Setting Up

Welcome to the 123 Sheets setting up guide

Follow the instructions below to get the most out of using 123 Sheets.

Any problems, please contact us at support.team@123sheets.co.uk.

Step 1
Get an Agent Services Account (ASA) (if you do not already have one)

If you already have an Agent Services Account (ASA), for example because you submit Making Tax Digital (MTD) VAT returns, you should not create a new ASA.

If you already have a working ASA and have linked VAT or Self-Assessment clients to it previously, please proceed directly to Step 3.

To create your firm’s Agent Services Account (ASA), sign in using your old agent Government Gateway ID and password. There is a specific page where you must do this, which you can access by Clicking Here.

During the sign-up process you will be required to:

• Sign in using your old agent Government Gateway ID and password.
• Enter the business type of your practice.
• Enter the business UTR and postcode of your practice. Use the address that HMRC holds for your old agent Government Gateway account.
• You may be asked who your money laundering supervisory body is. This could be, for example:
  • ICAEW
  • ACCA
  • If asked for the valid until date, it may be 31 December for ICAEW, or 12 months after you last paid your institute’s annual fees for ACCA.
  • If you are not a member of a professional body, your money laundering supervisory body is HMRC. You must arrange this with HMRC before proceeding.
• Make a note of your new Agent Services Account (ASA) Government Gateway ID and the password. You will need these to authorise our software with HMRC.
• The new Government ID is a 12-digit number. Also make a note of your "Account Number" which begins with four letters, as you may need this when speaking to HMRC.

Step 2
Link existing clients to your new Agent Services Account

Once your new account has been created, you can link your existing client relationships for both VAT and Income Tax to these new credentials. This will not remove client authorisations from your old Government Gateway account.

Follow these steps:

• Sign in to your newly created ASA by Click Here. Use your new ASA Government Gateway ID and password, then select the link:
  “Link your current Self Assessment and VAT clients to this account.”
• Enter your old Government Gateway User ID and password (not the new ASA credentials).
• Enter the Agent Reference Number (ARN) you received during the setup process.
• Enter the UTR or Corporation Tax reference for your accountancy/tax practice.
• Click Connect.

Once completed, your new Agent Services Account will have the same client authorisations as your old Government Gateway account. Any new authorisations added to the old account will also be automatically linked to your ASA.

Step 3
Register Clients for Making Tax Digital (MTD) for Income Tax

For MTD for Income Tax, you must register each affected client individually with HMRC.

You can only do this if you are already the registered Self-Assessment agent for the client.

When registering a client for MTD for Income Tax, you will be asked to select the tax year from which the client will begin using MTD. Please ensure the correct tax year is selected (for example “2026/27” for the tax year beginning 6 April 2026). If the wrong tax year is chosen, it can be very time-consuming to have HMRC correct this later.

You can register clients for MTD Income Tax by Clicking Here

Step 4
Authorise 123 Sheets Software

First, add a client in our software.

1. Click the “Clients” menu at the top of the page.
2. Click the green “Add Client” button in the top right corner.

If you have not yet purchased a licence, you will be prompted to do so.

Enter the client’s name and National Insurance Number (NINO) or VAT number, then click the green “Add” button.

Next:

• Go to the Clients menu.
• Click the Income Tax or VAT green button on the far right of the client’s row.

A message about HMRC authorisation will appear.

• Click the green “Continue” button and follow the instructions.
• You will be asked to sign in to your Agent Services Account using your new ASA Government Gateway credentials. Do not use your old tax agent government gateway credentials.
• Click “Grant Authority.”

You will then be redirected back to our website, and our software will be authorised to provide services for all your clients.

The client is now set up and the software has been authorised for all clients you add to our software.

If you are asked to authorise a client on our software even though you have already authorised the software correctly for another client, it is likely that:

• This new client is not registered for MTD or was registered for a later tax year, or
• Your HMRC client authorisation has expired

In these cases, you may need to re-establish the authorisation through your Agent Services Account or change the tax year for which the client is registered for MTD.

The software authorisation of 123 Sheets should last 18 months before you need to do it again.

Once you have authorised the software, you should in this order:

1. Add in all the clients you want on our software;
2. Add in staff membership access;
3. Create any sub-accounts where you want some clients to submit agreed quarterly filings
4. Consider if you wish to purchase one or both of our bookkeeping spreadsheets

You are now ready for MTD on 123 Sheets!