TM59 against Approved Document O: which applies

Approved Document O is a building control test. CIBSE TM59 is a design methodology that planning authorities borrow. They ask different questions, they allow different assumptions, and passing one does not demonstrate the other. On most residential schemes you will need both, which is why the modelling should be set up once to serve both rather than commissioned twice.

The short answer for an architect scoping fees: TM59 with future weather for the planning submission where the authority asks for it, Approved Document O for building control, one thermal model, two sets of inputs.

What each assessment is for

Part O of the Building Regulations deals with overheating in new residential buildings, and Approved Document O sets out how to show compliance. It is checked by building control, it applies at the point of construction, and it is a pass or fail against prescribed criteria.

TM59 is the CIBSE methodology for assessing overheating risk in homes. It defines occupancy profiles, internal gains and the criteria used to judge the result, drawing on the adaptive comfort approach in TM52 for naturally ventilated dwellings and a fixed threshold approach for mechanically ventilated ones. It is a method rather than a regulation, which is why policy can point at it.

The difference that matters is freedom of assumption. Approved Document O prescribes the weather data, the internal gains and the rules about when windows can be treated as open. A TM59 assessment for planning can use future weather years, the actual specification proposed, and assumptions agreed with the authority.

The simplified method and where it runs out

Approved Document O offers a simplified method alongside dynamic modelling. The simplified route works by limiting glazed areas and requiring minimum free areas for openings, with the limits varying by orientation, by whether the location is treated as high risk, and by whether cross ventilation is available.

It is quick, it needs no model, and it suits repetitive low rise housing with modest glazing and dual aspect units.

It runs out in four situations, and you can usually see all four at concept stage:

• Single aspect units, particularly those facing south or west.
• Glazing ratios above the limits, which almost any contemporary elevation exceeds.
• Locations where opening windows at night is not acceptable because of external noise or air quality, which removes the ventilation the simplified method assumes.
• Buildings with communal corridors contributing heat gains, which the simplified route does not handle well.

If any of those apply, go straight to dynamic modelling rather than testing the simplified route first.

Dynamic modelling inputs and weather files

The weather file is the input that changes results most, and it is the one most often reported without being stated.

Approved Document O prescribes which design summer year to use for the relevant location, and compliance is judged on that basis. A planning assessment under TM59 may use the same file, or a future weather projection representing a warmer climate period, depending on what the authority asks for. London policy has pushed schemes towards future weather for some time, and other authorities are following.

Other inputs worth agreeing before the model runs: the occupancy and equipment gains profile from TM59 rather than a bespoke one, the assumed opening behaviour and free area, whether blinds or shading devices are modelled as present and how they are controlled, corridor temperatures for flats, and the treatment of exposed thermal mass.

State every one of these in the report. An assessment that reports a pass without listing its assumptions is unverifiable, and officers have learned to ask.

Noise, pollution and security restrictions on openings

This is the point where overheating work and acoustics collide, and where a lot of schemes come unstuck late.

If the noise assessment concludes that bedroom windows must stay closed at night to meet internal noise levels, the overheating model cannot then assume those windows are open at night. Approved Document O addresses this directly by limiting reliance on openings where noise at night would be unacceptable, and by requiring a means of removing excess heat that does not depend on the occupant doing something unreasonable.

Security cuts the same way. Ground floor and accessible windows cannot be assumed open overnight, and restrictors limit the free area on upper floors.

The practical answer is to run the acoustic and overheating assessments together, with one agreed position on window openings, rather than sequentially. Where they conflict, the options are mechanical ventilation with a cooling element, acoustically attenuated background ventilation that allows a reduced opening, changes to the elevation, or a different unit mix. All of those cost far less at concept stage than at technical design.

Which one the planning authority asks for

Planning authorities cannot enforce Part O, because it is a building regulations matter. What they can do is require an overheating assessment as part of the sustainability evidence, and most that do so reference TM59 or the London Plan cooling hierarchy.

The cooling hierarchy is the substance of the planning argument: reduce the heat entering the building through orientation, shading and glazing ratios first, then manage the heat within the building through thermal mass and ventilation, then passive ventilation, then mechanical ventilation, and only then active cooling. A submission that arrives at mechanical cooling without demonstrating the earlier steps gets sent back regardless of its numbers.

An Overheating Risk Assessment is the document that carries this at planning stage, and our note on reporting TM59 results covers how the output should be presented. It sits alongside the Energy Statement, and the two should share one set of fabric and glazing assumptions. Where they do not, officers notice, as our note on the energy modelling assumptions officers challenge sets out.

Which one building control checks

Building control checks Approved Document O compliance, using the prescribed method, and it does so against the building as built rather than as consented.

That creates the risk worth flagging to clients. A scheme that passed a TM59 assessment at planning with future weather and generous shading assumptions can still fail Part O if the shading is value engineered out, if the glazing specification changes, or if the opening free areas are reduced by the window supplier's actual product.

Lock the parameters that the model depends on into the specification: glazing g-value, frame and opening free area, shading device type and depth, and any mechanical system. Then check them again at the technical design stage.

Presenting both without doing the work twice

Build one thermal model of the worst case units, then run it twice.

1. Identify the critical units: top floor, single aspect, west or south facing, smallest volume, highest glazing ratio.
2. Set the geometry, construction and system once.
3. Run the Approved Document O case with the prescribed weather file, gains and opening rules.
4. Run the planning case with the TM59 profiles and whichever weather file the authority requires.
5. Report both, with a short table setting out where the inputs differ and why.
6. Carry the differences into a risk note for the client, naming the specification items that must not change.

That table is the deliverable that saves arguments later. It shows the officer that the planning result is not a softer version of the compliance result, and it shows the contractor which assumptions are load bearing.

PlanScope drafts overheating and energy documents from your project details and flags site specific facts it cannot verify rather than inventing them. Councils expect a suitably qualified modeller to produce the dynamic thermal simulation itself, and professional review of any draft is strongly advised.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.