Overheating assessment beside a Part O compliance report

An overheating risk assessment for planning and a Part O compliance report answer different questions with the same software. Planning asks whether the design strategy manages overheating risk, following the cooling hierarchy in the development plan, and it is judged at application stage. Approved Document O asks whether the building as designed complies with the Building Regulations requirement to limit solar gains and provide a means of removing excess heat, and it is judged by building control. Same geometry, different assumptions, different weather files, different pass criteria.

Architects who commission one and assume it covers the other tend to find out at the building control submission, which is the expensive point to find out.

What planning asks and what building control asks

Planning policy on overheating is set locally. Many local plans, and the London Plan in particular, require applicants to demonstrate that the cooling hierarchy has been followed: reduce heat gains through orientation, shading, albedo, fabric and glazing before relying on ventilation, and rely on mechanical cooling only as a last resort. The evidence is usually dynamic thermal modelling to CIBSE TM59 for homes or TM52 for non-domestic space, reported alongside the energy strategy.

Approved Document O came into effect in June 2022 and applies to new residential buildings. It sets two compliance routes and no hierarchy. The question is binary: does the design meet the criteria.

TM59 against the simplified method

The simplified method in Approved Document O is a set of geometric limits: maximum glazing areas and minimum free areas of ventilation openings, varying by orientation, by whether the location is in a high risk zone and by whether the dwelling is cross-ventilated. It is quick, needs no modelling, and it is unforgiving on single-aspect flats with large south or west glazing.

The dynamic route uses CIBSE TM59 as its base method with modifications set out in Approved Document O, which is where the divergence starts. The planning assessment and the compliance assessment can use the same model geometry and still produce different answers, because the Part O route fixes several inputs that a TM59 assessment for planning may treat differently.

Where the two assessments diverge

Four differences account for most of the gap:

• Weather data. Approved Document O specifies the weather files to be used. Planning policy in some areas asks for future weather years to test resilience in the 2050s, which is a harder test.
• Internal blinds and curtains. The Part O route constrains what credit can be taken for occupant-operated shading, so a model that passes on the assumption that blinds are drawn may not pass for compliance.
• Window opening behaviour. Openings relied on for ventilation have to be realistic about security on ground floors and about restrictors at height, and the Part O route sets assumptions for this.
• Pass criteria. TM59 has its own criteria for living spaces and bedrooms. Compliance under Approved Document O is assessed against the criteria in the document itself.

The practical consequence is that a planning assessment reporting a comfortable pass is not evidence of compliance, and a building that complies may still fail the policy test where the development plan asks for future weather years. Read the current version of Approved Document O for the specified inputs rather than working from a note written when it was published.

Noise constrains both

Where the strategy relies on opening windows, the noise climate decides whether that is realistic. Approved Document O requires the effect of noise to be taken into account where openings are relied on, and planning policy on noise applies in parallel.

In practice this is one assessment serving two purposes. Measured or modelled external levels at the facade, internal levels in bedrooms at night with windows open to the free area required, and a judgement against the guideline internal values. Where the facade is exposed, the answer is usually mechanical ventilation with acoustic attenuation, which then changes the overheating model, which then changes the energy strategy. A Noise Impact Assessment and the overheating work should be commissioned together, by the same consultant where possible, because iterating between two firms is slow. The relationship between facade levels and internal values is worth understanding properly, and our note on BS 4142 rating levels or BS 8233 internal values sets out which standard does which job.

Conditions requiring overheating evidence

Councils commonly impose a condition requiring overheating modelling to be submitted and approved before the relevant part of the development starts, particularly where the planning submission contained a strategy rather than results. That condition is where projects lose weeks.

Two habits avoid it. Submit the modelling results with the application rather than a statement of intent, so there is nothing left to condition. And if a condition is imposed anyway, check its wording: a condition requiring compliance with Approved Document O is duplicating the Building Regulations and can be resisted, while a condition requiring TM59 results against local policy criteria is a planning matter and will stand. An Overheating Risk Assessment submitted up front also gives the officer something to point to when a neighbour raises the glazing area.

Timing each in the programme

The planning assessment has to be done on the design that is being submitted, which means after the elevations are fixed and before the application goes in. The Part O assessment has to be done on the design being built, which means after any value engineering of glazing specification, shading and ventilation hardware.

Those are usually different designs. Where the project changes between permission and construction, the Part O result changes with it, and a glazing substitution made for cost reasons is the commonest cause of a late failure. Flag the dependency to the client at the point the saving is proposed.

Producing one model for both

This is worth doing deliberately rather than by accident. Build one thermal model of the geometry, zoning and construction, then run it twice with two input sets: the planning set driven by local policy, including any future weather year, and the compliance set driven by the current Approved Document O inputs.

Ask the modeller for both outputs in one report with the assumptions tabulated side by side. The benefit goes beyond the fee. When building control queries an input, the tabulated comparison shows immediately whether the difference is a modelling choice or a design change. Where overheating mitigation affects the energy strategy, the Energy Statement has to be consistent with it, and a Sustainability Statement that contradicts the overheating report is the kind of inconsistency officers notice.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.