Working With a UK Tax Adviser From Abroad: Authorisation, AML Checks, Documents and Fees

An engagement letter appoints the adviser to your affairs, but it tells HMRC nothing. Until HMRC authorisation is in place, the adviser cannot see your record, speak to HMRC about you, or file through their agent account. Once authorised for Self Assessment, HMRC sends correspondence to the agent rather than to you, except tax bills and refunds, which still come to you (Appoint someone to deal with HMRC, GOV.UK). For personal tax there are three routes that matter, not interchangeable, and HMRC's agent guidance sets out which applies to which tax; a fourth route, through a business tax account, applies to business taxes such as employer PAYE (GOV.UK).

The 64-8 is the old faithful. It is paper only: fill it in, sign and date it, and post it to the address on the form (Authorising your agent (64-8), GOV.UK). From abroad, the adviser prepares it, you print, sign, scan and return it, and the adviser posts it to HMRC. No Government Gateway account is needed, which makes it the fallback for clients who have never used HMRC online services.

The online agent authorisation route is faster when the address HMRC holds for you is current. The adviser starts the request, HMRC posts a letter with an authorisation code within 7 working days, and the code must be passed back within the window stated in the letter, usually 30 days, after which it expires and the process restarts. Each tax service generates its own code (Online Agent Authorisation guidance, GOV.UK). Spot the trap for expats: the letter goes to the address on HMRC's file. If you left the UK three years ago and never updated it, the code lands on a doormat in a house you sold. Fix the address first, or use the 64-8.

The digital handshake covers the newer services. The adviser emails you a link; you follow it and sign in with your own Government Gateway details, never by giving your credentials to the adviser, which HMRC expressly prohibits. The link expires after 21 days, and you can revoke the authorisation at any time through your personal or business tax account (GOV.UK). It works well from abroad because nothing travels by post, but note the list: Making Tax Digital for Income Tax, Capital Gains Tax on UK property accounts, trusts and VAT are covered; plain annual Self Assessment is not.

This is the most common piece of onboarding confusion we see. A new client signs into their personal tax account, hunts for a button that says authorise an agent, and finds nothing of the kind. Nothing is broken. For Self Assessment there is no self-serve authorise-an-agent journey inside the personal tax account, because the process runs the other way round: the adviser initiates the request, and your role is to respond, by passing on the posted code or signing the 64-8. The personal tax account and the Self Assessment online service are also different views, and people often look in the wrong one. Your active role is limited to the digital handshake services, where you follow the emailed link and can later see and revoke the authorisation from your own account. If your adviser says they have requested authorisation and you cannot see anything to click, that is normal: for Self Assessment, watch the post.

Two practical points follow. Tell your adviser upfront whether you have a Government Gateway account, whether you can still access it (a UK mobile number you no longer own is the classic blocker for security codes), and what address HMRC holds for you. And do not create a new Gateway account just to authorise an agent unless asked: a second account on the wrong reference causes more confusion than it solves. If you have never been in Self Assessment, registration comes first; our expat Self Assessment guide covers registering from abroad.

Every UK firm providing accountancy or tax services must be supervised for anti-money laundering purposes, by a professional body such as the CIOT, ICAEW, ICAS, ACCA or ATT, or by HMRC directly where no professional body covers the firm (GOV.UK). Supervision carries legal duties. Before a firm takes you on, it must carry out customer due diligence, and GOV.UK spells out the minimum: your name, a photograph on an official document confirming your identity, and your residential address and date of birth, evidenced by documents such as a passport and recent bank statements or utility bills (Money Laundering Regulations: your responsibilities, GOV.UK).

Checks are required when a new business relationship starts, when an existing client's circumstances change, and whenever the firm doubts previous identification or suspects money laundering. Where a company is the client, the firm must also identify the beneficial owner, the person who owns or controls it, which is why an adviser taking on your overseas company asks about shareholders and not just directors. And the part that matters most here: enhanced due diligence applies to customers who are not physically present when identity is checked, and to politically exposed persons and other high-risk situations (GOV.UK). Almost every remote cross-border client is not physically present, so expect a certified copy of ID, electronic verification, or a short video call. It is routine, not suspicion.

Source-of-funds questions sit in the same framework. If your engagement involves significant money, the proceeds of a foreign property sale, a large family gift, or years of untaxed rent being disclosed, a supervised firm needs to understand where it came from before touching the work. Answer plainly and provide the paper trail; an adviser who never asks is the one to worry about. This matters double if the engagement is itself a disclosure, for example under the Let Property Campaign: the AML file and the disclosure need to tell the same story.

A first call works best when the adviser can see the shape of your position immediately. You do not need everything below to book it, but each item ready removes a follow-up loop, and across timezones each loop can cost days.

The residence timeline is worth singling out: almost every cross-border engagement starts with establishing residence year by year under the Statutory Residence Test, and clients who arrive with dates and day counts get answers on the first call, not the third.

The Self Assessment calendar is unforgiving in one direction only. You must tell HMRC by 5 October if you need to file for the previous tax year; paper returns are due 31 October; online returns and payment by 31 January, with a second payment on account deadline on 31 July (Self Assessment deadlines, GOV.UK). For 2025/26 that means paper by 31 October 2026 and online filing and payment by 31 January 2027.

Now stack the onboarding mechanics against that calendar. A posted authorisation code takes up to 7 working days to arrive and dies after roughly 30; an AML check on a client abroad may need certified documents; first-time registration adds its own wait for a UTR. Each step is quick in isolation, but they run in sequence, and in January they run while every firm is at maximum load. A client who makes contact in August has it all done with months to spare and knows their 31 January bill in the autumn.

August has a second advantage that has nothing to do with filing: the current tax year is still open. Planning points, timing a disposal, making an election, choosing which side of 5 April a payment falls, only exist while the year is live. By January the adviser is recording history; in August there is still time to change it. What rarely works is a first email in late January, when many firms have stopped taking new clients for that season, as our comparison of DIY filing versus using an accountant explains.

A UK adviser advises on UK tax; they do not prepare your French, Australian or American return. If your only foreign-country obligation is a simple employee return, or none at all, one UK adviser may be all you need. If you have filings on both sides, business interests, or a move in progress, you generally need one adviser in each country, and what matters is not finding one firm that claims to do both, but making sure the two advisers coordinate.

Coordination means three things. First, sequencing: where both countries tax the same income, one return usually needs to be finalised first so the foreign tax paid can be credited on the other, as our guide to double tax relief explains; the wrong order means filing on estimates and amending later. Second, consistency: the residence position, the dates of a move and the treatment of each income source must match across both returns, because tax authorities exchange information and inconsistency triggers questions; in a year you move, the UK side may also involve split-year treatment. Third, a clear division of labour recorded in each engagement letter, so no filing falls into the gap between two firms each assuming the other has it. Ask any UK adviser you are considering: have you worked alongside an adviser in my other country before? A cross-border firm says yes without hesitation, and can usually suggest a counterpart if you need one.

Fee models split broadly into hourly billing and fixed fees. Hourly billing prices the adviser's time and leaves you carrying the uncertainty; a fixed fee prices the defined piece of work and leaves the uncertainty with the firm, which can scope the work when you cannot. A fixed fee is only honest if it is set after the adviser understands your position, so the sequence matters: scoping conversation, stated fee, engagement letter, then the work. At Horizon the scoping conversation is a free 30-minute clarity call, and our published starting points are personal tax returns from £350, non-resident and expat returns from £550, and complex work from £750, always agreed as a fixed figure upfront.

The engagement letter is the contract, and a good one is specific enough that a stranger could tell whether the work was done. It should name the exact scope (which returns, years and advice questions), the fixed fee and what counts as a change of scope, what is excluded (the other country's filings, for example), each side's responsibilities, the timescales, how your data is held, and how to complain, including any professional body the firm answers to. Vague scope is where fee disputes are born: a letter that says general tax services protects the firm, not you.

Two smaller signals worth checking: professional indemnity insurance, which a regulated firm carries, and membership of a professional body, since supervision, complaints routes and technical standards all hang off it. Chartered Tax Advisers answer to the CIOT.

Horizon UK Tax Solutions is a Chartered Tax Adviser practice built for clients who are somewhere else. Onboarding is fully remote and deliberately boring. It starts with a free 30-minute clarity call, where we scope the work and quote a fixed fee. If you engage us, the engagement letter, AML checks and HMRC authorisation run in parallel: we tell you which authorisation route fits, prepare the 64-8 or send the digital handshake link, and warn you if a code is about to arrive in the post.

Fees are fixed and agreed upfront: personal tax returns from £350, non-resident and expat returns from £550, and complex work from £750. There is no clock running on emails and no surprise invoice. If your position spans two countries, we are used to working alongside advisers on the other side. Book the free clarity call, or see our expat tax adviser service page. The best month to start is the one you are in now, but if it happens to be August, so much the better.