MMTAR Tranche Two Weekend Checklist: ASA Prep Before 18 August Opens

As of 16 August 2026, HMRC’s first Modernising and Mandating Tax Adviser Registration (MMTAR) window has roughly 48 hours left for tranche one. At the same moment, firms that already hold a Self Assessment or Corporation Tax agent account but still lack an Agent Services Account (ASA) should treat this weekend as prep time: their three-month tranche-two window opens on Monday 18 August 2026.

HMRC’s GOV.UK guidance is clear that when you must register depends on your circumstances. If you already have a Self Assessment or Corporation Tax account, you need to register for an ASA from 18 August 2026 unless you already hold one. You can continue interacting with HMRC for clients during the three-month application window and while HMRC considers a timely application — but incomplete packs still stall access later.

Confirm you are in tranche two — not finished, not payroll-only

Use HMRC’s interactive checker before Monday. Typical tranche-two firms have an older Self Assessment or Corporation Tax online agent account, do not yet have an ASA, and are not solely third-party payroll providers (from 18 November 2026) or financial services organisations (from 31 December 2026).

If you already have an ASA, HMRC says you do not register again. HMRC will contact you through the existing ASA when it needs more information. Do not restart an application because tranche two opens.

Tranche one is not an extension into tranche two. New advisers and those without an ASA, Self Assessment, or Corporation Tax agent account still face the 18 August close of the first window — finish those applications before midnight if that is your cohort.

Weekend prep list so Monday is day one of filing, not day one of hunting documents

Map every agent ID the firm still uses across offices and acquisitions. Decide which legal entities need a new ASA application when the window opens.

Assemble AML supervision evidence and identity/ownership documents for the people HMRC will check. Incomplete AML supervision remains one of the most common application stalls.

Confirm HMRC registration conditions on GOV.UK and diary the end of the three-month window (professional bodies summarise the second non-ASA window as closing 17 November 2026). Do not wait until mid-November to discover missing evidence.

Split ownership from MFA work: opted-in agents still face 19 August MFA activation, and remaining IDs enter the 28 September–15 October automatic window. Registration and MFA are related firm-risk items, not the same task.

How FinnAccountings helps while ASA packs assemble

Client ledgers still need to move while registration admin runs. FinnAccountings helps Ireland and UK accountants and business owners prepare bookkeeping and tax packs with Chartered Accountant insight before qualified sign-off. Start a free trial for review-ready drafts — we prepare packs; we do not register tax advisers with HMRC or file client returns on your behalf.

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Related reading

Read the full tranche-two opening briefing for the three-month window rules, then the tranche-one 72-hour countdown if any IDs still sit in the first cohort.

MMTAR tranche two opens 18 August 2026 →

Tranche-one firms with hours left should use the final-day registration checklist published on 17 August.

MMTAR tranche one: final day before 18 August →

Sequence ASA, MFA, and the 20 August data consultation as three separate workstreams.

Agents’ critical fortnight: 18–20 August 2026 →