Energy modelling assumptions officers challenge
Energy statements are challenged on their inputs, not their headline percentage. Here are the assumptions officers query, from baseline edition and heat pump flow temperature to PV capacity and air permeability.
Energy statement assumptions decide the answer before any modelling happens. Change the assumed heat pump flow temperature from 55°C to 45°C and the carbon saving moves by several percentage points without a single design change. Officers and their consultants know this, which is why the questions that come back are almost never about the headline percentage and almost always about the inputs behind it.
The fix is unglamorous. Write the assumptions down, say where each one came from, and make sure the model and the drawings describe the same building.
Baseline: say what you are measuring against
The first line of any energy statement should state the regulatory baseline by name and edition. Approved Document L 2021 for England took effect on 15 June 2022, and it introduced targets for primary energy and carbon dioxide emissions alongside the fabric energy efficiency target. A statement that says "a 35 per cent improvement over Building Regulations" without saying which edition is unusable, because the same design produces very different numbers against the 2013 and 2021 baselines.
Say which calculation tool and version produced the figures. The Standard Assessment Procedure for dwellings, the Simplified Building Energy Model for everything else, and the version number in each case. Where a local policy sets a percentage target, state whether that target is expressed against the old or the current baseline, because several adopted plans still carry percentages written for the 2013 Regulations. The mismatch between plan-era percentages and current baselines is the single most common source of argument, and it is covered in more depth in the note on on-site carbon reduction targets in local plan policy.
In London, Policy SI 2 of the London Plan requires major development to be net zero-carbon, with a minimum on-site improvement and a cash-in-lieu contribution for the shortfall, assessed through the Be Lean, Be Clean, Be Green, Be Seen hierarchy. The Greater London Authority's energy assessment guidance sets the current baseline and percentages, and it has been revised since Part L 2021 took effect, so use the version in force rather than one from a previous scheme.
Finally, note the Future Homes Standard. It is expected to replace the 2021 uplift and will change both the baseline and the acceptable heating technologies. Check its commencement and transitional arrangements before fixing a design, rather than assuming today's targets will still apply at the point of building control submission.
Seasonal performance assumptions for heat pumps
Heat pump performance is where reviewers look first, and the two numbers that matter are the seasonal performance figure and the design flow temperature that produced it.
A claimed seasonal coefficient of performance has to come from the Product Characteristics Database entry for the specific unit, tested at the flow temperature the system will actually run at. A unit quoted at 35°C flow and installed on a system designed around 55°C radiators will not deliver the modelled figure. If the database has no entry, the calculation falls back on default values, and those defaults are deliberately conservative.
Three things should appear in the statement. The make and model, or a stated performance specification the contractor must meet. The design flow temperature, cross-referenced to the emitter schedule. And the assumed hot water strategy, because a heat pump raising cylinder temperatures for legionella control carries an immersion contribution that is easy to leave out.
Where the design is not fixed, model the performance you can defend and write a condition-friendly commitment to meet it. That is more persuasive than an optimistic figure with no source.
Glazing ratios and orientation in the model
Glazing area, orientation, g-value and frame factor all feed the result, and all four are checkable against the drawings.
A south-facing glazing ratio that helps the carbon calculation usually hurts the overheating one. Approved Document O applies to new residential buildings, and a scheme that passes Part L by way of solar gain and fails Part O by the same route has simply moved the problem. Run the two together from the outset. Where the simplified method in Part O will not work, dynamic thermal modelling is needed, and the scope of that work sits with the Overheating Risk Assessment rather than the energy statement.
State the assumed overshading. A default of average or heavy overshading on a site with open southern aspect will be spotted by anyone who opens the site plan.
Occupancy and hot water demand
For dwellings, occupancy is not a free variable. The Standard Assessment Procedure derives assumed occupancy from the total floor area, and hot water demand follows from it. An assessor who has manually adjusted occupancy has moved outside the methodology, and the statement should say why if that has happened.
For non-domestic buildings the activity database drives occupancy, operating hours and internal gains by space type. The judgement is in how spaces are classified. A flexible ground floor modelled as storage rather than retail will produce a different answer, and officers ask about it when the use class in the application description says something else.
Match the modelled schedule to the description of development. If the application says the building operates seven days a week, the model should not assume weekday occupancy only.
Renewable output and its basis
Photovoltaic output claims need four supporting facts: installed capacity in kWp, panel orientation, tilt, and the overshading category. A layout drawing showing the array on the roof plan, at the same panel count used in the model, settles most queries in one page.
Common problems are easy to avoid. An array that covers a roof plane already occupied by rooflights or plant. A capacity that assumes both roof slopes when only one faces within sensible bounds of south. Output figures quoted at standard test conditions rather than the modelled annual yield. Where an air source unit and an array both appear, be clear about which savings sit at the Be Clean stage and which at Be Green, because double counting between stages is a standard review finding.
Making the model match the drawings
| Assumption | Where a reviewer checks it | Frequent mismatch |
|---|---|---|
| Floor and envelope areas | Measured from the GA drawings | Model built from an earlier revision |
| U-values | Construction build-up notes | Values that need an insulation thickness the detail cannot hold |
| Thermal bridging | Junction schedule or default y-value | Accredited details claimed with no junction drawings |
| Air permeability | Design air test target | A tight figure with no mechanical ventilation strategy to suit it |
| Glazing areas and g-value | Window schedule and elevations | Glazing added at planning stage but not in the model |
| PV capacity | Roof plan | Panel count differs between drawing and model |
Air permeability deserves its own sentence. A design target below about 3 m³/(h·m²) at 50 Pa generally needs continuous mechanical ventilation with heat recovery to keep indoor air quality acceptable, so a tight assumed figure with natural ventilation on the drawings is a flag rather than a saving.
Presenting assumptions so they can be checked
Put every assumption in one table near the front of the Energy Statement, with a column for the value, a column for the source, and a column for the drawing or schedule it can be verified against. Reviewers who can find the inputs in ninety seconds ask fewer questions.
Add the output files as appendices. The full calculation output, the input summary and the specification sheets for any plant relied on. Where a figure is provisional, label it provisional and say what will fix it, rather than presenting a placeholder as a result.
One honest caveat is worth including. Planning-stage energy modelling is done on an incomplete design, and the building control submission will differ. Saying that openly, and setting out which assumptions are fixed commitments and which are indicative, tends to earn more trust than a statement that implies precision it cannot have. It also makes the eventual as-built comparison far less painful, particularly where the same evidence base feeds an EPC-linked planning requirement.
To see which supporting documents a scheme is likely to need alongside the energy work, enter your postcode on PlanScope and answer the guided questions.
This is general guidance, not legal advice, so 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
Which Part L baseline should an energy statement use?
Approved Document L 2021 for England, which took effect on 15 June 2022, unless transitional arrangements keep an earlier edition in play. State the edition explicitly, because a percentage improvement means nothing without it. Where a local plan policy was written against the 2013 Regulations, explain how the target has been translated to the current baseline.
Where should heat pump performance figures come from?
From the Product Characteristics Database entry for the specific unit, at the design flow temperature the system will run at. A figure quoted at 35°C flow does not apply to a system designed around 55°C emitters. Where no database entry exists, the calculation uses default values, which are deliberately conservative and should be stated as such.
Why do officers ask about glazing ratios?
Because solar gain cuts both ways. Glazing that improves the carbon result under Part L can push the same dwelling into failure under Approved Document O on overheating. Reviewers check the modelled glazing areas, orientation, g-value and overshading category against the window schedule and elevations, and a mismatch undermines both assessments at once.
Can I change the assumed occupancy in a SAP calculation?
Not freely. The Standard Assessment Procedure derives assumed occupancy from total floor area, and hot water demand follows from that figure. If an assessor has departed from the methodology, the statement has to explain why. For non-domestic buildings the activity database drives occupancy and hours, and the judgement lies in how each space type is classified.
What evidence supports a photovoltaic output figure?
Installed capacity in kWp, panel orientation, tilt and overshading category, supported by a roof layout drawing showing the same panel count used in the model. Check that the array does not clash with rooflights or plant, and make clear which savings belong to the Be Clean stage and which to Be Green so nothing is counted twice.
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.