Part L 2021 notional building and what it means for design

The Part L 2021 notional building is the reason two houses of identical floor area can have different compliance targets, and the reason a design that looks efficient on paper can still fail. Part L does not set a fixed number of kilowatt hours per square metre. It builds a hypothetical dwelling with the same size, shape and orientation as yours, gives it a specified fabric and services package, calculates its performance, and makes that the target your actual design has to beat.

Understanding the notional specification tells you where the slack is in a design and where there is none. It also explains the recurring argument on housing schemes about whether to add panels or change the heating system.

What the notional building is and how the target is set

Volume 1 of Approved Document L, the 2021 edition, applies to new dwellings and sets three targets. The primary energy rate is the principal one, expressed in kilowatt hours of primary energy per square metre per year. Alongside it sit an emission rate in kilograms of carbon dioxide per square metre per year and a fabric energy efficiency rate in kilowatt hours per square metre per year.

All three are calculated by running the notional dwelling through the national calculation methodology, the Standard Assessment Procedure, and recording its results. Your design is then assessed by the same method and has to meet or better each target.

Because the notional dwelling copies your geometry, a deep plan with a small external wall area starts from an easier target than a detached house of the same floor area with four exposed elevations. That is the system working as intended, and it is also why benchmarking against another project's numbers tells you very little.

Fabric values in the notional specification

The notional dwelling's fabric specification is tighter than the limiting values Part L allows, which is the point. In broad terms it assumes well-insulated opaque elements, windows and glazed doors around 1.2 W/m²K with a moderate solar transmittance, a party wall treated as having no heat loss, modest thermal bridging, and an air permeability of 5 m³/(h·m²) at 50 Pa.

Set against that are the limiting fabric parameters, the worst values any element in a real design may have. Walls at 0.26, windows and doors at 1.6 and air permeability at 8 are the figures commonly quoted, and they function as a floor rather than a target.

Read the specification tables in the current edition rather than relying on a summary, including this one, because the values and the way they are expressed have changed between editions and will change again.

The practical reading is this. You can build worse than the notional fabric in one element and compensate elsewhere, but only down to the limiting value, and only if the fabric energy efficiency target still passes.

The three targets and what each one controls

They behave differently, and designers who treat them as one number get caught.

The fabric target is the one that cannot be bought. Photovoltaics do nothing for it. A heat pump does nothing for it. Only insulation, air tightness, reduced thermal bridging and sensible glazing move it, which is why fabric decisions have to be made before the services strategy rather than after.

Where a design diverges from the notional building

The notional specification for houses includes on-site generation sized in proportion to the ground floor area, and the equivalent specification for flats does not. That single difference explains most of what happens on mixed schemes.

For a house type, the target has already assumed panels. Omitting them from your design means recovering the shortfall through fabric, plant efficiency or a low-carbon heat source, and on a compact house type there is not much fabric headroom left. So the practical choice on houses is usually panels, a heat pump, or both.

For flats, the target assumes no generation but does assume heat recovery from waste water. A flat block therefore has a different balance, and the compliance route often runs through the heat source and the communal system efficiency rather than through roof area.

Shape matters too. A terrace end unit and a mid-terrace unit of the same size have different targets, different exposed areas, and different answers. Modelling a single representative plot and applying the result across a scheme is how compliance failures get discovered during construction.

Heat pumps against gas in the calculation

Under the current calculation methodology the primary energy and carbon factors for grid electricity have fallen a long way relative to natural gas. The arithmetic consequence is that an air source heat pump improves both the primary energy rate and the emission rate substantially compared with a gas boiler.

Three cautions follow.

A heat pump shifts the design problem to flow temperature and emitter sizing, which affects room layouts, and to peak electrical demand, which affects the incoming supply. The connection capacity question is a real programme risk, and the implications of peak electrical demand with all-electric heating should be checked with the distribution network operator early.

It also increases the risk of summer overheating in a well-sealed dwelling, which brings Part O into the picture and usually an Overheating Risk Assessment with it.

And it does nothing for the fabric target, so a scheme that leans on the heat pump and relaxes the fabric will fail on the third target while passing the first two.

Evidence the planning submission needs

Planning is not building control, and an Energy Statement at application stage is a commitment rather than a compliance certificate. What authorities ask for, in rough order of frequency, is the predicted performance against each Part L target for representative plot types, the percentage improvement over the notional baseline, the specification assumed to achieve it, and the on-site generation capacity with its location shown on a roof plan.

Keep the planning commitment and the construction specification aligned. An energy statement promising an air permeability figure that the contractor has not priced is the origin of most late variations, and the way air tightness targets claimed at planning stage are tested later is worth understanding before the number goes in the report.

An illustrative example

A scheme of 24 houses and a 30 flat block models four plot types. The three-bedroom semi passes the primary energy and emission targets with a heat pump and no panels, but fails the fabric target because the design assumed 0.22 W/m²K walls. Improving the wall to 0.18 and reducing the thermal bridging allowance brings the fabric result inside the target without changing the services. This example is illustrative rather than a real assessment.

Transitional arrangements and when they bite

The 2021 edition applied from 15 June 2022, with transitional provisions for plots where the relevant building control notice had been given before that date and work on the individual dwelling started within a year. Those provisions have long since run out for new projects, but they still matter on long-running outline permissions where reserved matters are coming forward now.

The Future Homes Standard has been announced as the successor to Part L 2021 for new homes, with higher fabric and low-carbon heat expectations. As at October 2026 the commencement date and the transitional arrangements are the critical detail for anyone programming a scheme, so check the current position on gov.uk rather than relying on a date quoted in an older report.

This is general guidance, not legal advice. Check the position with your local planning authority, your building control body or a qualified professional before relying on it.

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