The tax adviser register: what to check in your practice before 18 November 2026

What is actually changing, and when

HMRC is phasing in mandatory registration for anyone paid to interact with HMRC on a client’s tax affairs, under a programme called Modernising and Mandating Tax Adviser Registration — MMTAR. It rolls out in windows rather than on a single date:

The second window is the one that lands this quarter, and 18 November 2026 is the deadline that matters to most accountancy practices: anyone who currently deals with HMRC on a client’s Self Assessment or Corporation Tax affairs without an ASA has to be registered by then. Miss it, and HMRC has said it may limit that adviser’s ability to act for clients, with enforcement action, including financial penalties, for anyone who carries on acting after being told to stop.

None of this is a new compliance regime layered on top of your existing tax work. It is a registration requirement, and most established practices already clear it because an ASA has been the gateway for MTD filing for years. The place this actually bites is narrower and more specific than “does the firm need to register” — and it is an operations question, not a tax-technical one.

It is the firm that registers — but HMRC checks the people

The detail owners get wrong first: MMTAR registration attaches to the legal entity that interacts with HMRC, not to each individual who happens to make a phone call. Partners, directors and employees do not each register in their own personal capacity. If your practice already holds an ASA, the firm itself is very likely already covered for this window.

That is where most owners stop reading, and it is the wrong place to stop. HMRC has been explicit that it will also run checks on “relevant individuals” connected with a registering business — the people who actually sit behind the registration, not just the entity’s name on the certificate. And separately, anyone who is themselves a distinct legal entity — a self-employed associate invoicing through their own limited company, a semi-retired partner working on consultancy terms, an outsourced preparer who logs into HMRC directly rather than through your systems — is not covered by your firm’s registration at all. Their own business is the one that has to register, or has to stop dealing with HMRC directly and route everything through yours.

That is a workforce-mapping exercise, not a tax question, and it is exactly the kind of thing that falls through the gap between “the partners assume the tax team has it” and “the tax team assumes it’s a firm-wide registration issue for someone else to own.”

The audit: who in your firm actually counts

Run this as a list-building exercise, not a policy discussion. The question for every name on it is the same: does this person or business deal with HMRC directly on a client’s Self Assessment or Corporation Tax affairs — by phone, webchat, post, or a login that is not your firm’s ASA?

• Employed staff at every level, not just partners — a semi-senior who rings the agent dedicated line on a query is doing exactly the thing this scheme is about, even if nobody thinks of them as “the adviser” on the file.
• Self-employed associates and consultants who invoice through their own company or as a sole trader, particularly ones brought in for specialist work — R&D claims, inheritance tax, HNW clients — where a direct relationship with HMRC is part of what you’re paying them for.
• Semi-retired partners kept on for a handful of legacy clients, often precisely the ones still dealing with HMRC the old way because nobody has migrated them onto the firm’s current systems.
• Outsourced or offshore preparation providers who occasionally step outside preparing figures and into direct HMRC contact — confirming a submission, chasing a repayment, resolving a coding notice.
• Anyone using a personal or legacy agent code rather than the firm’s ASA, which is usually a sign the relationship was set up before the current systems existed and has simply never been moved across.

In most practices this list is short — five or six names, not fifty — and it is almost never the partners. It is the people operating at arm’s length from the firm’s main systems, which is precisely why nobody has looked at it yet.

A worked example

Picture a twenty-two-person practice with a properly set-up ASA, used for every routine Self Assessment and Corporation Tax filing. On paper, the firm is covered. Illustrative figures and an invented firm, not a client of ours.

Running the audit above turns up two people the partners had not thought to check: a semi-retired former partner who still personally handles inheritance tax and probate work for eleven long-standing clients, ringing HMRC’s bereavement line directly under his own agent reference because that relationship predates the firm’s ASA by a decade; and a self-employed R&D specialist, engaged through her own limited company, who deals with HMRC’s R&D unit directly on live enquiries for four clients because that is the whole point of hiring a specialist.

Both are separate legal entities from the practice. Neither is covered by the firm’s ASA. Both fall squarely into the second window — a Self Assessment or Corporation Tax presence with HMRC, no ASA of their own — and both need to be registered, in their own right, by 18 November 2026, or brought inside the firm’s registration by having their client contact rerouted through it before that date.

The compliance fix — register, or reroute — takes an afternoon either way. What changes is whether it happens as a scheduled piece of work in September or as a scramble against eleven live probate files in the second week of November, with HMRC potentially unable to act on some of them until it is sorted.

What actually breaks in onboarding and your engagement letters

This is the part that rarely shows up in a tax-compliance summary, because it is not a tax problem. It is a process problem, and it sits in two places.

Engagement letters. Any letter that names a specific individual as the person who will “deal with HMRC on your behalf” — standard wording on plenty of specialist-work letters — is a promise about who has authority to act. If that named person is a subcontractor who has not registered and cannot be rerouted through your ASA in time, the letter is now promising something the firm cannot deliver, on a live file, not a new one. The fix belongs in your engagement letter review, not your tax file review: check whether any current template names an individual rather than the firm, and whether that individual is on your audit list.

Onboarding. A new-client checklist that confirms “agent authorisation submitted” without confirming which legal entity that authorisation sits under is checking the wrong thing. If a new client is being brought on with a named specialist as their day-to-day contact, the onboarding step that matters is confirming that specialist’s registration status before the file opens — the same discipline covered in how to onboard clients consistently, applied to a new checkpoint rather than a new client type.

Neither fix is difficult. Both are invisible until the deadline has passed and a client is asking why nobody can talk to HMRC about their return.

Do this week

1. Confirm your firm holds a working ASA covering Self Assessment and Corporation Tax. If it does, the firm itself is very likely already inside this window — but that is only the first line of the audit, not the whole of it.
2. List everyone who personally interacts with HMRC on a client’s tax affairs by phone, webchat, post or a login outside your ASA — employed staff, subcontracted associates, semi-retired partners and outsourced preparers alike.
3. For anyone on that list operating as a separate legal entity, establish whether they already hold their own ASA. If not, decide now: they register themselves by 18 November 2026, or their client contact is rerouted through the firm’s registration before that date.
4. Pull every engagement letter template — and any letters currently out for signature — that names a specific individual as the HMRC point of contact, and check that name against your list.
5. Add a single confirmation step to your onboarding checklist: which legal entity is registered to deal with HMRC for this client, checked before the file opens, not assumed afterwards.
6. Name one owner for closing any gaps, with a target date in early November — not 17 November — so nothing depends on HMRC’s portal being quiet in the last week before the deadline.
7. Put the same audit on your calendar for the payroll-only window closing 18 February 2027, and the financial services window closing 31 March 2027, if either applies to any part of your firm.

Where this sits alongside everything else

This is a one-off audit with a hard date, which is exactly the kind of task founder-led practices handle worst — not from incapacity, but because a one-off with no obvious owner defaults to whoever happens to notice, and often nobody does until a client complains. The same discipline that catches a stale SOP or a missed HMRC agent authorisation on an exiting client catches this too: one named owner, one list, a status reported at the same operating review as everything else that matters, rather than an inbox item that surfaces once, gets actioned once, and is never checked again when the next window opens. It is a smaller cousin of the audit already needed for Companies House ID verification — a different regulator, a different register, the same shape of problem: a one-off, firm-wide check with a date attached, that only ever gets done properly when someone owns it.

None of this needs a tax specialist to run. It needs someone to make the list, check it against the engagement letters and the onboarding checklist, and close the gaps before 18 November 2026 rather than during the week it lands.