Switching is straightforward. The handover is where it matters.
Around 1.4 million subcontractors are registered for CIS in the UK, and a significant proportion of them have never reviewed whether their accountant is doing the job properly. The most common reason for switching is not a spectacular failure. It is the slow accumulation of missed expenses, back years left unfiled, and questions that get vague answers rather than clear ones.
Switching accountants is legally uncomplicated. You authorise a new agent with HMRC, revoke the previous agent's authority, and hand over the records. The difficulty is ensuring the handover is complete and that nothing is left in a gap between the two accountants. This guide walks through the process.
Can you switch mid-year?
Yes. There is no rule that requires CIS subcontractors to wait until a particular point in the tax year to change accountants. The tax year runs from 6 April to 5 April, and the Self Assessment filing deadline is 31 January following the year end. But none of that prevents a mid-year accountant change.
The most practical switching windows are:
| Timing | Why it works well | Consideration |
|---|---|---|
| Shortly after 31 January | The previous year's SA return has been filed, so the outgoing accountant's work is complete for that year | Current year has just begun, so the new accountant starts fresh with a clean year |
| Shortly after 5 April | The tax year has closed, records are available for the full year just ended | The new accountant can immediately begin the year-end SA preparation |
| Mid-year (any time) | Appropriate when the relationship has broken down or a problem has been identified | Year-to-date records must be clearly allocated between the two accountants |
Mid-year switches work well where the impetus is urgent, such as discovering that a Self Assessment return is overdue or that a refund has been incorrectly calculated. In these cases waiting until January or April costs money.
What to request from your previous accountant
Before or alongside switching, request the following documents from your outgoing accountant. These are your records, not theirs, and you are entitled to them:
CIS deduction statements for the current year to date. These are the payment and deduction statements your contractors have issued, which your accountant should hold on file. They show the gross amount paid, the labour element, the deduction rate and the amount withheld. Without these, your new accountant cannot calculate your current-year position accurately or file a complete Self Assessment return.
Self Assessment position year to date. Has the return for the previous tax year been filed? If not, why not, and when is it being filed? Is there a return already in progress for the current year?
Details of any back-year claims. Many CIS subcontractors who have been in the scheme for several years are owed refunds for prior years that have not been claimed. The window is four years (so in 2026/27 the earliest claimable year is 2022/23). Confirm whether your previous accountant has reviewed back years and, if not, whether those years are still within the claim window.
Expense records. Mileage logs, receipts for tools and PPE, insurance documents and any use-of-home calculations your previous accountant was maintaining on your behalf.
Outstanding HMRC correspondence. Any letters, enquiries or compliance checks from HMRC that your accountant has received. You are legally the taxpayer and HMRC's correspondence is about you; it should not sit unresolved at an outgoing accountant's office.
How HMRC authorisation transfers: the 64-8 form
An accountant can only access your HMRC records, file returns on your behalf, and correspond with HMRC about your tax position if they hold a valid agent authorisation. The standard mechanism for this is the 64-8 form.
The 64-8 is an HMRC paper form. You complete it with your details and your new accountant's details, sign it, and the accountant sends it to HMRC. HMRC then grants the new accountant access and, where the same tax service was covered by your previous accountant's authorisation, revokes the previous agent's access at the same time.
For certain services, including Making Tax Digital for Income Tax and VAT, HMRC offers an online digital authorisation route through the agent services account. This involves the agent initiating a request, which you then approve via a link sent to your email. Online authorisation for qualifying services is typically processed within days rather than weeks. Your new accountant will advise you which route applies to your situation.
The key services covered by agent authorisation for a CIS subcontractor are:
- Self Assessment (the main vehicle for your annual income tax and refund)
- CIS (both as subcontractor and, where applicable, as contractor)
- PAYE (if you employ anyone)
- VAT (if you are VAT-registered)
Authorisation is service-specific. Your new accountant may need separate authorisations for Self Assessment and CIS. Confirm the full scope is covered in the initial engagement.
Timing: what not to assume
The most costly mistake in an accountant switch is assuming that because you have engaged a new accountant, everything that needed doing has been done. It has not, until someone has specifically confirmed it.
Check each of the following before treating the handover as complete:
- Is the previous year's Self Assessment return filed? If the switch happens in, say, November 2026, the 2025/26 return should have been filed by the outgoing accountant before you left, or explicitly agreed to be handed over to the incoming one.
- Are there outstanding back-year returns? The four-year lookback means years back to 2022/23 are in scope in 2026/27. If prior years were not reviewed, that work needs to happen now.
- Are there any penalties outstanding? Late filing penalties for Self Assessment or for any missed CIS returns do not disappear when you change accountants. Confirm whether any exist and whether there are grounds to appeal.
- Has HMRC received the 64-8? Until authorisation is confirmed, your new accountant cannot act. Ask them to confirm when the authorisation has been processed.
Red flags that suggest it is time to switch
Most CIS subcontractors who are underpaying or missing refunds are not aware of it, because they have never had an independent review of their position. The following are signs that your current accountant may not be optimising your CIS tax position:
CIS deductions applied to materials as well as labour. CIS is taken from the labour element of a payment only. If your accountant is basing your income figures on gross invoices without separating the materials, your taxable income is overstated and your refund is understated.
Mileage at 45p per mile rather than 55p. AMAP rates rose to 55p per mile for the first 10,000 business miles from 6 April 2026. If your 2026/27 return uses 45p, your expenses are understated by a tenth of a penny per mile, which across 15,000 business miles is £1,500 of unclaimed deduction.
No back-year review ever undertaken. A subcontractor who has been in CIS for five years without a professional review of back years may have missed four years of refund claims. The window is closing on each year as time passes.
Delays without explanation. A Self Assessment return that is not filed until close to the 31 January deadline every year, without any explanation of why, is a sign of a reactive rather than proactive practice.
Our guide to what a CIS accountant does and what to expect covers the full scope of what a good CIS accountant should be doing for you throughout the year.
After the switch: what your new accountant needs from you
Once the authorisation is live and the records have been transferred, your new accountant will typically run through:
- A review of the current year's CIS deduction statements and income to date
- A check on allowable expenses you have incurred (tools, PPE, mileage at 55p, insurance, use of home)
- A back-year assessment to see whether any years within the four-year window have unclaimed refunds
- Confirmation of your CIS registration status and rate (20% registered, or 0% GPS if you qualify)
The CIS refund is the most immediate financial benefit for most subcontractors. Because CIS is deducted from the labour element at 20% before any expenses or your £12,570 personal allowance are applied, most registered subcontractors overpay across the year and the Self Assessment return produces a refund rather than a bill. Understanding your full Self Assessment position is covered in our complete guide to CIS and Self Assessment.
Your new accountant will also want to confirm your record-keeping approach going forward: how you track mileage, what receipts you keep, and how you document materials separated from labour on invoices. Good records from the point of switching make the year-end process faster and reduce the risk of HMRC queries about the refund claim. See our CIS record keeping guide for what HMRC expects you to retain and for how long.