As far as I’m aware HMCTS doesn’t yet accept HB appeal responses by email, only the Valuation Tribunal does (actually, they insist!).
At the last Tribunal User Group I attended, in May this year, there were no plans to digitise any of the legacy benefit appeals and HB appeals are out of scope for any of the digital reform being applied to other benefits. They intended to keep the ability to file paper appeals, and judges still expect paper bundles at hearings.
The address for everything to be sent to is currently:
Her Majesty’s Courts & Tribunals Service
Social Security & Child Support
Direct Lodgement Centre
Phoenix House
Rushton Avenue
Thornbury
Bradford
BD3 7BH
Bradford will still issue the AT37 to the appellant and deal with all the admin, and any additional papers you need to send also have to go there.
