Checklist for taking on a new client

Before accepting the work

Ask the client for written authority to contact their existing accountant (ICAEW: change of professional appointment, incoming accountant)
Write to the existing accountant asking for any information that bears on accepting the appointment
Ask for the handover information: last accounts and returns, working papers the client is entitled to, and open matters with HMRC
Check the work falls within the practice's skills and professional indemnity cover

Identify the client and verify their identity (Money Laundering Regulations 2017, regulation 28)
For an individual, record their name, photo identity document, home address and date of birth (GOV.UK: money laundering regulations, your responsibilities)
For a company, record its name, company number, registered office and the names of its directors (regulation 28)
Identify the beneficial owners and take reasonable steps to verify who they are (regulation 28)
Record the purpose and intended nature of the business relationship (regulation 28)
Assess the client's risk against the practice's firm-wide risk assessment (ACCA: firm-wide risk assessment)
Apply enhanced due diligence where the client was identified without meeting them or is a politically exposed person, with senior management approval for a politically exposed person (GOV.UK: your responsibilities)
Keep the due diligence records for 5 years from the end of the business relationship (GOV.UK: your responsibilities)

Send an engagement letter with a schedule for each service, the terms of business and the privacy notice
Agree the fee and billing arrangements in writing
Have the client sign and return the letter before work starts

Request authorisation through the HMRC agent services account for each tax the practice will deal with (GOV.UK: agent services account)
Use form 64-8 only where an online authorisation route is unavailable (GOV.UK: form 64-8)
Get the company authentication code, a 6-character code Companies House posts to the registered office, which can take up to 10 working days (GOV.UK: company authentication codes)
Confirm whether the directors and persons with significant control have verified their identity with Companies House (GOV.UK: when you need to verify your identity)

Record the UTR, VAT number, company number, PAYE reference and National Insurance number
Put every deadline into the diary: accounts, confirmation statement, CT600, corporation tax, VAT, payroll and Self Assessment
Check whether the client is in scope for Making Tax Digital for Income Tax
Collect opening balances from the last accounts
Give the client the practice's privacy notice when their personal data is collected (ICO: the right to be informed)

In Accountin, adding a client records its company number, UTR, VAT number and year end, and its deadlines then appear on the deadlines dashboard.