signed up yet, use Add manually. You can reconnect later without losing your setup.

Troubleshooting

If you see "client or agent not authorised", the client isn't registered for MTD ITSA yet, or the authorisation isn't in place. Check that:

  • Sign-up (Step 1) is complete and at least 72 hours have passed.

  • You signed in at HMRC with the correct Government Gateway account — your ASA if you're an agent, or the business owner's own Gov Gateway if self-signing up.

  • If you're an agent, your Self Assessment authorisation for this client is active in your ASA. Existing Old Self Assessment authorisations are recognised for MTD ITSA; if the authorisation is only in the Old Self Assessment service, add it to your ASA first.

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

Further reading

Questions? Email support@briefcase.so.