Why the button you are looking for is not there
This is the most common piece of onboarding confusion. A new client signs into their personal tax account, hunts for an authorise-an-agent option, and finds nothing. For Self Assessment the adviser starts the process from their agent account, and the personal tax account and the Self Assessment online service are also different views, so 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: watch the post. The full walkthrough is in our guide to working with a UK tax adviser.
The three routes and when each applies
The paper 64-8 covers Self Assessment, individual PAYE, Corporation Tax and services with no online route; you sign and date it and it goes to HMRC by post, so allow several weeks. The online agent authorisation code is quicker: the adviser requests it, HMRC posts the code to you within 7 working days, and you pass it to the adviser within 30 days before it expires. The digital handshake covers Making Tax Digital for Income Tax and the Capital Gains Tax on UK property account: you approve an emailed link, valid for 21 days, using your own Government Gateway sign-in. If you live abroad, the posted code is the route that catches people out, because it goes to the address HMRC holds for you; if you have never been in Self Assessment, registration comes first, which our expat Self Assessment guide covers.
What to tell your adviser upfront
Three facts unblock most authorisations: 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 currently holds for you. Getting these right first time matters because each failed loop costs weeks by post, and authorisation sits in front of every piece of paid work, from a non-resident return to the SA109 residence pages that HMRC's free online service does not support. Horizon UK Tax Solutions handles the whole authorisation sequence as part of onboarding, on fixed fees agreed upfront; book a free 30-minute clarity call at /book and we will tell you exactly which route applies to you.