The notional building and fabric energy efficiency explained
Part L targets come from a notional dwelling with your geometry and a fixed specification. Why renewables can offset two of the three targets but never the fabric one.
The notional building in Part L is not a real house and it is not a specification you have to copy. It is a calculation device: a dwelling of exactly the same size, shape and orientation as yours, given a fixed fabric and services specification set out in Approved Document L Volume 1. Run that imaginary dwelling through the approved methodology and it produces the three targets your actual design has to beat. Get this straight early and the rest of Part L stops feeling arbitrary.
Three rates come out of it. A target emission rate in kilograms of carbon dioxide per square metre per year, a target primary energy rate in kilowatt hours of primary energy per square metre per year, and a target fabric energy efficiency rate in kilowatt hours per square metre per year. Your design has to meet all three, and the fabric one is the awkward member of the family.
What the notional building is
Take the drawings of the proposed dwelling. Keep the floor area, the storey heights, the shape, the orientation and the total area of glazing. Now strip out the actual construction and services and substitute the specification given in the Approved Document.
That substituted dwelling is the notional building. The calculation is run on it, and the answers become the targets. Because it takes its geometry from your design, a compact terraced house and a sprawling single-storey barn-shaped dwelling get different targets, which is the point. The standard adjusts for form rather than punishing it outright.
What it does not adjust for is bad form factor relative to its own assumptions. A design with a high ratio of external envelope to floor area is harder to pass, because heat loss scales with envelope while the target scales with floor area.
The specification it assumes
The 2021 notional dwelling is broadly a well-insulated, airtight, gas-heated house. In outline it assumes external wall, floor and roof U-values in the region of 0.18, 0.13 and 0.11 watts per square metre kelvin, windows around 1.2, air permeability of 5 cubic metres per hour per square metre at 50 pascals, waste water heat recovery on showers, and, for houses rather than flats, an allowance of photovoltaic panels.
Check the exact figures in the tables of Approved Document L Volume 1 before you use them, because the notional specification changes with each edition and the version in force is the one that counts.
Two design consequences follow immediately. First, if your fabric is worse than the notional fabric, something else has to make up the difference. Second, the notional includes renewables for houses, so a design with no renewables starts behind.
How the fabric target works alongside the emissions targets
The fabric energy efficiency rate measures space heating and cooling demand only. No boiler, no heat pump, no solar panels, no grid carbon factor. It is a pure measure of how much energy the building needs to stay comfortable.
That isolation is deliberate. Before the fabric target existed, a developer could offset thin insulation with a large array on the roof and still pass on carbon. The fabric rate closes that route. You can trade services and renewables against the emission and primary energy targets. You cannot trade them against the fabric target.
So the sequence for a self-build design is fabric first, then services, then renewables. Reversing that order produces a design that passes on carbon, fails on fabric, and has to be reworked at the point where the windows are already ordered.
| Target | What it measures | Can renewables help |
|---|---|---|
| Target emission rate | Carbon dioxide emissions per square metre per year | Yes |
| Target primary energy rate | Primary energy demand per square metre per year | Yes |
| Target fabric energy efficiency rate | Space heating and cooling demand from fabric alone | No |
Trade-offs the method allows and forbids
Within the fabric calculation you can still trade between elements. A better roof can compensate for a slightly worse wall, provided every element stays inside the limiting fabric parameters, which are the worst values the Approved Document will accept regardless of how good the rest of the design is.
The limiting parameters are looser than the notional values, and that gap is where most confusion lives. Meeting the limiting U-value for a wall does not mean the wall is compliant. It means the wall is not automatically non-compliant. The dwelling still has to beat the targets overall.
Thermal bridging is the trade most often missed. The calculation uses linear thermal transmittance values at junctions, and using default values rather than calculated or accredited details imposes a penalty that can be worth more than an upgrade to the wall build-up. On a self-build with unusual junctions, getting the details assessed is often the cheapest route to compliance.
Air permeability is the other lever, and it is the one that depends on site workmanship rather than on the specification. A design that relies on 3 cubic metres per hour per square metre and achieves 6 on test will fail, and the remedy at that stage is expensive.
Where a self-build design typically falls short
Four patterns recur.
- Large areas of glazing on north and east elevations, which lose heat without the compensating solar gain that south glazing brings.
- Complex roof geometry with many junctions, dormers and valleys, which raises both envelope area and thermal bridging.
- Single-storey or part-single-storey forms with a high ratio of roof and floor to internal volume.
- A late decision to swap the heating system, which moves the emission and primary energy results but leaves the fabric result untouched.
The overheating check under Approved Document O sits alongside all of this and pulls in the opposite direction on glazing. An Overheating Risk Assessment prepared at design stage will often be the thing that decides window sizes on a south elevation, so run it before the fabric calculation is finalised rather than after.
Where a ground source system is proposed, the borehole consenting position needs checking early as well, and our note on ground source heat pumps and borehole permitting covers the abstraction and ground investigation points.
Evidence to keep for building control
Compliance is demonstrated twice: at design stage and as built. Keep the design stage calculation output, the as-built calculation, the air pressure test certificate, commissioning records for the heating and ventilation systems, and the photographic evidence required by the standard reporting format that accompanies the calculation.
The photographic requirement catches people out on self-builds where the builder is also the client. Insulation continuity at wall plate, at the floor perimeter and around openings has to be photographed as it is installed, and a wall that is already plastered cannot be evidenced retrospectively. Brief whoever is on site in week one.
Keep the file. It is the evidence a buyer's surveyor will ask for, and it is what a warranty provider will want.
Explaining the figures to a planning officer
Planning and building control are separate, and a case officer is not assessing Part L compliance. What they are assessing is any local plan policy on energy, and whether an Energy Statement demonstrates the reduction the policy requires.
Write it so that the baseline is unambiguous. State the edition of the Approved Document, state that the baseline is the notional dwelling for that edition, and give the percentage improvement for each of the three rates separately rather than quoting one headline figure. Officers have seen too many statements that quote an improvement without saying what it is measured against.
Where the policy asks for a percentage improvement over Part L, check the current national position on local energy standards before you design to it. Government has intervened in this area more than once, and the weight of a local policy depends on the position as at the date of your submission. A Sustainability Statement is often the right home for the wider commitments that sit beyond energy.
Illustrative example: a four-bedroom self-build with 0.15 walls, 0.10 roof and triple glazing passes fabric energy efficiency comfortably, then fails primary energy because the heating system is direct electric. Swapping to an air source heat pump fixes both the primary energy and the emission results without touching the fabric figures, which is exactly what the separation of targets is designed to show.
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. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Enter your postcode on PlanScope to see which energy and sustainability reports your self-build is likely to need.
Frequently asked questions
What is the notional building in Part L?
A calculation device, not a specification to copy. It is a dwelling with the same size, shape, orientation and glazing area as your design, given the fabric and services specification set out in Approved Document L. Running it through the approved methodology produces the three target rates your actual dwelling has to beat.
Can solar panels make up for poor insulation?
Only for two of the three targets. Renewables and efficient services can offset a weaker fabric against the target emission rate and the target primary energy rate. They cannot help with the target fabric energy efficiency rate, which measures space heating and cooling demand from the fabric alone and deliberately excludes services and generation.
What is the difference between the notional values and the limiting fabric parameters?
The notional values are the assumptions behind your targets. The limiting fabric parameters are the worst U-values the Approved Document will accept for any element, whatever the rest of the design does. Meeting a limiting value does not make an element compliant, it only stops it being automatically non-compliant.
Why does my single-storey design struggle to pass?
Because heat loss scales with the area of the external envelope while the target scales with floor area. A single-storey or complex form has more roof, floor and junction length per square metre of living space than a compact two-storey box, so it needs better fabric and better detailing to reach the same result.
What evidence does building control want at completion?
The design stage and as-built calculation outputs, the air pressure test certificate, commissioning records for heating and ventilation, and photographic evidence of insulation continuity taken as the work proceeds. The photographs cannot be produced retrospectively once surfaces are closed up, so brief whoever is on site before work starts.
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.