Costing an overheating risk assessment under TM59
The modelling is a day or two of work. Settling the inputs and re-running after each facade change is where a TM59 fee actually goes.
A TM59 assessment cost is driven by the number of model runs and the number of design iterations, not by the length of the report. The modelling itself is a day or two of work once the inputs are settled. Settling the inputs, and then re-running the model after the facade changes, is where the fee goes. Architects who budget for one assessment and get three are the normal case rather than the unlucky one.
The method is the CIBSE dynamic thermal modelling approach for domestic overheating, and it is one of the two compliance routes in the building regulations for limiting overheating in new homes.
What the dynamic method requires
An overheating risk assessment under the dynamic route builds a thermal model of representative dwellings and runs it across a summer period against two criteria. The first limits the hours during which the operative temperature exceeds a comfort threshold across occupied rooms in the May to September period. The second applies to bedrooms at night and limits hours above a fixed temperature between 10pm and 7am.
Dwellings designed for occupants who cannot regulate their own environment are assessed against a fixed threshold instead, which is a materially harder test. Care settings and some affordable housing products fall into this category, so confirm the occupancy assumption before any modelling is commissioned.
Not every room is modelled. The assessment selects worst-case dwellings by orientation, floor level, glazing ratio, exposure and plan depth, then models every occupied room in those dwellings. The selection is where an experienced assessor earns the fee, because a badly chosen sample either understates risk or models rooms that were never going to fail.
Weather files, scenarios and the number of runs
The model is run against a design summer year weather file for the nearest location, with the emissions scenario and percentile set by the guidance. Where the building sits in a dense urban area, an urban weather file matters, and for London schemes the relevant file is the one the guidance names.
The number of runs multiplies quickly. Count the number of dwelling types selected, times the number of orientations, times the number of ventilation strategies tested, times the number of weather scenarios the authority or the client wants. A single block with two orientations and three dwelling types is six base runs before any iteration. A phased scheme of four blocks can reach forty.
So the first conversation with an assessor should be about the run count, not the report. Ask what sample they propose and why, and ask what happens to the fee if the facade changes.
Model inputs and what the design team has to supply
| Input | Who supplies it | Why the fee moves without it |
|---|---|---|
| Glazing areas, orientations, reveal depths | Architect | The model cannot be built at all |
| Openable free area of each window, after restrictors | Architect and window supplier | Restrictors cut free area and change the result |
| G-values, U-values and frame factors | Architect or facade consultant | Assumed values get challenged at building control |
| Shading from balconies, fins, reveals and neighbours | Architect | Omitted shading overstates the risk |
| Ventilation strategy and system type | Services engineer | Decides which ventilation modes can be modelled |
| Internal gains and occupancy profile | Guidance defaults, varied for the product | Non-standard gains need justification |
| Night-time noise levels at the facade | Acoustician | Decides whether windows may be modelled as open |
That last row is the one that most often stalls a project. The input detail for a deep single aspect plan is worked through under TM59 inputs for a single aspect flat.
Failed rooms and the iteration loop
Failures cluster. Single aspect south and west facing rooms with a high glazing ratio, a shallow reveal and a balcony overhead fail first, and bedrooms fail the night criterion before living rooms fail the daytime one.
The loop runs like this. Report the failures, agree a change, re-run, report again. Each turn of that loop is chargeable, and three turns is common on a scheme where the facade was designed before anyone thought about solar gain. The cost control is to run an early indicative model at the point the elevations are first drawn, accept that it is not a compliance report, and use it to size the glazing and the shading.
Interventions in increasing cost order: deeper reveals and recessed windows, external shading or brise soleil, reduced glazing area, higher specification solar control glass, cross ventilation by replanning, and mechanical cooling as the last resort. Mechanical cooling is the one that changes the energy strategy and the capital cost, and some local policies treat it as a design failure to be justified.
The simplified route and when it is the better option
The regulations offer a simplified compliance route with limits on glazing area and minimum free area for ventilation by orientation and risk category. For a simple plan with modest glazing and cross ventilation, that route is faster and far less costly than modelling, and there is no reason to model at all.
The dynamic route earns its fee where the simplified limits cannot be met: single aspect flats, deep plans, large glazed areas, high internal gains, restricted openings for fall protection, or a noise constraint that prevents night ventilation. If the scheme passes the simplified route comfortably, do not pay for modelling.
Mechanical ventilation and the acoustic interaction
Where night-time noise prevents windows being opened, the model has to assume them closed, and almost everything then fails without mechanical intervention. That makes the acoustic input a cost driver for the thermal work.
Order the facade sound insulation assessment and the overheating assessment together. A Noise Impact Assessment establishing the free-field levels and the internal design targets lets the thermal modeller fix the ventilation assumption once rather than twice. Where the answer is mechanical ventilation with a summer bypass, or cooling to selected rooms, the services engineer needs that information before the riser layout is fixed.
The honest difficulty is that the acoustic and thermal strategies pull against each other, and resolving them late usually costs a riser, a ceiling void or a glazing specification.
Reporting for planning and for building control
Planning and building control want different documents from the same model. A planning submission wants the policy context, the sample selection, the results against each criterion, the mitigation and any residual risk, written so a case officer can follow it. Building control wants the compliance demonstration against the approved document, with the input schedule and the as-designed specification.
Produce both from one model and say in the planning report that the building control submission will follow. Where the design changes between permission and construction, the model has to be re-run, and the question of who can prepare an overheating assessment arises again for the revised version.
If you are budgeting a whole application pack rather than a single report, see PlanScope pricing: one free report, £50 per report, or Professional for unlimited reports, or browse the 66 report types by category.
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.
Frequently asked questions
How many dwellings have to be modelled?
Not all of them. The assessment selects worst-case dwellings by orientation, floor level, glazing ratio, exposure and plan depth, then models every occupied room within that sample. The selection drives both the cost and the credibility of the result, so ask any assessor to explain which dwellings they propose to model and why before agreeing a fee.
When is the simplified route better than modelling?
Where the scheme has a simple plan, modest glazing, cross ventilation and no night-time noise constraint, the simplified limits on glazing area and free ventilation area are quicker and less costly to demonstrate. Modelling earns its fee on single aspect flats, deep plans, large glazed areas, restricted openings or a noise constraint that prevents night ventilation.
Why does a noise assessment affect the overheating fee?
Because night-time noise at the facade decides whether windows can be modelled as open. If they cannot, almost every bedroom fails the night criterion without mechanical intervention. Commissioning the acoustic work alongside the thermal work lets the modeller fix the ventilation assumption once instead of revisiting it after the first set of failures.
Which rooms fail most often?
Single aspect south and west facing rooms with a high glazing ratio, a shallow reveal and a balcony overhead. Bedrooms fail the night-time criterion before living rooms fail the daytime one. Running an indicative model as soon as the elevations are first drawn, before any compliance report, is the least costly way to avoid repeated iterations.
Do planning and building control need the same report?
They need different documents from the same model. A planning submission sets out policy context, sample selection, results against each criterion and the mitigation. Building control wants the compliance demonstration against the approved document with the full input schedule. If the design changes after permission, the model has to be re-run for the building control version.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.