HMRC’s July 2026 reforms introduce new Pillar Two safe harbours, extend transitional relief and materially change the position for some US-parented groups. But simplification does not mean that multinational tax governance has become simpler.

In my previous article, I argued that HMRC’s approach to the Global Information Return, or GIR, confirmed a fundamental change in international tax risk.

The challenge is no longer limited to reaching the correct technical answer. Multinational groups must be able to execute that answer across jurisdictions, systems, filing mechanisms, local notifications and evidential files.

HMRC’s latest Pillar Two announcement takes that argument one stage further.

On 13 July 2026, the UK published draft legislation implementing the OECD’s Pillar Two Side-by-Side Package, together with further amendments to the UK’s Multinational Top-up Tax and Domestic Top-up Tax regimes. See 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

The headline will inevitably be “Pillar Two simplification”.

That is accurate, but incomplete.

Pillar Two is not being dismantled. It is being segmented.

Different groups, jurisdictions, incentives and accounting periods may now fall within different safe harbours. The result may be lower top-up tax or reduced computational work in some cases, but it also creates a new classification, election and governance exercise.

The groups that treat the package as an exemption risk misunderstanding it.

The groups that treat it as a strategic redesign of their Pillar Two operating model will be in a much stronger position.

What has the UK announced?

The UK proposals affect multinational groups with annual consolidated revenues exceeding €750 million and UK business activities.

The draft legislation implements the OECD Side-by-Side Package through four principal changes:

1. a Side-by-Side Safe Harbour and an Ultimate Parent Entity Safe Harbour;
2. a permanent Simplified Effective Tax Rate Safe Harbour;
3. an extension of the Transitional Country-by-Country Reporting Safe Harbour; and
4. a Substance-based Tax Incentive Safe Harbour.

The legislation also contains technical amendments concerning discontinued operations, distressed companies, de minimis elections, international expansion, and the allocation of controlled foreign company, hybrid and flow-through taxes for Domestic Top-up Tax purposes.

This is not merely a drafting correction.

HMRC’s published impact assessment forecasts an Exchequer effect of £590 million in 2026–27, increasing to £740 million in 2030–31. Across the published forecast period, the negative Exchequer impact is approximately £3.5 billion.

That is a strong indication that the package may materially change the tax outcome for affected groups rather than merely reduce administrative inconvenience. (GOV.UK)

1. The Side-by-Side Safe Harbour: the most politically significant change

The most prominent reform is the Side-by-Side Safe Harbour.

Under the OECD architecture, a multinational group whose ultimate parent entity is located in a jurisdiction with a qualifying Side-by-Side regime may elect for top-up tax to be treated as nil under the Income Inclusion Rule, or IIR, and the Undertaxed Profits Rule, or UTPR.

Eligibility is not based simply on the headline corporate tax rate.

A qualifying jurisdiction must have both:

• an eligible domestic minimum taxation system; and
• an eligible worldwide taxation system covering foreign branch and controlled foreign company income.

The OECD criteria also test whether the jurisdiction’s system creates a material risk that domestic or foreign profits will ultimately be taxed below 15%.

This is important because the package recognises that the Pillar Two policy objective may, in some circumstances, be achieved through a different domestic tax architecture.

In other words, the GloBE rules are no longer treated as the only possible route to minimum taxation.

However, this is not a general exemption from Pillar Two.

The Side-by-Side Safe Harbour does not switch off a qualifying domestic minimum top-up tax in countries where the group operates. QDMTTs remain relevant, and the OECD package expressly preserves the primary taxing rights of jurisdictions applying them.

That distinction is fundamental.

A group may be protected from an IIR or UTPR charge while remaining exposed to a local QDMTT calculation and filing obligation.

The safe harbour therefore changes where the tax may be collected. It does not necessarily eliminate the tax or the compliance process.

The US provision deserves immediate attention

The UK draft legislation contains a particularly significant provision for US-parented groups.

For UK legislative purposes, the United States is to be treated as specified as a qualified Side-by-Side territory for accounting periods beginning on or after 1 January 2026, until it is formally specified or the Treasury provides otherwise.

The UK test also refers to recognition through the OECD framework, so the precise application must be monitored against the OECD Central Record and the final UK legislation.

Nevertheless, the policy direction is clear.

For many US-headquartered multinational groups, the package could materially reduce exposure to the UK IIR or UTPR architecture.

It does not, however, justify closing the Pillar Two workstream.

A US-parented group with UK and other international operations must still determine:

• whether the relevant accounting period qualifies;
• whether the jurisdiction remains recognised for the period concerned;
• whether an election has been validly made;
• which entities and profits are covered;
• whether any local QDMTT remains payable;
• what must be reported in the GIR; and
• what UK registration, return and notification obligations continue to apply.

The relevant conclusion is not that US groups are “out of Pillar Two”.

It is that their exposure must now be mapped through a different route.

2. The UPE Safe Harbour is narrower

The Ultimate Parent Entity Safe Harbour should not be confused with the broader Side-by-Side Safe Harbour.

It applies where the UPE jurisdiction has a qualifying domestic taxation regime but does not necessarily meet all the worldwide taxation criteria required for full Side-by-Side status.

Where the election applies, the UPE jurisdiction’s domestic profits are protected from the UTPR.

It does not provide the same group-wide protection from both the IIR and UTPR.

That distinction is likely to create practical classification issues for groups operating through intermediate parent entities, partially owned parent entities and complex regional holding structures.

A group may need to determine not only whether a jurisdiction is recognised, but which safe harbour it has been recognised for and which part of the Pillar Two charging architecture is actually displaced.

3. The Simplified ETR Safe Harbour: permanent relief, but not a simple calculation

The Simplified Effective Tax Rate Safe Harbour is likely to have the broadest operational relevance.

It is intended to provide a permanent alternative to performing the full GloBE calculation in jurisdictions where the group can demonstrate a simplified effective tax rate of at least 15%.

The calculation begins primarily with financial accounting information used for the group’s consolidated financial statements. It applies a jurisdictional approach and reduces the number of detailed constituent-entity adjustments required under the full GloBE methodology.

The OECD describes the calculation as relying on simplified income and simplified taxes, subject to specified adjustments and integrity protections.

That will be helpful.

But the word “simplified” should be treated carefully.

The UK draft provisions dealing with the safe harbour remain extensive. They address, among other matters:

• deferred tax;
• permanent establishments;
• hybrids and flow-through entities;
• international shipping;
• revaluation gains and losses;
• tax credits;
• pension adjustments;
• intra-group financing;
• foreign currency movements;
• prior-period errors;
• duplicate deductions; and
• cross-border tax allocations.

The safe harbour is therefore better described as a reduced GloBE calculation, rather than a calculation requiring no GloBE analysis.

For UK purposes, the permanent provisions generally apply to accounting periods commencing on or after 31 December 2026. Transitional access may be available for certain periods commencing from 31 December 2025 where specified conditions are satisfied.

The UK draft also introduces a 24-month look-back condition. Broadly, the safe harbour must have been used in the reference periods or the jurisdiction must not have produced a top-up amount during those periods.

This creates a critical point for 2026 decision-making.

An election made, or not made, in one accounting period may affect the group’s ability to use the safe harbour later.

Safe-harbour elections should therefore not be treated as mechanical return-preparation decisions. They need to be considered as part of a multi-year Pillar Two strategy.

4. The Transitional CbCR Safe Harbour has been extended

The Transitional Country-by-Country Reporting Safe Harbour has been one of the most important practical simplifications during the initial Pillar Two implementation period.

The UK draft legislation extends its availability by one year.

The relevant end date is moved from 31 December 2026 to 31 December 2027, with the long-stop date moving from 30 June 2028 to 30 June 2029.

For many groups, this creates a temporary choice between:

• continuing to rely on the Transitional CbCR Safe Harbour; and
• moving to the new Simplified ETR Safe Harbour.

That choice should not be based solely on which calculation appears easier this year.

Groups should consider:

• data quality and availability;
• the source of qualified CbCR information;
• purchase accounting adjustments;
• consistency between jurisdictions;
• the safe-harbour look-back rules;
• acquisitions and disposals;
• the expected ETR trajectory;
• deferred tax movements; and
• whether the chosen approach can be sustained and evidenced.

The extra year is useful.

It is not an extra year in which to postpone Pillar Two systems work.

It is a transition period in which groups should decide which permanent operating model they intend to adopt.

5. The Substance-based Tax Incentive Safe Harbour changes how incentives enter the ETR

The Substance-based Tax Incentive Safe Harbour may have a significant impact on multinational groups benefiting from R&D incentives, manufacturing incentives and other expenditure- or production-linked tax reliefs.

Under the existing GloBE framework, some tax incentives can reduce covered taxes and therefore reduce the jurisdictional ETR, potentially generating top-up tax.

The new safe harbour permits qualifying tax incentives to be treated as an addition to covered taxes, subject to an election and a substance-based cap.

Broadly, the incentive must be:

• generally available;
• calculated by reference to expenditure incurred in the jurisdiction or qualifying production; and
• connected to substantive economic activity.

The standard substance cap is the greater of:

• 5.5% of eligible payroll costs; and
• 5.5% of relevant depreciation, amortisation or depletion.

A long-term election may instead use 1% of adjusted eligible tangible asset values, excluding land and certain non-depreciating assets.

This may improve the Pillar Two treatment of certain incentives.

It does not mean every tax relief becomes GloBE-friendly.

Groups will need an incentive-by-incentive analysis covering:

• the legal mechanism of the incentive;
• whether it is expenditure- or production-based;
• general availability;
• whether it constitutes a grant or subsidy;
• the accounting treatment;
• the amount elected;
• payroll and asset substance;
• the relevant cap; and
• interaction with qualified refundable and marketable transferable tax credits.

For tax teams, this means the group’s tax incentive register can no longer be separated from the Pillar Two data model.

For governments, it means that the design of corporate tax incentives will increasingly be judged by whether the relief rewards genuine payroll, assets, expenditure and production.

Simplification creates more elections—and more elections create more governance

The package reduces some calculations, but it also introduces a larger number of judgement points.

A group may now need to decide, territory by territory and period by period:

• whether to rely on the Transitional CbCR Safe Harbour;
• whether the Simplified ETR Safe Harbour is available;
• whether the look-back condition is satisfied;
• whether to make a qualifying tax incentive election;
• which substance cap method to adopt;
• whether the UPE jurisdiction qualifies for Side-by-Side treatment;
• whether the narrower UPE Safe Harbour applies;
• whether a QDMTT continues to take priority; and
• how each conclusion is reflected in the GIR and local returns.

That is why the package continues the governance theme identified in my earlier GIR article.

Pillar Two tax risk is increasingly determined by classification, elections, data lineage and execution.

A technically available safe harbour has no value if the group cannot establish that the conditions were met.

An election may save substantial computational work, but it must be made by the correct filing member, for the correct period, in relation to the correct entities and using supportable data.

A group may reach the right numerical answer and still have a weak position if it cannot demonstrate:

• why the safe harbour applied;
• which version of the legislation was used;
• which OECD recognition list was checked;
• how the underlying data reconciled;
• who approved the election; and
• what would happen if the safe harbour ceased to apply in the following period.

The interaction with GIR reporting cannot be ignored

Safe-harbour eligibility does not remove the need for a controlled GloBE Information Return process.

The GIR must identify the basis on which the group has determined its Pillar Two position. Safe-harbour elections, jurisdictional classifications and relevant calculations must therefore be supported by information capable of being reported, exchanged and reconciled.

This reinforces the point made in my previous article.

Central filing is not the same as central responsibility.

A group’s headquarters may determine the global safe-harbour strategy, but the UK team still needs to understand:

• the treatment of UK entities;
• whether the UK Domestic Top-up Tax applies;
• whether the UK is relying on an overseas GIR;
• what elections have been made;
• whether the UK return is consistent with the GIR; and
• which evidence has been retained locally.

The new package makes that local visibility more important, not less.

The M&A impact may be substantial

The Side-by-Side Package must also be incorporated into tax due diligence, deal modelling and post-acquisition integration.

A buyer assessing a target’s Pillar Two position should no longer ask only whether the target’s jurisdictional ETR is below 15%.

The buyer should also establish:

• whether the target relies on a transitional safe harbour;
• whether that safe harbour remains available following the acquisition;
• whether the buyer’s qualified CbCR data can incorporate the target;
• whether previous top-up amounts affect the Simplified ETR look-back test;
• which tax incentives the target receives;
• whether those incentives satisfy the new substance-based conditions;
• whether the UPE jurisdiction changes after completion; and
• whether a Side-by-Side or UPE Safe Harbour becomes available or ceases to apply.

These issues may affect the forecast tax rate, tax indemnities, purchase-price modelling, deferred tax, integration costs and post-completion filing risk.

Pillar Two due diligence performed before the July 2026 proposals may therefore need to be refreshed.

What multinational groups should do now

The correct response is not to rebuild every Pillar Two model immediately.

It is to perform a structured impact assessment.

First, map the group by UPE jurisdiction and accounting period

Confirm the location of the ultimate parent entity, the start date of each relevant accounting period and the potential availability of the Side-by-Side or UPE Safe Harbour.

For US-parented groups, the UK deeming provision should be analysed specifically, but not in isolation from the OECD recognition requirements.

Second, compare the available safe harbours territory by territory

Model the Transitional CbCR Safe Harbour, Simplified ETR Safe Harbour and QDMTT Safe Harbour rather than assuming one approach will apply globally.

The best answer may differ between jurisdictions.

Third, create a Pillar Two tax incentive inventory

Identify R&D reliefs, manufacturing incentives, production credits, accelerated deductions, exemptions and reduced tax rates.

Determine which are potential qualifying tax incentives and whether sufficient payroll and tangible asset substance exists.

Fourth, establish an election register

Every Pillar Two election should record:

• the legal provision;
• period of application;
• territories and entities covered;
• annual or long-term status;
• approval owner;
• filing location;
• data source; and
• interaction with previous and subsequent periods.

Fifth, update the GIR and local reporting control framework

The GIR, UK Pillar Two return, Domestic Top-up Tax analysis, statutory accounts, tax provision and safe-harbour files should tell the same story.

Sixth, revisit acquisitions, disposals and restructuring projects

Changes to the UPE, group perimeter or jurisdictional data may change which safe harbour applies.

A transaction that appears neutral under the full GloBE calculation may have a different effect once safe-harbour continuity and elections are considered.

The wider lesson

The Side-by-Side Package represents a significant political compromise.

It acknowledges that the original GloBE framework imposed substantial compliance costs and that some jurisdictions may achieve comparable minimum tax outcomes through different domestic systems.

It also gives greater recognition to incentives connected with real economic substance.

Those are important developments.

But multinational groups should resist the temptation to interpret “simplification” as “less control required”.

The opposite may be true.

The architecture now contains more routes to a nil top-up tax outcome, but each route has its own conditions, dates, elections, evidence requirements and interactions with local taxing rights.

The future Pillar Two question will therefore not simply be:

What is our jurisdictional effective tax rate?

It will increasingly be:

Which calculation applies, why does it apply, who elected it, what data supports it, and can we defend that conclusion consistently across the GIR and every affected jurisdiction?

That is the next phase of Pillar Two.

Not repeal.

Not retreat.

A more fragmented, elective and governance-dependent global minimum tax system.

How Vectigalis AC Tax can help

Vectigalis Tax advises UK and international groups on:

• UK Pillar Two and OECD GloBE implementation;
• Multinational Top-up Tax and Domestic Top-up Tax;
• Side-by-Side and UPE Safe Harbour eligibility;
• Simplified ETR and Transitional CbCR Safe Harbour modelling;
• substance-based tax incentive reviews;
• GIR reporting and Pillar Two governance;
• Pillar Two due diligence and post-deal integration; and
• cross-border tax data, elections and evidential files.

Where a group is US-parented, benefits from significant tax incentives, is considering which safe harbour to use, or is revisiting its 2026–27 Pillar Two compliance strategy, the starting point should be a focused impact assessment before elections and filing positions become embedded.

For a confidential discussion, contact Angelo Chirulli at: angelo@vectigalistax.co.uk

This article reflects the UK draft legislation and policy documents published on 13 July 2026 and the OECD Side-by-Side Package. The provisions may change before enactment, and their application depends on the group structure, UPE jurisdiction, accounting periods, elections, local QDMTTs, tax incentives and OECD qualification status.