Energy statements where a local plan sets a percentage reduction

Where a local plan policy requires new homes to achieve a percentage reduction in carbon emissions beyond the Building Regulations, the compliance question turns on two definitions before it turns on any technology. Which emissions the percentage applies to, and which edition of the Building Regulations sets the baseline. Get either wrong and a self-builder can spend money on measures that do not count towards the target at all.

The reduction almost always applies to regulated emissions against the target emission rate for the relevant edition of Part L. Not to total energy use, not to the building's actual bills, and not to the emissions of the household living there.

Finding the policy and its baseline

Start with the adopted local plan rather than with a search result. The policy is usually in the climate change or sustainable design chapter, and it will state the percentage, the development types it applies to and the baseline.

Three things to extract and write down.

The percentage and what it applies to. Some policies apply to all new dwellings, some only to schemes above a threshold such as ten units, some only to major development. A single self-build plot may sit outside the policy entirely.

The baseline edition. A policy adopted before the 2021 uplift to Part L will often refer to an earlier edition, and the council may have published a position statement explaining how it applies the policy against the current edition. Ask the planning policy team directly, in writing, and keep the answer.

Whether the reduction must be achieved on site. Some policies allow a shortfall to be met through a carbon offset payment. Others do not.

Check for supplementary planning documents too. Design guides and sustainable construction guidance often carry the detail on how the council expects the calculation to be presented, and they are material considerations in the decision.

Regulated against unregulated emissions

This is the distinction that decides what counts.

Regulated emissions come from fixed building services: space heating, hot water, ventilation, pumps, fans and fixed lighting. These are what the Part L calculation covers, and they are what the percentage is measured against.

Unregulated emissions come from everything else the occupant plugs in: cooking, appliances, televisions, computers. They are not part of the Part L compliance calculation and, in most local policies, not part of the target either.

The practical consequence is that a self-builder who buys efficient appliances has done something worthwhile that does not appear in the energy statement. And a large glazed living space that increases heat demand does appear, whatever the appliances are.

Some policies now ask for a space heating demand figure in kilowatt hours per square metre per year alongside the carbon reduction, which is a fabric measure and is harder to meet with technology. Read the policy carefully to see which measure you are being held to.

What counts towards the reduction

The calculation runs on the approved software for the dwelling type, producing a target emission rate for the notional building and a dwelling emission rate for yours. The reduction is the gap between them.

Measures that move the number, roughly in order of reliability on a small scheme:

• Fabric improvements: wall, roof and floor insulation beyond the notional specification, better windows, reduced thermal bridging using calculated junction details rather than default values.
• Airtightness better than the limiting value, with a pressure test to prove it.
• Heat pump in place of a gas boiler, which changes the fuel carbon factor applied to the heat demand.
• Mechanical ventilation with heat recovery, where the airtightness justifies it.
• Photovoltaics, which reduce the dwelling emission rate directly.
• Waste water heat recovery, which is low cost and appears in the calculation.

Measures that do not count include efficient appliances, low energy behaviour, a wood burning stove used as a secondary heat source in most cases, and anything installed after the calculation was submitted unless the calculation is revised.

Orientation and form matter more than the list suggests. A compact house with sensible glazing distribution starts several percentage points ahead of a sprawling one with the same specification.

Proving the figure with a calculation output

Councils want the calculation output, not a statement that the target has been met.

Submit the full output from the approved software showing the target emission rate, the dwelling emission rate, the percentage improvement, the fabric parameters used and the services specified. Include the software name and version. Where the design has changed since an earlier run, submit the current run and say which drawing revision it matches.

Two common validation failures on small schemes. An output produced for a different plot or a different house type, which happens on multi-plot self-build sites. And an output that assumes an airtightness figure far better than anything the builder will actually achieve, which surfaces at the as-built test and leaves a condition undischargeable.

The energy statement should carry the calculation as an appendix and explain in the body which measures produce the reduction and in what proportion. A short table works better than prose here.

Where a small scheme meets the target by fabric alone

On a well-formed house, a fabric-first route can reach a meaningful reduction without any renewable technology, and it is usually the most durable answer because there is nothing to maintain or replace.

The measures that do the work are unglamorous. Insulation thicknesses a step beyond the notional dwelling. Triple glazing or high performance double glazing with a low whole-window heat loss value. Calculated thermal bridging details rather than the default allowance, which on a small dwelling can be worth more than people expect because the ratio of junction length to floor area is high. And an airtightness target the builder commits to, with a test.

Where the target is higher, a heat pump usually becomes necessary, and the decision has consequences for the hot water strategy, the emitter sizing and the space required. It also changes the overheating position, so run the Part O check on the same design rather than afterwards, using the overheating risk assessment to confirm it.

Where the local plan target is high and the plot is constrained, say so in the statement and set out what is achievable and why. A reasoned case for a lower on-site reduction with an offset contribution is better received than an implausible specification.

Conditions the council will attach

Expect a condition requiring the development to be built in accordance with the approved energy statement, and often a second requiring evidence of compliance before occupation.

That evidence normally means the as-built calculation and the pressure test result, submitted and approved. Some councils also ask for photographs of insulation and junction detailing during construction, or a commissioning certificate for the heat pump and ventilation system.

Plan for it. Tell the builder at tender stage that an as-built calculation and a pressure test are required, because retrofitting airtightness after plastering is expensive and difficult.

Keeping the as-built position honest

The gap between the design calculation and the finished house is the real risk, and on a self-build it falls on you rather than on a developer.

Three habits close it. Specify the actual products in the calculation rather than generic performance values, so the builder is pricing what was modelled. Inspect the insulation and airtightness detailing at the points where they are usually compromised, which are service penetrations, the wall to roof junction and around window reveals. And run an interim air test before the finishes go on, when faults can still be found and sealed.

If a change becomes necessary during construction, revise the calculation and tell the council before completion rather than after. A condition discharged on an honest revised figure is straightforward. A condition discharged on a figure nobody can reproduce is not.

A sustainability statement may also be required alongside the energy work where the policy covers water use, materials or overheating, and the two should be drafted together so the assumptions match.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.