Energy statement for a conversion where the fabric is fixed

A conversion energy statement has to answer a policy written for new build using a building you cannot substantially change. The walls are where they are, the windows are the size the elevation allows, the roof faces the way it faces, and on a listed or locally distinctive building half the obvious measures are unavailable. Officers know this. What they will not accept is a document that quietly applies the new build template and reports a shortfall without explaining what was tested.

The workable approach is to be explicit about the constraint, then show the search for performance within it.

How energy policy applies to an existing building

Two regimes run in parallel and they are frequently confused.

The Building Regulations treat a conversion as a material change of use, which brings Part L into play for the elements affected, with the standard set by what is technically, functionally and economically feasible. That test has real content: it lets a designer decline a measure that would fail in that building, provided the reasoning is recorded.

Planning policy runs separately. Local plans commonly require a percentage reduction in carbon emissions against the Building Regulations baseline, and some require a carbon offset payment for the residual. Most of those policies were drafted with new construction in mind, and many include qualifying words such as "where feasible" that do the same work as the Building Regulations feasibility test.

Read the policy wording closely before drafting. The difference between a policy that requires a reduction and one that requires a reduction where practicable is the whole argument.

For conversions under a prior approval route such as commercial to residential, remember that the prior approval matters are limited and energy is generally not among them, while the Building Regulations and the space standard still apply. That narrows what an energy statement is for, and it is still worth preparing where the local plan or a condition asks for one.

Establishing the baseline honestly

The baseline is where most conversion energy statements go wrong, because the existing building's real performance is unknown and the temptation is to assume a default.

Gather what evidence exists. An existing EPC gives assumed rather than measured values and should be treated as a starting point. Construction drawings, where they survive, tell you the wall build-up. Opening up during strip-out gives actual cavity condition and insulation presence. Thermal imaging in cold weather shows cold bridging and missing insulation, and it is inexpensive on a small building.

Where a value has to be assumed, say so and say which direction the assumption errs in. A statement that assumes an uninsulated solid wall and then discovers a filled cavity has overstated the improvement it claims. Reviewers in London and in authorities with carbon offset funds check this, because the baseline drives the offset payment.

Be similarly careful with air permeability. An old building with a new internal lining can perform very differently from either the old or the new assumption, and the as-built test will settle it.

Measures available without touching the envelope

Work through the fabric options in order and record why each was adopted or rejected.

• Roof and loft insulation, usually the highest return and the least disruptive.
• Floor insulation where floors are being lifted anyway.
• Internal wall insulation, which costs floor area and brings condensation risk at junctions that has to be designed out rather than assumed away.
• Draught sealing and airtightness improvements at junctions, service penetrations and around openings.
• Secondary glazing, which often outperforms replacement in heritage buildings and avoids consent issues.
• Window and door replacement where the existing units are not of interest and the elevation allows it.

Internal insulation on a historic solid wall needs particular care, because a vapour-closed system on a breathable wall causes damage that appears years later. Historic England publishes technical advice on this and it should be cited where the building is listed or in a conservation area. External insulation is usually unavailable on a building of any character, for the reasons set out in external wall insulation in a conservation area.

Services, controls and metering

When the envelope is fixed, services carry the load.

A heat pump is often the largest single improvement available in a conversion, and the questions are practical: is there space for the unit and the cylinder, can the emitters run at a lower flow temperature, will the external unit be acceptable in the setting, and does the electrical supply have capacity. Each of those is answerable at design stage and expensive to discover later. The wider reasoning about leaving gas behind is covered in the all-electric decision on gas connections.

Mechanical ventilation with heat recovery is worth assessing in a tightened building, particularly where windows cannot be enlarged and airtightness has improved sharply.

Controls, zoning, hot water distribution losses, pipework insulation and LED lighting are unglamorous and they add up. Metering matters for a different reason: it lets a landlord demonstrate performance after occupation, which is the only way to answer a claim that the conversion underperforms.

Renewables on a constrained roof

Roof area is usually the binding constraint, followed by orientation and shading from surrounding buildings.

Model the actual available area rather than the plan area of the roof. Deduct the north-facing slopes, the areas shaded by neighbouring buildings in winter, the space needed for access and for existing plant, and the structural limits of an older roof that was never designed for a distributed load. What remains is often a fraction of what a policy calculation assumes.

Where the building is listed or in a conservation area, visibility of the panels from the street governs. A rear slope array invisible from the street is a different application from a front elevation one, and the heritage case has to be made on its own terms rather than on carbon grounds alone.

Justifying partial compliance

A statement that reports a shortfall and stops will be refused or conditioned. A statement that documents the search will usually be accepted.

Set out the policy target, the measures considered, the reason each was adopted or rejected with the evidence behind it, the performance achieved, the residual gap, and what is proposed for that gap. Where the authority operates a carbon offset fund, calculate the payment and say you accept it. Where viability is the reason a measure was rejected, provide figures rather than an assertion, because an unevidenced viability claim tends to attract more scrutiny than the measure would have cost.

A Sustainability Statement alongside the Energy Statement lets the wider case, on water, materials and overheating, carry some of the weight where the carbon number is uncomfortable.

Conditions and post-completion evidence

Expect a condition requiring the approved measures to be installed and evidence submitted after completion: as-built energy calculations, the new EPC, an air permeability test result, and photographs of insulation and services before they are covered.

Take those photographs during the build. Reopening a completed conversion to prove what is behind the plasterboard is the sort of task that makes a condition impossible to discharge.

This is general guidance, not legal advice; check the position with your local planning authority 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.