How to register clients MTD income tax: the agent process, step by step
How to register clients MTD income tax is a question a lot of agents are now asking under real time pressure, not as a future planning exercise. MTD ITSA is live, and HMRC's own guidance, while accurate, is not written specifically for an agent managing this process across dozens or hundreds of clients at once. This is that version: the actual steps, in order, with the specific points where the process commonly stalls.
Step 1: Confirm your Agent Services Account is set up and current
An Agent Services Account, distinct from the older HMRC online services for agents account many practices already have, is required to sign clients up for MTD for Income Tax. If your practice has not created one specifically for this purpose, that is the first blocking step, and it is worth doing well before you need to register your first client, since account setup itself can take time to process.
If your practice already has an ASA for other services, such as MTD for VAT, confirm it covers Income Tax specifically rather than assuming the same account automatically extends to every MTD service.
Step 2: Authorise for each client, even existing ones
You must hold specific authorisation to act for a client within your ASA before you can sign them up to MTD for Income Tax, and this is a separate step from any general authorisation your practice may already hold for that client under older HMRC systems. An existing 64-8 authorisation for self assessment does not automatically transfer into ASA authorisation for MTD.
In short: do not assume an existing client relationship means you are already authorised in the system that actually matters here. Check ASA authorisation specifically, for every client, before attempting to register them.
Main agent vs supporting agent
Within the ASA, authorisation can currently be requested as either a main agent or a supporting agent (worth reconfirming against live HMRC guidance, as this distinction has shifted during MTD ITSA's rollout), a distinction worth understanding before you request it, not after, alongside which software you intend to run each client's records on once they are signed up. A main agent typically holds primary responsibility for a client's MTD obligations; a supporting agent has a narrower role, often used where a bookkeeper handles quarterly submissions while a separate accountant handles annual finalisation. Decide which role fits your practice's actual relationship with each client before submitting the authorisation request, since changing it later adds an extra step you can avoid by getting it right the first time.
Step 3: Request authorisation and manage the client's part of the process
Requesting authorisation through the ASA generates an authorisation code that the client must use to approve the request themselves. This is the step in the entire process an agent cannot complete on the client's behalf, and it is also the step most likely to stall a batch registration effort, because it depends on a client actually receiving, understanding, and acting on a code.
Under HMRC's current process, the code must be entered within 30 days of being issued, and it expires if not used in that window, requiring the whole authorisation request to be reissued; this window is worth reconfirming against live HMRC Agent Services Account guidance, since the process has changed before as MTD ITSA has rolled out. For a practice registering many clients at once, this is worth planning around directly: batch your authorisation requests with client communication sent at the same time, not the request first and a follow-up chase weeks later once codes have already started expiring.
Step 4: Complete the sign-up, and confirm the tax year
Once authorisation is confirmed, sign-up can be completed from within the ASA or via gov.uk. A specific decision point here is easy to overlook: sign-up can be for the current tax year or the next one, and getting this wrong for a client means either backdating obligations that were never met in real time, or delaying MTD compliance a full year when the client should have started sooner. Confirm explicitly, in writing, which tax year each client is being signed up for as part of this step, not as an assumption carried over from a general conversation.
Step 5: Connect MTD-compatible software
The final step links the client's records in your chosen software to HMRC's MTD systems, typically through an API connection. This is where the record-keeping side of MTD actually becomes live and functional, and it is worth confirming the connection is genuinely active, not just configured, before considering a client's registration complete. A software integration that is set up but not actually submitting or syncing data leaves a client technically registered but not actually compliant.
The five steps at a glance
| Step | What happens | Where it commonly stalls |
|---|---|---|
| 1. ASA setup | Create or confirm your Agent Services Account covers Income Tax | Assuming an existing ASA for other services already covers this |
| 2. Client authorisation | Confirm ASA-specific authorisation, choose main or supporting agent | Assuming an existing 64-8 authorisation transfers automatically |
| 3. Authorisation code | Client receives and enters a code within 30 days | Code expires before the client actions it |
| 4. Sign-up | Complete sign-up, confirm current or next tax year | Wrong tax year selected, creating backdated or delayed obligations |
| 5. Software connection | Link MTD-compatible software via API | Connection configured but not actually live or submitting |
Troubleshooting the most common stalls
The client never receives, or ignores, the authorisation code
This is the single most common point of failure in a practice-wide rollout. Confirm the code is being sent to an email address or contact method the client actually checks regularly, not an outdated one on file from years ago. For clients who are digitally hesitant generally, a brief phone call flagging that the code is coming, and what it is for, meaningfully increases the chance it gets actioned inside the 30-day window rather than sitting unread.
Authorisation conflicts with an existing agent relationship
Where a client has an existing agent relationship on file, whether from a previous accountant or an overlapping arrangement, the authorisation request can behave unexpectedly. Confirm there is no stale or conflicting agent authorisation still active for that client before submitting a new request, since an unresolved conflict is a common, quiet cause of an authorisation that appears to succeed but does not actually grant the access expected.
The wrong tax year gets selected under time pressure
When registering many clients in a short window, the tax year decision at Step 4 is the one most likely to be rushed. Build a simple rule into your process: confirm the intended tax year with the client, in writing, before the registration session, not as a judgement call made in the moment while working through a list.
What happens after registration: this is the start, not the finish line
Registration is the entry point into an ongoing quarterly obligation, not a one-off task to complete and move past. Once a client is registered and their software connection is live, the practice's actual workload shifts to managing quarterly submissions consistently across the client list, and separately to fielding the client questions that start arriving once the process is actually live, both distinct from the registration process itself. Treating registration as the finish line, rather than the start of a new recurring process, is a common planning gap worth avoiding from the outset.
Managing this across a full client list without it becoming chaos
The single-client version of this process is straightforward once you have done it once. The practice-wide version, running it across dozens or hundreds of clients simultaneously, is where the real operational risk sits, and HMRC has directly encouraged agents to start authorisations well ahead of any deadline to avoid a late bottleneck.
A few practical steps that keep this manageable at scale: batch clients by urgency, starting with anyone already over or close to the current income threshold, rather than working through your client list alphabetically. Track authorisation status explicitly, requested, code sent, code entered, sign-up complete, software connected, in a shared tracker your whole team can see, rather than relying on individual memory of where each client sits in the process. And build in a standard follow-up cadence for clients who have not entered their authorisation code within the first two weeks of the 30-day window, rather than waiting until it has already expired.
A practical tracker for this needs only a handful of columns to be genuinely useful: client name, income threshold status, date authorisation requested, date code entered (or a flag if still pending past day 14), tax year confirmed, and software connection status. That is enough for anyone on the team to see, at a glance, exactly where every client sits without needing to reconstruct the picture from memory or dig through individual client files.
For a practice with a genuinely large client list, staggering registration in weekly batches of a manageable size, rather than attempting to register everyone in a single week, keeps the authorisation code follow-up work proportionate to what the team can actually chase. A batch too large to follow up on properly within the 30-day window defeats the purpose of batching in the first place.
Receiptflow keeps client records MTD-ready ahead of each quarterly submission, so registration is only the first step, not the whole job. Start a free trial and see how the record side of MTD compliance runs once registration is done.
The bottom line
Registering a client for MTD Income Tax is a five-step process that HMRC's own guidance describes accurately but not with an agent's practice-wide volume in mind. The genuine risk is not in any single step's complexity. It is in managing the authorisation code window across many clients simultaneously, getting the tax year decision right for each one, and confirming software connections are actually live rather than just configured. A tracked, batched process, rather than working client by client from memory, is what keeps this from becoming chaos as the deadline pressure increases.
Receiptflow keeps client records MTD-ready ahead of each quarterly submission. See how it fits your registration process.
