Overheating Assessments Knowledge Hub

An overheating assessment is a report that evaluates whether a building is at risk of becoming too hot in summer and what mitigation is needed to reduce that risk. In building-regulations terms, it is the process used to show compliance with Part O for qualifying residential buildings. In planning terms, it is often a dynamic thermal modelling exercise used to show how the scheme manages heat risk through design rather than relying on late mechanical cooling.

Yes, in most UK project language those terms usually mean the same thing. Both refer to an assessment of summer heat risk using either the simplified method or dynamic thermal modelling under Part O, or using TM59/TM52 at planning stage. The real distinction is usually not the title, but whether the report is being prepared for Building Regulations, planning, or both.

It is important because overheating is a health and welfare issue, not just a comfort complaint. Approved Document O says the aim of Requirement O1 is to protect occupants by reducing the occurrence of high indoor temperatures. CIBSE’s TM59 overview also highlights the health and wellbeing impacts of overheating in homes, including sleep disruption and heat-wave vulnerability.

Not always. A Part O assessment is specifically the Building Regulations route for qualifying new residential buildings. An overheating assessment can also be a planning-stage exercise, especially in London, where dynamic modelling is required on major developments even if the design team later uses a different compliance route for Building Regulations.

Not necessarily. A planning overheating report is usually broader and earlier-stage than a pure Building Regulations Part O submission. The GLA requires dynamic overheating modelling at planning application stage for major developments, while Part O also has its own statutory compliance routes and checklists for Building Control. On many schemes the same modelling informs both, but the submission purpose is different.

The strongest use cases are new residential buildings, student accommodation, care-related residential buildings, major London planning applications, and mixed-use schemes with residential elements. Under Part O, the scope includes dwellings, residential institutional buildings and residential other buildings such as halls of residence and student accommodation. In London planning, major proposals are expected to submit dynamic overheating modelling.

Yes, in current England guidance it does. Approved Document O says the guidance applies to new residential buildings only. The same core position exists in Wales, where the current Approved Document O says it applies when a new residential building is erected. That makes Part O a new-build residential overheating regime, not a blanket rule for all building work.

No, not in current England guidance. The official England FAQ says Part O only applies to new residential buildings, and that it does not apply to extensions or conservatories added to residential buildings after they are built. A conservatory forming part of a new residential building is different, but later add-ons are outside current Part O scope.

No, not currently in England. The official England FAQ says Part O does not apply to buildings undergoing a change of use, and the 2023 consultation documents also state that MCU dwellings are currently outside scope. In Wales, the current Approved Document O applies when a new residential building is erected, so the live Welsh scope is also not a blanket change-of-use route.

Yes. Planning and Building Regulations are separate. Even where Part O does not currently apply to a change of use, planning authorities can still ask for an overheating assessment through policy or validation requirements. In London, major development proposals require dynamic overheating modelling, and borough guidance such as Haringey’s requires overheating assessments on all major planning applications.

Yes. The GLA Energy Assessment Guidance says London Plan Policy SI 4 requires all major development proposals to undertake dynamic overheating modelling. It also says this dynamic modelling is required at the planning application stage, regardless of which method may later be used to demonstrate compliance with Approved Document O.

Under the current England guidance, the scope includes dwellings, residential institutional buildings, and residential other buildings. That means houses, flats, care-type residential buildings where people sleep on the premises, and residential colleges, halls of residence and other student accommodation. Wales uses the same broad residential categories in its current Approved Document O.

No, hotel rooms are excluded. The wording of Requirement O1 in England applies to dwellings, institutions and buildings containing rooms for residential purposes, other than a room in a hotel. So if the building is genuinely hotel accommodation rather than student, care or residential-type accommodation, it is not in the normal Part O scope.

No, not by Part O itself. Part O is a residential building regime. However, planning-stage overheating assessments can still apply to non-residential buildings, especially in London, where the GLA requires TM52 dynamic overheating modelling for non-residential developments on major applications. So non-residential overheating assessment exists, but it usually sits in the planning/TM52 space rather than Part O.

Yes, in residential buildings they can be. England’s Approved Document O says shared communal rooms and common spaces of buildings containing more than one residential unit fall within scope. Wales says the same. In planning guidance, the GLA also says communal corridors should be included in residential overheating analysis where community heating pipework runs through them.

Sometimes, yes. England and Wales both say a unit containing living accommodation and commercial space should be treated as residential if the commercial part can revert to residential use, there is direct access between the spaces, and both sit within the same thermal envelope. That makes some live/work units Part O jobs rather than pure commercial assessments.

The simplified method is the prescriptive Part O route based on glazing, location, shading and free-area limits. Dynamic thermal modelling is the performance route using a simulated building model to predict overheating risk. England’s Approved Document O says compliance can be demonstrated by either the simplified method in Section 1 or the dynamic thermal modelling method in Section 2. Wales also offers the same two broad routes.

The simplified method can be used where the design can actually meet the prescriptive limits on glazing, shading and opening area. England says the simplified method is suitable for any building within Part O scope, while Wales says the simplified method can be used where the prescriptive measures in its Section 1 are achievable. If they are not, the dynamic route is needed.

It is the better route when the design is too complex or too constrained for the simplified method, or where more design flexibility is needed. England’s Approved Document O specifically points to dynamic modelling for buildings with very high insulation and airtightness, unusual site conditions, or heavy shading by neighbouring buildings. It is also the route you usually end up on when noise, pollution, security or façade constraints make simplified assumptions unrealistic.

TM59 is CIBSE’s Design methodology for the assessment of overheating risk in homes. CIBSE’s Knowledge Portal still lists TM59 (2017) as active, and England’s Approved Document O says the dynamic thermal modelling route should follow CIBSE’s TM59 methodology. In practical terms, TM59 is the standard residential dynamic overheating method most teams mean when they ask for an overheating assessment.

TM52 is CIBSE’s guidance on the limits of thermal comfort and avoiding overheating in buildings. The CIBSE portal describes it as predicting overheating in buildings and providing criteria for overheating risk. In London planning it is the standard dynamic-overheating reference for non-residential developments, while TM59 is used for residential projects.

Yes. The GLA Energy Assessment Guidance says TM59 should be used for residential developments and TM52 for non-residential developments on major planning applications. CIBSE’s Knowledge Portal still lists both TM59 and TM52 as active. So, as at March 2026, TM59/TM52 remain the live planning-stage overheating standards most teams are working to.

The cooling hierarchy is the policy sequence used to reduce heat risk before falling back on active cooling. The GLA guidance sets it out as: reduce heat entering the building, minimise internal heat generation, manage heat within the building, provide passive ventilation, provide mechanical ventilation, and only then provide active cooling systems. In practical terms, it is a design-first approach, not an air-conditioning-first approach.

In England, the simplified method uses high-risk and moderate-risk locations. Appendix C of Approved Document O identifies the high-risk part of London by postcode, and the official FAQ also notes that central Manchester may justify the higher-risk approach even though the minimum requirement there is still the moderate-risk route. Wales uses a different Approved Document O framework and does not use the same England postcode split.

No. England’s Approved Document O is explicit that the guidance is written to protect health and welfare, and that following it does not guarantee comfort for occupants. That is an important real-world point: a design can comply with the minimum regulatory threshold and still not feel ideal during extreme weather or unusual occupancy patterns.

It means the ability to ventilate a dwelling using openings on opposite façades, so heat can be purged more effectively. The GLA guidance says dual aspect in this context means openings on opposite façades, and that openings on façades that are not opposite, such as in a corner flat, do not count as cross-ventilation. This distinction matters because it directly affects both simplified-method allowances and dynamic-model assumptions.

They work by capping the proportion of glazing allowed based on orientation, whether the dwelling has cross-ventilation, and whether the site is in a high-risk or moderate-risk area. England’s simplified method checks both the overall glazing area of the building or part of the building and the glazing area of the most glazed room, using the orientation of the façade with the largest glazing area.

The g-value is the measure of how much solar energy passes through the glazing. In the official England FAQ, the stated Part O g-values are centre-pane g-values and should not include the effect of frames. In practical terms, lower g-values reduce summer solar gain, which is why they are often part of the overheating strategy where glazing areas are high.

It is the effective area of the opening available for removing excess heat. In England, Approved Document O says openings should be designed so their equivalent area meets or exceeds the required free area, either by testing to BS EN 13141-1 or by calculation using Appendix D. In Wales, the FAQ explains that free area is typically the geometric open area adjusted by discharge characteristics, and equivalent area can be used if it achieves the required performance.

Then the overheating strategy has to account for that restriction properly. England’s Approved Document O says where external noise is an issue, the strategy should take account of the likelihood that windows will be closed during sleeping hours, and where the standards in Section 3 make the simplified free-area assumptions unrealistic, dynamic thermal modelling should be used instead. Wales takes the same broad usability-first approach through Section 2 of its guidance.

Usually, yes. England’s guidance says where the standards in the usability section mean the simplified method cannot be met, dynamic thermal modelling should be used. The GLA goes further at planning stage, requiring applicants with window-opening limitations from noise, air quality or security to demonstrate that all passive measures have been explored and, in some cases, to submit both an open-window and closed-window overheating analysis.

No, not for demonstrating compliance. England’s Approved Document O says internal blinds and curtains should not be taken into account when considering whether Requirement O1 has been met, and the GLA guidance says applicants should confirm they have not been included in TM59 assumptions for residential overheating analysis.

Not for Part O compliance in England. Approved Document O says foliage such as tree cover can reduce solar gains, but it should not be taken into account when considering whether Requirement O1 has been met. Permanent adjacent buildings or permanent shading structures are a different matter and can be part of the strategy.

Yes, but only as a last resort. Approved Document O says the building should meet Requirement O1 using passive means as far as reasonably practicable, and that mechanical cooling is expected to be used only where the requirement cannot be met using openings. The GLA cooling hierarchy makes the same point by putting active cooling at the end of the sequence.

Yes. The official England FAQ says that if mechanical cooling is used to pass Part O, the SAP calculations must reflect that cooling is installed, and changes may then be needed to ensure the dwelling still complies with the energy-efficiency requirements. This is why overheating and energy compliance cannot be treated as separate silos.

England’s Approved Document O sets limits on the choices available when using TM59. During the day, openings should start to open at 22°C and be fully open at 26°C, then close back down as temperatures fall. At night, qualifying upper-floor, non-easily-accessible openings are modelled fully open if the temperature still exceeds 23°C at 11pm. Wales sets similar opening-regime rules in its dynamic modelling section.

Yes. England’s Approved Document O says that when a ground-floor or easily accessible room is unoccupied, windows, patio doors and balcony doors may be modelled open in the day if that can be done securely, but at night they should be modelled closed. This is one of the key practical limits that stops overheating models from assuming unrealistic night cooling on vulnerable façades.

They are the fall-protection rules for openings that form part of the overheating strategy. In England, outward-opening window handles must be no more than 650mm from the inside face of the wall at maximum opening angle, and where the change in level is more than 600mm, the guarding height is 1.1m. Wales has the same 650mm control-position rule in its current guidance.

That depends on the route used. For the dynamic method in England, the Building Control body should be provided with a report showing that the building passes TM59, plus the compliance information in Appendix B. Wales also uses an approved-document checklist/reporting route for demonstrating compliance. In simple terms, Building Control needs more than a one-line “pass” statement.

In both England and Wales, Regulation 40B requires the person carrying out the work to give the owner sufficient information about the overheating strategy within five days after completion. The guidance says this should include the overall strategy, the location of the elements, operating instructions, maintenance information and, for new dwellings, the relevant Home User Guide content about staying cool in hot weather.

At planning stage, a good report should clearly state the software used, site location, orientation, weather files, internal gains, occupancy profiles, fabric and glazing performance, shading, thermal mass, ventilation strategy, and the sample units modelled. The GLA guidance lists these assumptions explicitly and requires evidence of how the development performs against the overheating criteria together with the mitigation strategy.

The consultant needs enough information to understand the layout, façade orientation, glazing strategy, shading, ventilation method, fabric performance, occupancy type and services strategy. For London planning work, the GLA specifically expects assumptions on weather file, internal gains, occupancy, U-values, g-values, shading features, thermal mass, ventilation strategy and sample units. Without that, the model is guesswork.

Yes, often it can, but the sample selection has to be justified. England’s Part O checklist asks for the number of sample units modelled and an explanation of why that size and selection has been chosen. The GLA also requires model images indicating the sample units modelled. The important point is that the sampled units genuinely represent the highest-risk and relevant typologies.

Sometimes, yes. Under Part O, shared communal rooms and common spaces in multi-unit residential buildings are in scope. The GLA guidance adds that communal corridors should be included in residential overheating analysis where community heating pipework runs through them. On schemes with heat networks or long pipe runs, leaving corridors out can miss a real problem.

Typical problems are too much glazing, weak or unshaded west/south façades, single-aspect layouts, unrealistic window-opening assumptions, noise/security constraints, and heat gains from building services such as communal heating pipework. TM59’s overview also highlights excessive glazing, inadequate natural ventilation and mechanical systems not delivering intended air-change rates as common contributors to overheating risk.

The most effective measures are usually the passive ones used early: orientation changes, reduced glazing where needed, external shading, better cross-ventilation, sensible thermal mass, and cleaner control of internal gains. The GLA cooling hierarchy and Approved Document O both point in this direction, with external shading, glazing design, balconies, louvres, opening strategy and passive ventilation all sitting ahead of active cooling.

Yes. The GLA guidance says overheating analysis should continue to be incorporated into design discussions as the design evolves, and that substantive changes from earlier stages require revised overheating analysis. In practical terms, changing glazing, shading, plant, layout or ventilation assumptions late can invalidate an earlier report.

There is no single national timeframe. A simple residential assessment following a clear route is quicker than a phased mixed-use scheme with multiple sample units, communal spaces, planning-stage revisions and London-specific reporting requirements. The main time drivers are usually the amount of modelling, the number of scenarios, and the quality of the design information rather than the report writing alone.

Cost is driven mainly by complexity, not just floor area. The biggest factors are whether the job is planning, Building Regulations, or both; whether it uses TM59, TM52, or the simplified method; how many sample units and communal spaces are included; and how many revisions the design goes through. The more scenarios and coordination the project needs, the more involved the service becomes.

Treat overheating as an early design issue, not a late compliance add-on. The strongest approach is to review heat risk before the façade and services strategy are fixed, follow the cooling hierarchy, model the right sample units, and keep the analysis live as the design evolves. The GLA and CIBSE guidance both point the same way: early passive design decisions are what stop overheating becoming a late-stage problem.