Notional building comparison in a Part L submission
The notional building in Approved Document L generates three separate targets per plot. Fabric energy efficiency is the one renewables cannot fix, and primary energy is where all-electric designs run tight.
A dwelling does not comply with Part L by beating one number. The notional building comparison that sits at the heart of Approved Document L Volume 1 (2021 edition) generates three separate targets for your plot, and the design has to beat all three: the target emission rate, the target primary energy rate and the target fabric energy efficiency rate. Pass two and fail one and the SAP output says fail. Architects who have only ever looked at the carbon figure are the ones caught out, usually by primary energy on an all-electric house or by fabric efficiency on a heavily glazed one.
The comparison is per plot, not per scheme. A terrace of eight gets eight sets of targets, because each notional dwelling takes the real dwelling's size, shape and orientation.
What the notional building is
The notional dwelling is a modelled twin of the house you have drawn. Same floor area, same shape, same orientation, same number of storeys, same ratio of glazing area in the version used for the target calculation. What changes is the specification: fabric U-values, thermal bridging, air permeability, services efficiency and the renewable contribution are all replaced with the values tabulated in Approved Document L Volume 1. Run SAP on that twin and the outputs become your three targets.
Read the current published Approved Document for the actual notional values rather than carrying them in your head. They are specific, they differ between new dwellings and work to existing dwellings, and they changed materially between the 2013 and 2021 editions.
Two features of the 2021 notional dwelling drive most design conversations. It includes a renewable contribution on houses, which is why so many house types fail primary energy with no photovoltaic array on the roof. And its fabric and airtightness assumptions are tight enough that a design relying on plant efficiency alone has very little headroom.
The three targets a dwelling must meet
| Target | What it measures | What it responds to |
|---|---|---|
| Target emission rate | Regulated carbon dioxide emissions per square metre per year | Fuel choice, plant efficiency, renewables |
| Target primary energy rate | Primary energy demand per square metre per year | Fabric, plant efficiency, renewables, and the primary energy factor of the fuel |
| Target fabric energy efficiency | Space heating and cooling demand per square metre per year | Fabric, thermal bridging, airtightness, glazing area and orientation only |
The fabric target is the one that cannot be bought back. Space heating and cooling demand is calculated from the building envelope, so photovoltaics, a better boiler or a heat pump make no difference to it at all. If a design fails fabric efficiency, the fix is in the envelope or in the glazing area, and nowhere else.
Alongside the three rates sit minimum standards for individual elements and for services, and the separate overheating requirement under Approved Document O. A design can satisfy all three target rates and still fail Part O on a south-facing single-aspect flat, which is why the two assessments belong in the same conversation rather than in sequence.
Reading the comparison table in a SAP output
Every SAP compliance output presents the same three pairs: dwelling emission rate against target emission rate, dwelling primary energy rate against target primary energy rate, dwelling fabric energy efficiency against target fabric energy efficiency. Each pair carries a percentage, and the percentage is where misreadings start.
A 20 per cent improvement on emissions and a 2 per cent improvement on primary energy is a compliant dwelling with no margin. Build it, miss the air test, and you have a non-compliance that needs a redesign at plot level rather than a quick swap of a boiler. Assessors usually ask for a margin of several per cent on each of the three before they will sign a design off for construction, and that is a judgement call rather than a rule.
Look also at the as-designed assumptions behind the numbers. Air permeability entered at a value the site has never achieved, thermal bridging taken as accredited details rather than calculated psi values, and photovoltaic output modelled with no overshading are the three inputs that collapse at the as-built stage.
Where a design fails one target and passes another
The characteristic all-electric failure is primary energy. SAP 10 brought the carbon factor for grid electricity down sharply to reflect decarbonisation of generation, while the primary energy factor for electricity remains above one. A heat pump house therefore looks comfortable against the emission target and much tighter against the primary energy target, especially where the heat pump's seasonal efficiency has been assumed optimistically rather than taken from a product record.
The characteristic architectural failure is fabric energy efficiency. Large glazed openings, roof lights, and three-sided bay projections all raise space heating demand, and no amount of plant will recover it. Where a planning-stage design has been driven by elevational quality, this is the target that forces the conversation about glazing area back open after permission has been granted.
The third pattern is a design that passes everything on paper with a gas boiler and fails when the scheme is switched to electric heating late in the day for policy reasons. The three targets do not move together, and the comparison has to be re-run in full.
Design responses ranked by cost
- Reduce thermal bridging with calculated junction details rather than default accredited values. The lowest cost point on the list and often worth more than any single product change.
- Improve airtightness through the detailing and the site process, then enter the value you can actually test to.
- Reduce glazing area, or redistribute it, where the fabric target is the binding constraint.
- Improve the glazing specification and the frame, which helps fabric and primary energy together.
- Add waste water heat recovery, a low-cost measure that reads well on both energy targets.
- Add or extend the photovoltaic array, which does nothing for the fabric target.
- Change the heating system, which is the largest intervention and the one most likely to unsettle the other two results.
That order is a rule of thumb for a typical two-storey house and it inverts on flats, where glazing area per unit is smaller and the heating system dominates.
What to submit at planning against building control
The three target rates are a building regulations matter, assessed by a SAP assessor and signed off through building control. Planning asks a different question. Local plan policies commonly require a percentage reduction against a Part L baseline, sometimes with an energy hierarchy applied in order, and that case is made in an Energy Statement submitted with the application rather than in the SAP output itself.
The two documents must agree. Name the edition of Approved Document L used as the baseline, state whether the percentage is measured on emissions or on primary energy, and set out the assumptions you have modelled. Where wider policy asks for carbon, water and materials commitments together, those belong in the Sustainability Statement alongside it.
Roof strategy is where planning and Part L meet most visibly. An array sized to close a primary energy gap competes with green roof and drainage requirements, and a biosolar roof is often the only way to satisfy both without losing area.
Keeping the energy statement consistent with the SAP
Conditions requiring post-construction evidence are now routine, and they are discharged against the figures in your planning submission. Three habits prevent the awkward discharge meeting.
State the baseline edition and the metric in the energy statement, in one sentence, in plain terms. Keep a copy of the design-stage SAP outputs for each plot type with the statement, so the comparison can be reproduced later by someone else. And flag in the statement where a figure depends on a product selection that has not been made, instead of presenting an assumption as a commitment.
Where a draft statement is produced from structured inputs, the sensible discipline is the same one a good assessor applies: name the edition, show the baseline, and leave anything unverified visibly marked rather than quietly filled in. A draft prepared that way is a starting point for the assessor and the design team, not a replacement for either.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. If you want to see which supporting documents a scheme is likely to need, enter your postcode on PlanScope.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What is the notional building in Part L?
It is a modelled twin of your dwelling with the same size, shape and orientation but a specification fixed by Approved Document L Volume 1. Running SAP on that twin produces the three targets your real design has to beat. Because the twin takes its geometry from your plot, every house type in a scheme has its own set of targets.
Which of the three Part L targets is hardest to meet?
It depends on the design. Heavily glazed houses tend to fail the fabric energy efficiency target, which measures space heating and cooling demand and cannot be improved by renewables or better plant. All-electric designs more often run tight on primary energy, because the primary energy factor for grid electricity remains above one.
Can photovoltaics fix a fabric energy efficiency failure?
No. The fabric target is calculated from the building envelope, covering U-values, thermal bridging, airtightness, glazing area and orientation. An array on the roof changes the emission and primary energy results but leaves fabric efficiency untouched. The remedy is in the envelope: better detailing, better glazing or less of it.
Do I submit SAP calculations with a planning application?
Usually not as the main document. The three target rates are assessed through building control. Planning policies that ask for a percentage reduction against a Part L baseline are answered in an energy statement, which should name the edition used as the baseline and say whether the percentage is measured on carbon or on primary energy.
How much margin should a design hold against each target?
There is no figure in the Approved Document, so this is a judgement for the assessor. Most will want several per cent of headroom on each of the three rates before signing off for construction, because airtightness, overshading of an array and thermal bridging assumptions all tend to worsen between design and completion.
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.