Mixed use energy statements: two compliance routes

A mixed use scheme sits under both volumes of Approved Document L, so a single energy statement has to carry two separate sets of figures that are never added together. Volume 1 covers dwellings and is modelled in SAP. Volume 2 covers buildings other than dwellings and is modelled in SBEM or a dynamic simulation tool. The baselines differ, the target metrics differ, and the percentage improvement against each baseline has to be reported on its own terms.

Every avoidable problem in a mixed use energy statement comes from blending the two. A combined carbon saving figure, a single improvement percentage over a notional building that does not exist, or a shared plant allocation that quietly credits the residential element with savings earned by the commercial one.

Why two compliance routes apply

The split is a Building Regulations split, not a planning one. A block with flats above a ground-floor retail unit contains two regulated building types. The flats are assessed individually and collectively under Volume 1 against a target emission rate, a target primary energy rate and a target fabric energy efficiency rate, with minimum backstops for fabric and services. The retail unit is assessed under Volume 2 against a building emission rate and a primary energy target derived from a notional building of the same geometry.

Both volumes of the 2021 edition took effect in June 2022 and set a stepped improvement over the previous edition, with different step sizes for dwellings and for other buildings. Check the current edition and any transitional arrangements before fixing the modelling approach, since the Future Homes Standard and the equivalent non-domestic uplift will change both baselines.

What this means for a planning submission is simple to state and often ignored. The energy statement reports two compliance cases, each against its own baseline, and the planning policy percentage improvement is applied to each separately.

Residential and non-residential targets side by side

Present the two cases in parallel so that a reviewer can read across without reconstructing your arithmetic.

London schemes have a further layer. The London Plan sets a net zero-carbon expectation for major development through its energy policy, applies the Be Lean, Be Clean, Be Green, Be Seen hierarchy, and requires a minimum on-site reduction beyond the Building Regulations baseline with a cash in lieu contribution for the shortfall. The percentage and the baseline year are set in the GLA's own energy assessment guidance, which has been revised since Part L 2021, so verify the current requirement and apply it to each use separately rather than to a blended total.

Shared plant and how it gets apportioned

This is where mixed use energy statements get challenged. A single air source heat pump array, a shared substation, a communal hot water plant or a photovoltaic array on one roof serving two building types all need an apportionment that is stated, consistent and defensible.

Three rules keep it honest.

First, apportion by demand rather than by floor area where the demands differ sharply. A ground-floor restaurant with a large hot water load and a floor of flats above it do not share plant in proportion to their square metres. Where demand data is not yet available, apportion by floor area and say so as an assumption.

Second, allocate the output of a renewable system once. Photovoltaic generation credited to the residential emission rate cannot also appear in the non-domestic calculation. Where a single array serves both, split the installed capacity explicitly, label which panels are allocated where, and carry that split through to the drawings.

Third, keep the apportionment the same in the energy statement, the carbon calculation and the metering strategy. A reviewer who finds the array split 70:30 in one document and 50:50 in another will ask for both to be redone.

Where the plant competes for roof area with other requirements, the trade-offs are set out in roof space competition between plant, solar panels and green roofs. And where the strategy is all-electric, the diversified peak demand across two very different load profiles is worth checking early, as described in peak electrical demand and all-electric heating.

Metering strategy and what policy expects

Mixed use schemes need a metering strategy on the drawings, not a paragraph promising one later. Volume 2 of Approved Document L already requires sub-metering for non-domestic buildings above a size threshold, broken down so that energy use can be attributed to end uses. Residential units are metered individually as a matter of course.

The gaps that cause arguments are the shared ones. Landlord supplies for lifts, corridor lighting, car park ventilation and communal plant need their own meters, and the boundary between landlord and tenant supply needs to be fixed before the electrical design is frozen. Where a heat network serves both uses, heat meters at each interface unit and at the plant are needed to apportion cost and to produce the data any monitoring obligation will demand later.

Draw a metering schematic showing every meter, what it measures, who owns it and where the data goes. That one drawing answers more consultee questions than several pages of text.

One strategy, two sets of figures

The structure that reads well in a mixed use energy statement keeps the narrative single and the numbers separate.

1. Scheme description with the floor areas of each use class, stated in m2 and as a percentage of the total.
2. The energy hierarchy applied once to the whole scheme as a design narrative, since fabric decisions, orientation and plant selection are shared.
3. Be Lean results for each element separately, against its own baseline.
4. Be Clean, covering heat network connection or the absence of one, with the connection decision evidenced.
5. Be Green, with renewable capacity and its apportionment between elements.
6. Residual emissions for each element, and the offset or contribution calculation where local policy requires one.
7. Be Seen, setting out the monitoring and reporting commitment.

Keep the summary table on one page. The planning officer, the sustainability officer and the committee will read that page and nothing else, so it needs the two improvement percentages, the two baselines and the residual figures clearly labelled by use.

Overheating across different uses

Approved Document O applies to new residential buildings and does not cover offices, shops or most commercial floorspace, where overheating is assessed against CIBSE criteria instead. So a single building can contain flats tested under Part O and a ground-floor unit tested under a different method entirely.

Two interactions matter in a mixed use block. Heat rejection from commercial plant can raise the local air temperature at residential window openings and at ventilation intakes, which Part O's simplified method does not capture and a dynamic model does. And the acoustic constraint on opening windows above a busy frontage, often the reason flats fail the simplified method, is worse where a servicing bay or plant enclosure sits below.

Model the residential element dynamically where either of those applies, and state the external conditions assumed. An overheating risk assessment that ignores the commercial plant on the same roof will not survive a technical review.

Conditions that cover both elements

Conditions drafted for a single-use scheme cause trouble on a mixed use one. Watch for three patterns.

A condition requiring the development to achieve a stated percentage reduction, without saying which baseline or which element, is ambiguous and should be refined at drafting stage rather than at discharge. Ask for wording that names each element and its baseline.

A condition requiring the renewable installation to be in place before first occupation can be unworkable where the commercial shell is let later than the flats. Phased wording tied to occupation of each element is the fix, and it is easier to negotiate before the decision notice is issued.

A condition requiring post-construction verification will need the as-built models for both routes, which means retaining the modeller through to completion. Budget for that at the start, because the alternative is a condition that cannot be discharged and a sale that cannot complete.

Where the scheme is large enough to need a whole life carbon assessment, the same apportionment logic applies to the embodied carbon of shared structure and plant.

This is general guidance, not legal advice. Energy and overheating modelling should be carried out by a suitably qualified assessor, and councils usually expect a named author for this work. Check the current Approved Documents and any local energy policy before fixing the strategy.

To see which energy and sustainability documents a mixed use scheme is likely to need, enter your postcode on PlanScope and work through the guided questions. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.