Guidance

Self Assessment technical specifications (2026) for individual returns

Use this technical documentation for individual returns if you're a software developer working with the Self Assessment online service.

From:
HM Revenue & Customs
Published
30 December 2025
Last updated
18 February 2026 — See all updates

Documents

Self Assessment Individual RIM Artefacts version 1.1

ZIP, 4.99 MB

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Calculate Tax and NIC MTR version 1.5.0

ODT, 559 KB

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Payments on Account (Stage 13) Tax and NIC calculation v1.0.0

ODT, 11.4 KB

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Test Case Generator (Tester) MTR version v1.5.2

XLSM, 4.31 MB

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MTR Mapping Version 1.0

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Details

You should use this technical documentation for individual returns if you’re a software developer working with the Self Assessment online service.

Updates to this page

Published 30 December 2025
Last updated 18 February 2026 href="#full-history">+ show all updates

  1. Updated documents added for 'Calculate Tax and NIC MTR version 1.5.0', 'Payments on Account (Stage 13) Tax and NIC calculation v1.0.0', 'Test Case Generator (Tester) MTR version v1.5.2' and 'MTR Mapping Version 1.0'.

  2. First published.

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