Energy and overheating modelling: who holds what

Energy and overheating modelling for a planning submission is done by two different kinds of modeller, and conflating them is how schemes end up with an energy statement the authority accepts and an overheating result that collapses at detailed design. Compliance modelling for dwellings is carried out by an accredited domestic energy assessor working in the government's Standard Assessment Procedure. Dynamic thermal modelling, which is what overheating assessment to the dynamic method actually requires, needs a modeller competent in dynamic simulation, and no domestic accreditation on its own covers it.

So when you ask a consultant whether they can do the energy statement and the overheating assessment, the useful follow-up is which software and which accreditation, not whether they have done one before.

Compliance modelling against design modelling

Compliance modelling answers a regulatory question with standardised inputs. The Standard Assessment Procedure for dwellings, and the Simplified Building Energy Model for most other buildings, use fixed occupancy profiles, fixed internal gains and standard weather, so that two buildings can be compared on the same basis. The output is a pass or a margin against a target.

Design modelling answers a physical question. It asks how a specific building with specific glazing, orientation, ventilation strategy and occupancy will behave, usually hour by hour across a year, and it gives temperatures rather than a compliance figure.

Both have a place in a planning submission. The emissions reduction figures in an energy statement, one of the energy and sustainability report types, come from compliance modelling, because that is the basis policy targets are written against. The overheating conclusions come from design modelling, because a compliance tool cannot predict internal temperature.

Accreditation for domestic and non-domestic assessment

For dwellings, the relevant qualification is an on-construction domestic energy assessor, accredited through one of the government-approved schemes and able to produce the SAP calculation and lodge the energy performance certificate. The accreditation is the thing to check, along with the assessor's registration number, because an unaccredited SAP output cannot be lodged.

For non-domestic buildings, assessors work at defined levels. The higher levels cover more complex buildings and, at the top level, dynamic simulation modelling rather than the simplified tool. A modeller accredited only for the simplified tool cannot produce a compliant dynamic simulation result for a complex building, and the distinction between levels is the single most useful question to ask.

In London, the energy assessment also has to follow the Greater London Authority's guidance and the Be Lean, Be Clean, Be Green, Be Seen hierarchy, with reporting at each stage. That guidance adds expectations about the modeller's outputs and the format of the submission that sit on top of the national position.

Who performs dynamic simulation

Dynamic simulation is carried out by building services engineers and energy modellers using recognised simulation software. The competence markers worth asking for are membership of the Chartered Institution of Building Services Engineers, the low carbon consultant registers that body maintains, and demonstrable experience of the specific assessment method being used.

Ask three questions at appointment:

1. Which simulation tool will be used, and is it accepted for the method proposed.
2. Which weather file will be used, and whether a future climate projection is being applied.
3. Who will carry out the quality check on the model, and what that check covers.

The weather file question matters more than it sounds. A result generated against a historic average year and a result generated against a projected warmer year are not comparable, and a planning authority asking for climate resilience will want to know which was used.

Overheating assessment and the applicable method

For new dwellings in England, Approved Document O sets out two routes: a simplified method based on glazing areas and opening areas, and a dynamic method following the Chartered Institution of Building Services Engineers methodology for assessing overheating risk in homes.

The simplified method is quick and will pass some designs. It fails buildings with large glazed areas, single aspect flats, restricted opening because of noise or security, and corridor-fed apartment plans. For those, the dynamic method is the only route, and it requires a modeller with dynamic simulation competence.

The honest position is that the simplified method is not a lower standard applied to simpler buildings. It is a conservative screening route, and schemes that are borderline on it often fail outright. If the design has deep plan flats, a south or west facing glazed elevation, or a mechanical ventilation strategy, go straight to the dynamic method and budget for it.

Non-domestic overheating and comfort assessment uses the separate guidance for other building types, and the choice of criteria needs agreeing with the client before modelling begins.

Inputs the modeller needs and when

Modellers are frequently blamed for late results when the real problem is late information. The minimum package to start is:

• Plans, sections and elevations at a consistent revision, with glazing sizes and opening types shown.
• Construction build-ups with intended U-values, or a stated design intent.
• The ventilation strategy, including whether openings are restricted and why.
• Shading proposals, including external shading, reveals, balconies and any neighbouring obstruction geometry.
• Occupancy assumptions for non-domestic buildings.

Noise constraints are the item most often missing. Where the acoustic strategy requires windows to stay closed, the overheating model has to reflect that, and the two consultants need to be speaking to each other. Discovering the conflict after planning submission usually means a design change.

Reviewing outputs for plausibility

An architect reviewing a report does not need to rebuild the model. Four checks catch most problems.

Compare the modelled areas and glazing ratios against the drawings. Check the orientation in the model matches the site plan, because a rotated model is a common error. Look at which rooms fail and ask whether they are the ones you would expect, since a result where a north facing room overheats and a south facing one does not usually indicates an input error. And read the assumptions appendix, which is where restricted opening, internal blinds and optimistic occupancy hide.

Where two consultants reach different conclusions on the same building, the approach set out in the note on statements of common ground and expert conferencing is a practical way to narrow the dispute.

Post completion verification and sequencing

The planning stage figures are not the end of it. As-built calculations and the energy performance certificate follow completion, and in London the Be Seen requirements call for post-construction and in-use reporting against the figures submitted at planning stage. An energy statement that promised a reduction the building cannot deliver becomes a problem years later.

Sequence the appointment early. Commission the energy and overheating modelling at the point the massing and glazing are settled but before the elevations are finalised, so the result can still change the design. Commission it after planning submission and the only available fix is a condition.

Where a modeller is replaced partway through, the handover needs the model files rather than just the report, for the reasons set out in the note on changing consultant mid-application. For review and sign-off of a draft before submission, the professional directory is the route to a reviewer with the right accreditation.

Draft an energy or sustainability statement on PlanScope, then have an accredited assessor check the modelling before it is submitted.

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. For specialist technical reports, qualified professional input may be required.