Tax Planning
Planning for the UK’s Side-by-Side Package: Pillar 2 Changes Multinationals Need to Know
With the UK's Pillar 2 rules entering a new phase from 1 January 2026, multinationals must understand the Side-by-Side package’s safe harbours and amendments to avoid surprises.
By NomadicTax Research Team • 5-8 min read • September 4, 2026
## What is the Side-by-Side Package?
The Side-by-Side package refers to the UK’s implementation of the global minimum tax framework under Pillar 2 of the OECD Inclusive Framework. It introduces several **safe harbours** (e.g. Ultimate Parent Entity Safe Harbour, Substance-Based Incentive Safe Harbour, Simplified Effective Tax Rate Safe Harbour) and technical changes to how Multinational Top-up Tax and Domestic Top-up Tax are calculated. These rules ensure UK law aligns with OECD model rules and administrative guidance. ([gov.uk](https://www.gov.uk/government/publications/introduction-of-the-side-by-side-package-and-amendments-to-multinational-top-up-tax-and-domestic-top-up-tax/pillar-2-side-by-side-package-and-further-amendments-to-multinational-top-up-tax-and-domestic-top-up-tax?utm_source=openai))
## Effective Dates & Operative Periods
- The Side-by-Side package takes effect for **accounting periods beginning on or after 1 January 2026**. ([gov.uk](https://www.gov.uk/government/publications/introduction-of-the-side-by-side-package-and-amendments-to-multinational-top-up-tax-and-domestic-top-up-tax/pillar-2-side-by-side-package-and-further-amendments-to-multinational-top-up-tax-and-domestic-top-up-tax?utm_source=openai))
- Further technical amendments, including advanced rules for cross-border tax allocation, hybrids, distressed entities, are proposed to apply for accounting periods beginning on or after **31 December 2026**. ([gov.uk](https://www.gov.uk/government/publications/introduction-of-the-side-by-side-package-and-amendments-to-multinational-top-up-tax-and-domestic-top-up-tax/pillar-2-side-by-side-package-and-further-amendments-to-multinational-top-up-tax-and-domestic-top-up-tax?utm_source=openai))
## Key Changes and What You Must Do
| Reform | What It Means | Action Items |
|---|---|---|
| **Safe Harbours** | New safe harbours for Multinational Top-up Tax and Domestic Top-up Tax reduce compliance burdens and calculation complexity. | Review if your group qualifies under any safe harbour to simplify tax reporting. |
| **Cross-Border Tax Allocation Cap for Hybrids / Flow-Throughs** | Tightened technical rules to ensure appropriate use and prevent abuse. | Model out your structure to identify if income shifts or allocations are affected. |
| **Discontinued Operations Treatment** | Entities held for sale or discontinued operations will get clearer treatment under Pillar 2 rules. | If you have such entities, ensure separation in accounting and tax modelling. |
| **Companies in Distress** | Adjustment rules clarified for distressed entities. | Assess whether your group meets distress thresholds and document supporting evidence. |
## Practical Example
Imagine a multinational company with its **ultimate parent entity (UPE)** in the UK and foreign subsidiaries. Under the Side-by-Side rules:
- If the UPE submits the **GloBE Information Return (GIR)** in the UK, some overseas jurisdictions may waive requirements for local GIR filing. This aligns with OECD’s common understanding on GMT compliance. ([oecd.org](https://www.oecd.org/en/about/news/announcements/2026/05/global-minimum-tax-release-of-a-common-understanding-of-implementing-jurisdictions-and-further-administrative-guidance-to-support-compliance.html?utm_source=openai))
- If one of your subsidiaries is in distress or disposal, its operations must be clearly distinguished as discontinued, so that top-up taxes or adjustments are handled correctly. |
## What Global Tax Planners Should Be Aware Of
- **Timelines matter**: Accounting periods straddling the transition dates need careful mapping. |
- **Safe harbour eligibility**: Failing qualification can lead to full rule application with more complexity. |
- **Data and reporting systems**: Need robust systems for 2026 GIR filings and cross-border allocations. |
- **Tax incentive reviews**: As part of the Side-by-Side package, substance-based incentive safe harbour encourages aligning incentives with real economic activity. |
## Conclusion
The UK’s Side-by-Side package under Pillar 2 represents a major shift for multinationals. Early alignment on safe harbours, restructuring, and reporting systems can mean the difference between compliance and costly pitfalls. Have you audited your corporate structure yet?