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United KingdomGOV.UK

ADR ground rules add written disclosure duty before mediation

Applicants commit to full participation, both sides disclose all relevant facts in writing before mediation, and undisclosed facts are formally recorded.

By Taxxa AI OyPublished 11 September 2026

Legal & Corporate

Industry: Across industries

Anyone applying for HMRC Alternative Dispute Resolution must now be prepared to commit fully to the process. The application asks taxpayers and their representatives to agree to a set of principles up front, and breaking those terms at any point allows HMRC to remove the dispute from ADR.

Responsibility for the process and for the outcome is split. The mediator runs the ADR process; the taxpayer and HMRC together bear responsibility for deciding the tax outcomes. You and your representative must be available within 90 days of your application being accepted for ADR for a mediation meeting, which may last a full day and takes place by phone, video call or face to face.

The central new obligation is disclosure. Before the mediation meeting, both parties must disclose in writing all facts and evidence relevant to the dispute.GOV New information discovered before a meeting goes to the mediator and the caseworker as quickly as possible, and information the mediator asks for must arrive within 15 days. If it emerges during mediation that a relevant fact was not disclosed, that information is formally recorded.GOV Every document exchanged during ADR is treated the same way as information exchanged with HMRC at any other time.

Discussions aimed at resolving the dispute are conducted on a without prejudice basis, with the meaning of that term signposted to a dedicated section. A taxpayer who makes a formal complaint about the process should expect the mediator to be asked to comment.

Several features of the previous ground rules have gone. The old rule under which any disclosed tax fact affecting another HMRC tax or duty could be relied on, including through assessments on that other tax or duty, no longer appears. The prohibition on any communication being off the record has been dropped. The expectation that cases are generally concluded within four months has been removed, as has the statement that mediators police delay and a flagging or improperly engaged mediation will be addressed and in some cases ended.

The previous stand-alone duty to tell the mediator as soon as possible where a deadline cannot be met no longer appears. What survives is timing discipline around information: new information still goes over as quickly as possible and requested information still within 15 days. What is new is the written, advance duty on both parties to discloseGOV, and the rule that where it becomes clear during mediation that a relevant fact has not been disclosed, that information will be formally recordedGOV.

The ground rules sit in HMRC's Alternative Dispute Resolution guidance, Ground rules for Alternative Dispute Resolution (ADRG01300).

Before an ADR application, assemble every relevant fact and piece of evidence in writing — undisclosed material will be formally recorded — and diarise availability within 90 days of acceptance.

Sources

Ground rules for Alternative Dispute Resolution