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: 19 May 2026
  Show all updates

  1. 19 May 2026

     Updated document 'Self Assessment Individual RIM Artefacts version 1.2' has been added.

  2. 17 April 2026

     Updated document 'Special cases version 1.1' has been added.

  3. 31 March 2026

     The following attachments have been added: 'Special cases version 1.0' and 'Exclusion cases version 1.0'.

  4. 24 March 2026

     Updated documents added for ‘Calculate Tax and NIC MTR version 1.5.1’ and ‘Test Case Generator (Tester) MTR version v1.5.4’.

  5. 18 February 2026

     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'.

  6. 30 December 2025

     First published.

Documents

Self Assessment Individual RIM Artefacts version 1.2

ZIP, 4.94 MB

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Calculate Tax and NIC MTR version 1.5.1

ODT, 837 KB

This file is in an OpenDocument format

This file may not be suitable for users of assistive technology.

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If you use assistive technology (such as a screen reader) and need a version of this document in a more accessible format, please email different.format@hmrc.gov.uk. Please tell us what format you need. It will help us if you say what assistive technology you use.

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

ODT, 11.4 KB

This file is in an OpenDocument format

This file may not be suitable for users of assistive technology.

Request an accessible format

If you use assistive technology (such as a screen reader) and need a version of this document in a more accessible format, please email different.format@hmrc.gov.uk. Please tell us what format you need. It will help us if you say what assistive technology you use.

Test Case Generator (Tester) MTR version v1.5.4

4.3 MB

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If you use assistive technology (such as a screen reader) and need a version of this document in a more accessible format, please email different.format@hmrc.gov.uk. Please tell us what format you need. It will help us if you say what assistive technology you use.

MTR Mapping Version 1.0

ZIP, 24.6 KB

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If you use assistive technology (such as a screen reader) and need a version of this document in a more accessible format, please email different.format@hmrc.gov.uk. Please tell us what format you need. It will help us if you say what assistive technology you use.

Special cases version 1.1

ODT, 47.9 KB

This file is in an OpenDocument format

This file may not be suitable for users of assistive technology.

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If you use assistive technology (such as a screen reader) and need a version of this document in a more accessible format, please email different.format@hmrc.gov.uk. Please tell us what format you need. It will help us if you say what assistive technology you use.

Exclusion cases version 1.0

ODT, 34.2 KB

This file is in an OpenDocument format

This file may not be suitable for users of assistive technology.

Request an accessible format

If you use assistive technology (such as a screen reader) and need a version of this document in a more accessible format, please email different.format@hmrc.gov.uk. Please tell us what format you need. It will help us if you say what assistive technology you use.

Details

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