Accounting Guide

Published: 2026-10-30 · Reviewed: 2026-11-04

Client Onboarding Paperwork for Accountants

Why this matters

A new client arrives in March with a carrier bag of receipts and a deadline in April. The engagement was never put in writing, nobody asked for the prior year’s figures, and the payroll login details are “with my cousin.” Every accountant knows this client. Onboarding paperwork exists to make sure you never take them on again, or rather, to make sure that when you take them on, the terms are written, the documents are listed, and the gaps are visible before the deadline pressure starts. Good onboarding is a filter: it tells you which clients will be a pleasure and which will be a problem, while there’s still time to price accordingly.

The engagement letter: first, always

The engagement letter is the contract for the relationship. It states what you’re doing, what you’re not doing, what it costs, and what you need from the client. Non-negotiable contents:

  • Scope, precisely. “Annual accounts preparation and tax return filing for [business name], year ending [date].” Not “accounting services.” Every service included is named; everything else, bookkeeping cleanup, payroll, advisory, is either included or explicitly excluded. Scope creep is the engagement letter’s enemy, and vagueness is how it gets in.
  • Fees and billing. Fixed fee or hourly, what’s included, what triggers extra charges, payment terms. “Additional work required due to incomplete records will be charged at $X per hour”, write it now, while everyone’s friendly.
  • Client responsibilities. What the client must provide, by when, and in what form. “Bank statements, sales records, and expense receipts for the full year, provided by [date], in [format].” This clause is the one you’ll point at in February when the records still haven’t arrived.
  • Deadlines both ways. Your commitment to deliver by a date, conditional on receiving complete records by an earlier date. Deadlines without the client’s reciprocal obligation are just promises you can’t keep.
  • Termination terms. How either side ends the engagement, notice period, and what happens to work in progress. Professional relationships end; the letter decides whether they end cleanly.

Signed before work begins. An engagement letter sent “for your records” after three months of work is a souvenir, not a contract.

The document checklist: what clients always forget

Every new client gets a document checklist tailored to the engagement. The items clients always forget to send:

  • Prior year accounts and tax returns. Without these, you’re starting blind, and prior-year errors become your problem to discover mid-deadline.
  • Bank statements for the full period, all accounts. Not “most of them.” All of them, including the account they “hardly use.”
  • Loan and financing agreements. Interest, terms, balances, needed for correct treatment and often forgotten because “that’s with the bank.”
  • Payroll records and employment details. Staff numbers, pay rates, benefits, the client who “does payroll themselves” usually means a spreadsheet last updated in June.
  • Asset purchases and disposals. The van bought in March, the laptop sold in September. Clients remember the big things and forget the medium ones, which are exactly the ones with tax implications.
  • Login credentials and authorizations. Tax portal access, bank feed authorizations, payroll software logins. Collect these at onboarding with a secure method, not by email, and never stored in plain text.

Send the checklist with the engagement letter. Tick items off as they arrive. What’s still missing two weeks before your internal deadline gets a formal chase, in writing, referencing the engagement letter’s client-responsibilities clause.

Practical steps

  • Engagement letter signed before any work. No exceptions, not even for referrals, not even for “an old friend.” Especially not for old friends.
  • Checklist issued with the letter, chased on a schedule. Issue at onboarding, first chase at 30 days, second at 60, formal notice per the letter’s terms after that. The schedule is what makes it happen.
  • Record the client’s systems. What accounting software (if any), who does the bookkeeping, how records arrive. The client on a shoebox needs a different workflow than the client on cloud software, know which one you have.
  • Note the red flags at onboarding. Incomplete prior records, reluctance to sign, “my last accountant was terrible” with no specifics, unrealistic deadlines. Write them in the file. They’re either manageable (price them in) or disqualifying (decline politely).
  • Store credentials properly. A password manager or secure vault, access-controlled, never in the client file itself. Check your local regulations on data protection, client financial data is regulated in most jurisdictions, and “it’s on a spreadsheet” is not compliance.

Common mistakes

  • Starting work before the letter is signed. The scope was “understood,” the fee was “discussed,” and six months later both are disputed. Signed first, work second.
  • The generic checklist. A one-size checklist that doesn’t match the engagement teaches the client that your paperwork is box-ticking. Tailor it: sole trader, limited company, and partnership checklists are different documents.
  • Chasing documents by memory. “I think we’re still waiting on the bank statements”, the checklist says exactly what’s missing and when it was last chased. Use it.
  • Accepting the carrier bag without comment. Taking whatever the client hands you, however it arrives, trains them to keep doing it. The checklist sets the standard; the engagement letter enforces it.

Templates that help

Bottom line

Engagement letter signed before work, tailored document checklist issued with it, missing items chased on a schedule, credentials stored securely. Onboarding is where you set the terms of the whole relationship, the clients who sign the letter and send the documents on time are the clients you’ll still want in three years. Check your local regulations on data protection and record-keeping, then make the pack non-negotiable.