Government Confirms Full Review of Part O Overheating Regulations

The government has confirmed that Approved Document O, the overheating regulation first introduced in 2021, will undergo a full technical review. The decision was set out in the Future Homes and Buildings Standards consultation response published on 24 March 2026 and has since been highlighted by Elmhurst Energy, which collated the main industry concerns that drove the outcome. Part O was treated as a call for evidence in the consultation rather than a proposal for immediate regulatory change, so while revised versions of Part L and Part F were published on 24 March 2026, Part O will now follow its own, slower trajectory.

What Part O Actually Does

Approved Document O applies to all new residential buildings in England, including private dwellings, institutional residences, student halls of residence and care homes. It gives guidance on Requirement O1 of Schedule 1, which asks for reasonable provision to limit unwanted solar gains in summer and to provide an adequate means of removing heat from the indoor environment. Mechanical cooling may only be used where passive strategies cannot do the job.

The guidance offers two compliance routes:

• The simplified method: a prescriptive approach with limits on glazing area, cross-ventilation and openable area, differentiated between high-risk and moderate-risk locations.
• The dynamic thermal modelling method: an assessment based on CIBSE TM59 with bedroom and living room temperature limits, using the TM59 scenarios for high-risk locations.

Requirement O1 also says that the overheating mitigation strategy must account for noise at night, pollution, security, protection from falling and protection from entrapment. Those five real-world constraints sit at the heart of the criticism industry has now raised against the 2021 document.

Why a Review Became Unavoidable

The consultation response confirms that Part O received a high volume of feedback, much of it technical and much of it critical. Elmhurst's summary groups the themes into a handful of recurring problems, all of which the government now intends to address through the review rather than leaving them to case-by-case interpretation.

Questions Over the Simplified Method

The simplified method was designed as a faster, cheaper route for lower-risk projects. Consultation feedback argued that in practice it omits too many real design variables: thermal mass, internal shading, passive design strategies, and triple glazing are among the factors respondents said are not properly captured. A common observation was that developments failing the simplified method frequently pass dynamic thermal modelling without any changes to the design, which suggests the simplified method is flagging false positives.

As a result, many assessors now skip the simplified method entirely and move straight to dynamic thermal modelling. That defeats the purpose of having a two-tier system and raises the cost and timescale of assessments on developments where a genuinely simple check ought to suffice.

Conflicts With Other Building Regulations

The sharpest criticism concerns interactions with other parts of the Building Regulations. Requirement O1 already asks the designer to account for security and protection from falling, but respondents said the guidance does not give enough practical help to resolve the tensions that creates on real sites:

The core issue is that Part O is a relatively recent addition to the Building Regulations framework and its interactions with older, better-established parts were not fully worked through before it came into force. The review will now look at those interactions explicitly, particularly noise and security.

Assessor Competency and Modelling Consistency

Another recurring theme was inconsistency between assessments of the same building by different assessors using the same TM59 method. The source of that inconsistency sits in the assumptions the modeller is permitted to make: window opening schedules, internal gains, internal shading, and weather file selection can all drive a building from pass to fail. Respondents called for formal approval of modelling software, recognised competency for assessors, and standardised reporting templates. Elmhurst notes that it already operates an overheating competency scheme for dynamic thermal modelling assessors with an NHBC-approved reporting template.

Extensions, Conservatories and Highly Glazed Additions

Because Part O applies only to new residential buildings, extensions and material change of use (MCU) works fall outside its scope. Respondents argued that this is a growing blind spot: large extensions with bifold doors, rooflights or highly glazed wraps around existing homes can present an obvious overheating risk without any regulatory check. The consultation response confirms the government will explore extending Part O to MCU dwellings, which would be a material change in scope.

What the Review Will Cover

Taken together, the issues above shape the scope of the review. Based on the government response as summarised by Elmhurst, the workstreams are:

• Adoption of the updated CIBSE TM59 methodology for dynamic thermal modelling (the current Approved Document O 2021 references the 2017 edition of TM59).
• Improvements to the simplified method to address the gaps in thermal mass, internal shading, triple glazing, and passive design that respondents highlighted.
• A review of noise and security guidance so that Part O's night-ventilation assumptions can be reconciled with Parts Q and F in practice.
• Consideration of extending Part O to material change of use dwellings, closing the gap on large domestic extensions and conversions.
• Analysis of conflicts with other parts of the Building Regulations and guidance on how to resolve them at design stage.
• Updated weather files that reflect future climate scenarios, rather than a historic baseline that may already be understating risk.

One change is already visible in the Part L package published on 24 March 2026: where air conditioning or reduced g-value glazing is installed to comply with Part O, the Part L energy efficiency standards will account for the resulting energy use so that developers are not penalised twice for the same design choice. This is a narrow fix rather than a restructuring of either document, but it signals the direction of travel.

Part O, SAP 10.3 and HEM

A related question raised in the consultation concerns the relationship between overheating assessment and the underlying energy calculation methodology. Before Part O was introduced in 2021, overheating risk was checked through an appendix inside SAP. Splitting it out into a standalone document gave it a legal footing but also created duplication of effort, since assessors now run a SAP (soon SAP 10.3) or HEM calculation for Parts L and F and then a separate overheating assessment for Part O.

Some respondents asked whether overheating ought to be brought back into the main calculation, either inside SAP 10.3 or as a module of HEM. The government has not committed to this. HEM is already modular by design, so adding an overheating module at some point in future is not out of the question, but nothing in the consultation response suggests it is an immediate priority. For now the two calculations will continue to run in parallel.

What It Means for the Industry

For energy assessors and overheating modellers, nothing changes immediately. The 2021 edition of Approved Document O is still the reference document, the simplified method and the TM59-based dynamic thermal modelling method are both still valid, and the 15 June 2022 in-force date for Part O is unchanged. Projects in design should continue to be assessed against the current rules.

For developers and architects, the more useful signal is the direction of travel. Designs that rely on a marginal pass of the simplified method will face more scrutiny in future revisions, particularly where the design hinges on night opening of windows on noisy or insecure elevations. Projects where overheating is managed through robust passive design, real thermal mass, and effective shading are likely to weather the transition better than those that rely on assumptions the updated method may remove.

For builders and handover teams, the Regulation 40B duty to provide overheating mitigation information to occupants is unchanged. Any revision to Part O will almost certainly reinforce this, not weaken it. A reliable handover pack for how to operate ventilation and shading is already the best defence against a complaint that a building overheats in practice.

For everyone else, the significance of this announcement is its separation from the main Future Homes Standard timeline. The 24 March 2026 package was the headline event. Part O is now a second, quieter track running behind it, and the detail of where it lands will shape how practical new homes are to live in during hot summers.

Sources

• Elmhurst Energy: Government Announces Full Review of Overheating Regulations Following Industry Feedback (9 April 2026)
• MHCLG: Future Homes and Buildings Standards consultation response (24 March 2026)
• GOV.UK: Approved Document O, Overheating (2021 edition, corrected February 2022)
• Related: Future Homes Standard Published (24 March 2026)
• Related: FHS Published, What Actually Changed

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