Part O compliance or a CIBSE TM59 assessment

Approved Document O and CIBSE TM59 are not alternatives in the way the question is usually asked. Part O of the Building Regulations is the legal requirement for new residential buildings in England, and it offers two routes to compliance: a simplified method based on glazing and opening areas, or dynamic thermal modelling. TM59 is the modelling methodology that the dynamic route uses. So you are not choosing between Part O and TM59. You are choosing between the simplified method and modelling, and then dealing separately with whatever the planning authority asks for, which is frequently more.

For an architect the practical decision comes at concept stage, because the simplified method puts hard limits on how much glass you can draw.

What Part O requires

Part O applies to new residential buildings, covering dwellings, residential institutions and rooms for residential purposes. It requires reasonable provision to limit unwanted solar gains in summer and to provide an adequate means of removing excess heat from the indoor environment.

Approved Document O sets out how to meet that. It divides England into moderate and high risk locations, with parts of London and a small number of other urban areas treated as high risk. It then offers the two compliance routes.

It also imposes constraints that often matter more than the temperature criteria themselves. Where night-time noise means windows cannot reasonably be opened, the strategy has to work with them closed. Openings have to be usable without compromising security, particularly at ground and first floor. Guarding and entrapment have to be addressed where openings are large.

Conversions and material changes of use generally fall outside Part O, which is why a permitted development office-to-residential scheme can produce dwellings that would fail the standard if they were new build. That gap is well known and it is one reason planning authorities have stepped in with their own policies.

The simplified method and what it costs you

The simplified method works by limiting glazing area in each orientation and requiring minimum free areas for openings, with different figures for cross-ventilated and single-aspect dwellings, and stricter figures in high risk locations.

It is quick, it needs no specialist, and building control accepts it without argument. The price is design freedom. South and west facing glazing gets constrained, single-aspect flats in high risk locations are difficult to make work, and deep plan layouts with one openable facade struggle.

A rule I would apply: if the scheme has single-aspect units, a glazed south elevation, or a noise or air quality constraint that limits opening windows, assume the simplified method will not carry the design and budget for modelling from the start.

Dynamic thermal modelling under TM59

The dynamic route models the building hour by hour across a summer period using specified weather data, and tests the results against TM59 criteria.

For predominantly naturally ventilated homes, TM59 applies two tests. The first uses the adaptive comfort criterion from TM52, limiting hours of exceedance in occupied rooms across the May to September period. The second is a fixed bedroom test, limiting the hours between 22:00 and 07:00 during which operative temperature exceeds 26 degrees. Mechanically ventilated homes are assessed against a fixed threshold instead.

Both tests have to pass. A scheme that satisfies the living room criterion and fails the bedroom criterion has failed, and bedroom night-time performance is where most designs fall over.

The modelling needs real inputs: construction build-ups, g-values, opening free areas rather than nominal window sizes, shading geometry including balconies and reveals, and internal gain profiles. Who is competent to run it is a live question, and we deal with it in the note on who can prepare a TM59 assessment.

Where planning policy asks for more than the regulations

This is the part that catches design teams out. Building regulations and planning are separate consent regimes, assessed by different people at different times.

In London, policy on managing heat risk requires major development to follow a cooling hierarchy, starting with reducing the amount of heat entering the building and only reaching active cooling at the end, and expects the evidence to be presented at application stage. That is a planning requirement, and satisfying Part O does not discharge it.

Outside London a growing number of local plans include overheating policies, often tied to the same modelling methodology and sometimes applying to conversions that Part O does not reach. Conditions requiring an overheating risk assessment before commencement are now common on flatted schemes.

The two regimes also ask slightly different questions. Building control asks whether the building complies. The planning officer asks whether the design has minimised overheating risk in the first place, which is a design question about orientation, depth, shading and aspect rather than a compliance calculation.

Ventilation, security and noise pulling against each other

Resolve these together. An acoustician, an air quality consultant and a thermal modeller working from different assumptions will produce three reports that contradict each other, and the case officer will notice.

Evidence submitted at each stage

At planning, expect to submit an overheating assessment where policy or a condition requires it, showing the cooling hierarchy, the passive measures adopted, the modelling results by room type, and any residual reliance on mechanical cooling. How those results are presented matters, and the note on reporting TM59 results sets out what a reviewer looks for.

At building regulations, expect either the simplified method calculations or the dynamic modelling output, plus evidence on removal of excess heat, noise, security and entrapment.

The two submissions should be consistent. Where the planning assessment modelled a shading fin that later disappeared from the tender drawings, the building control submission will show it, and the discrepancy is exactly the kind of thing an enforcement officer follows up after occupation.

Choosing the method early

1. Establish at concept stage whether the site is in a high risk location and whether night-time noise or air quality will limit openable windows.
2. Test the worst unit, usually a single-aspect south or west facing flat, before the elevations are fixed.
3. Decide simplified or dynamic, and tell the client what each implies for glazing ratios and cost.
4. Coordinate the energy strategy with the overheating strategy, because insulation and airtightness improvements that help the energy statement can worsen summer performance.
5. Model the building you will actually build, including restrictors, reveals and balcony soffits.

The honest uncertainty here is how far planning policy will keep diverging from the building regulations on this topic. For now, assume the planning ask is the harder one and design to it.

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