Introduction
On 1 June 2026, HMRC published its long-awaited guidance on Mandatory Tax Adviser Registration (MTAR) — the new requirement, introduced by the Finance Act 2026, for businesses that deal with HMRC on behalf of clients to register before they can continue doing so. The guidance has been issued as an internal manual in two parts: the scope and requirement to register, and the checks HMRC carries out against the registration conditions. For many in the profession the headline question has been a simple one: does this apply to ordinary solicitors' firms, or only to accountants and dedicated tax advisers? The answer in the guidance is unambiguous, and for small and medium-sized firms in particular it is one that needs attention now rather than later.
This article summarises what the guidance says, draws out the points that matter most for small to medium sized law firms, and reflects the practical issues already being raised by practitioners in the immediate aftermath of publication.
The regime in a sentence
The principle at the heart of MTAR is straightforward. A business must register with HMRC where, in the course of business and for payment, it interacts with HMRC in relation to another person's tax affairs. Two elements have to be present: an interaction with HMRC on someone else's behalf, and payment for the service. Where both are present and no exception applies, registration is mandatory.
What makes the regime far wider than its "tax adviser" label suggests is the breadth of what counts as an interaction. The guidance lists contacting HMRC by phone, post or email; sending a message through GOV.UK or the HMRC app; filing any return, claim, notice or disclosure; making tax payments; and communicating with HMRC in any other way about a person's tax position. Crucially, it does not matter whether the interaction is routine, one-off, or simply part of a wider service that the firm would never describe as tax advice. As HMRC puts it, a business may be required to register even if it does not view itself as a tax adviser at all.
The corollary, which firms should hold on to, is that pure advice with no HMRC contact is outside the trigger. Explaining the tax consequences of a transaction to a client, or giving general information about rates, reliefs and deadlines, does not by itself require registration. The hinge is always whether the firm deals with HMRC on the client's behalf.
Conveyancing is the clearest example — and the one HMRC chose to spell out
Of the two detailed worked examples in the guidance, the first is conveyancing, and it leaves little room for doubt. A conveyancer is likely to be acting as a tax adviser, and required to register, if in the course of business they submit Stamp Duty Land Tax (SDLT) returns to HMRC for clients, deal with HMRC about a client's SDLT liabilities, pay SDLT or other property-transaction taxes to HMRC on a client's behalf, or otherwise interact with HMRC in connection with those taxes. Submitting an SDLT return is treated as assisting with a document HMRC relies on to determine a person's tax position, and so is squarely an interaction in relation to tax.
Two further points from the conveyancing example deserve emphasis because they close off what some had hoped would be escape routes. First, the guidance confirms that a single in-scope client is enough — there is no de minimis. Second, and more significantly, outsourcing does not solve the problem. Some outsourcing providers had suggested they could take the submission of the return away from the firm. HMRC has made clear that the act of paying the tax is itself an interaction, so splitting filing from payment does not remove the requirement; both businesses are caught. It is doubtful whether firms would in any event outsource the payment of sums of that size, given their personal responsibility to lenders, since doing so would simply add another element of risk to the transaction. For the great majority of firms, then, the realistic position is that they will register.
What remains out of scope in conveyancing is genuinely advice-only work: explaining the SDLT consequences of a purchase without dealing with HMRC, providing general information about rates and reliefs, or handling a transaction where the client or another registered adviser completes all the returns and payments. In practice, because almost all conveyancing teams file at least some returns themselves, this carve-out will rarely keep a property firm out altogether.
Why this reaches well beyond the conveyancing department
It would be a mistake for firms to treat MTAR as a conveyancing issue alone. The guidance contains a rule that effectively pulls in the whole firm: where a business carries out any in-scope activity — even for a small number of clients — it must register, and it cannot rely on an exception covering part of its work to avoid registering for the rest. Registration attaches to the legal entity, the firm, partnership or LLP, not to individual departments or fee-earners. There is one registration per firm, not one per team.
That matters because HMRC interaction on a client's tax affairs is far more common across a typical high-street or regional practice than the "tax adviser" framing implies. A probate or private client team that submits inheritance tax accounts, pays IHT for an estate, or files trust and estate income tax or capital gains tax returns is interacting with HMRC. A corporate team that submits instruments to HMRC's Stamp Taxes for stamping, makes employment-related securities or share-scheme filings, or applies for statutory clearances is interacting with HMRC. A family team that files an SDLT return on a property transfer, or deals with HMRC on capital gains arising from a financial settlement, is interacting with HMRC. Any one of these activities is enough to require the firm to register — at which point the question for every other department is no longer whether the firm registers, but how it manages its obligations once it has.
For a small full-service firm, the upshot is that it is very likely to be caught somewhere, most obviously through conveyancing or probate. The sensible first step is therefore not to debate department by department whether each is in scope, but to map every point at which the firm touches HMRC on a client's behalf, across all teams, and proceed on the basis that registration will be required.
What registration actually involves
Registration is more than a form-filling exercise; it is the mechanism through which HMRC enforces a set of conditions the firm and certain of its senior people must meet. To register and remain registered, a firm must, broadly, have no outstanding tax returns and no unpaid tax (unless covered by an agreed Time to Pay arrangement); not be subject to an HMRC decision refusing to deal with it; have no anti-avoidance sanction, stop notice or anti-avoidance penalty in the previous twelve months; have no disqualified director; have no relevant unspent convictions for fraud or tax offences; have no insolvency practitioner acting in relation to it; and not be suspended or banned. The firm must also provide evidence of anti-money laundering (AML) supervision, and all registrants are expected to behave consistently with the HMRC standard for agents.
There is a point of self-interest buried in those conditions that firms should not overlook: the firm's own tax compliance now gates its ability to act for clients. A practice that is behind on its own returns or payments, or that has fallen foul of an anti-avoidance measure, could in principle find itself unable to register and therefore unable to interact with HMRC for clients. Getting the firm's own house in order is part of the preparation, not an afterthought.
On AML supervision, the news is mostly reassuring. Firms regulated by the SRA are already supervised for AML purposes by the SRA as a professional body supervisor, so the condition will generally be satisfied; the firm simply needs to produce the evidence at the point of registration. A business that does not sit within an AML-supervised legal or accountancy practice would have to obtain supervision first, and HMRC will not complete registration on the strength of a pending application — only actual supervision counts.
The relevant individuals sting for smaller firms
One feature of the regime bears particularly on smaller firms. As well as registering the firm, HMRC checks certain senior people, called "relevant individuals," against the same conditions. The test is deliberately substance-based: it targets those who genuinely set the strategy, structure, risk standards or HMRC-engagement approach for the firm's tax-advice activities, rather than anyone with a particular job title. People whose only role is delivering day-to-day client work — even if they are the ones who actually contact HMRC — are not, on that basis alone, relevant individuals.
The complication for smaller practices lies in a statutory backstop. Where a business has fewer than six officers — partners, LLP members or directors — the legislation treats every one of them as a relevant individual, regardless of how responsibilities are actually divided. A four-partner firm therefore finds all four partners brought into HMRC's checking population automatically. Larger firms with six or more officers apply the substance test and, if fewer than five officers qualify, nominate additional officers up to a minimum of five. In short, the smaller the firm, the more likely it is that all of its principals will be individually checked.
Timing: which window applies, and what to do about the agent services account
Registration is being introduced in tranches, each with a three-month window, and which window a firm falls into depends on its existing footprint with HMRC. The first window, running from 18 May to 18 August 2026, applies to new advisers and to those who interact with HMRC without an agent services account (ASA), Self Assessment or Corporation Tax account. A second window, from 18 August to 18 November 2026, covers those with a Self Assessment or Corporation Tax account but no ASA. Later windows deal with payroll-only providers and financial services organisations.
For most established firms the key practical message is the simplest. Firms that already hold an agent services account do not need to apply again; HMRC will contact them in April 2027 to gather details of their relevant individuals. Firms without an ASA have three months from the start of their window to apply for one, and can continue to interact with HMRC on behalf of clients in the meantime, including while their registration is being processed. HMRC has published an interactive tool on GOV.UK to help businesses work out which tranche they fall into.
The point that should concentrate minds at smaller firms is that the first window is already open. A conveyancing-led practice that has been filing SDLT returns without an agent services account is in the 18 May to 18 August 2026 tranche and needs to act within that period.
The process itself is still bedding in
If the substance of the guidance is now reasonably clear, the mechanics of registering are still catching up. HMRC has reported thousands of online applications since the service launched in May, but practitioners have flagged that a small number of applicants have been unable to complete the process online, largely because of confusion over which Government Gateway credentials to use. HMRC's clarification is that a business should apply for its agent services account using its business Government Gateway credentials. Where a sole trader is asked to provide relevant-individual information, they should give their personal details and then log into their personal Government Gateway account, where they will receive a link to confirm those details.
HMRC has acknowledged that more help is needed and has said it will update its guidance to support applicants, including on the use of Government Gateway credentials, and that it is working with representative bodies to help businesses register smoothly. In other words, further updates are expected, and firms should treat the current guidance as the first instalment rather than the final word. The regime's stated purpose — more reliable advice for taxpayers, a fairer market, and support for advisers who play by the rules — is not in doubt, but the operational detail is still being refined.
What firms should be doing now
The sensible response for a small or medium-sized firm is to treat MTAR as a near-term compliance project rather than a wait-and-see issue. That means mapping every HMRC touchpoint across all departments — SDLT, IHT, stamp duty and SDRT, capital gains, share-scheme filings, clearances, disclosures and payments — and working on the realistic assumption that the firm will need to register. It means checking the firm's ASA status to identify the correct window, and applying for an agent services account promptly where one is not already held. It means confirming that AML supervision evidence is readily available, usually through the SRA. It means making sure the firm's own tax affairs are in order, since they are now a precondition to acting for clients. And it means identifying the firm's relevant individuals, bearing in mind that in a firm of fewer than six partners or members, that will be all of them.
None of this is insuperable, and for firms that already hold an agent services account the immediate burden is light. But the combination of a live registration window, a regime that reaches almost every full-service practice, and a process that is still being smoothed out means this is a matter to address in the coming weeks, not to file away until 2027.
