Self Assessment document checklist for UK clients: what accountants should ask for (2025/26)
A practical SA100, SA103 and SA105 records checklist you can send to clients, with the deadlines and what Making Tax Digital changes.
October 23, 2026
Self Assessment is a records problem before it is a tax problem. The return takes an hour; waiting three weeks for a client to find a dividend voucher is what fills January. This is the checklist we would send a client, split by the situation they are in.
The dates that drive the chase
For the 2025/26 tax year, online returns and the balancing payment are due on 31 January 2027. A second payment on account is due on 31 July. Paper returns had an earlier date of 31 October 2026. Confirm any date on gov.uk before you quote it to a client.
Work backwards from 31 January. If you want everything in by the middle of December, the request has to go out in November, with a reminder every week after that.
Every client: identity and authority
- Photo ID (passport or driving licence) and proof of address from the last three months, for your anti-money-laundering checks
- National Insurance number and Unique Taxpayer Reference
- Authority for you to act as their agent with HMRC (Form 64-8 or the online authorisation code)
- Last year's return or tax calculation (SA302), if you did not prepare it
Employed clients (SA100)
- P60 or P45 from each employer
- P11D if they received benefits in kind
- Employment expenses they want to claim, such as professional subscriptions or work-from-home costs
- Pension income statements, including the State Pension
- Student or postgraduate loan plan, and Child Benefit received (for the High Income Child Benefit Charge)
Savings, dividends and gains
- Dividend vouchers from every company
- Bank and building society interest statements
- Details of any shares, property or crypto sold in the year, with purchase cost and sale proceeds
- Gift Aid donations and personal pension contributions
Self-employed clients (SA103)
- All sales invoices or an income summary for the tax year
- Business expenses by category, with receipts for the larger items
- Business bank statements for the full year, or a bank feed export
- Mileage log and home office costs, if they are claiming them
- CIS deduction statements for subcontractors in construction
- Equipment bought in the year
Landlords (SA105)
- Rent received per property
- Letting agent annual statement, if there is an agent
- Repairs, insurance, ground rent, service charges and agent fees
- Mortgage interest statements for each loan
- Purchase or sale documents for any property bought or sold, and the ownership split
What Making Tax Digital changes
HMRC guidance says Making Tax Digital for Income Tax applies from April 2026 to sole traders and landlords with qualifying income above 50,000 pounds, from April 2027 above 30,000 pounds and from April 2028 above 20,000 pounds. Qualifying income is measured before expenses. For those clients the question is no longer "send me your records in January" but "send me your figures every quarter", so the collection routine has to run all year. Check eligibility for each client on gov.uk.
Stop rebuilding this list in every email
Typing this list into an email for every client is how documents arrive as eleven separate messages with names like "scan0043". Build each list once as a checklist, send each client one link, and let them upload each item from their phone. You can see what has arrived and what is missing without opening your inbox, and the reminders go out on their own.
idutax has these checklists ready to use, including onboarding with photo ID and proof of address. See the UK page, or start free with up to 5 clients, no credit card. If you are comparing options, here is our comparison with Intuit Link.
Frequently asked questions
What documents do I need for a Self Assessment tax return?
Employment income records (P60, P45, P11D), self-employment income and expenses, rental income and costs, dividend vouchers, bank interest, pension income and contributions, capital gains disposals, Gift Aid donations, and details of student loans and Child Benefit. Your accountant will also need your UTR and National Insurance number.
When is the Self Assessment deadline for 2025/26?
Online returns and the balancing payment are due by 31 January 2027. Paper returns were due by 31 October 2026. Check gov.uk for the current dates.
Does Making Tax Digital for Income Tax apply to my clients?
HMRC guidance says it applies from April 2026 to sole traders and landlords with qualifying income above 50,000 pounds, from April 2027 above 30,000 pounds and from April 2028 above 20,000 pounds. Qualifying income is measured before expenses. Confirm eligibility on gov.uk.