The energy hierarchy applied to an existing building

A retrofit energy statement written to the new build template reads badly, because the hierarchy it follows assumes choices a refurbishment does not have. You cannot set the orientation, you cannot choose the form factor, and the wall build-up is whatever the Victorians left. The sequence still holds as a principle, demand reduction before efficient supply before renewable generation, but the constraints bite at every step and the statement has to explain them rather than pretend they are not there.

Start by describing the building you actually have, in enough detail that the reviewer can see why the obvious measure is unavailable.

Why a new build hierarchy fits a retrofit badly

On a cleared site, the fabric target is a design decision. On an existing building it is a negotiation with the structure, the floor area, the windows, the heritage significance and the occupied use.

The practical consequence is that the energy statement's first section should not be a table of target U-values. It should be a constraints analysis: what the walls are made of, what the floor construction is, where the thermal bridges are, what the window openings can accept, what plant space exists and what the roof can carry.

Only then does the hierarchy make sense, because each step can be assessed against what is possible rather than against what would be possible on a different building. A reviewer who can see the constraint accepts a lower fabric outcome. One who is handed a generic table assumes the design team has not looked.

Fabric measures constrained by the existing structure

Insulation is limited by where it can go and what it displaces.

External wall insulation changes the appearance of the building, which in a conservation area or on a building of any quality is a planning question before it is a technical one. It also needs a detail at the eaves, at the reveals, at the plinth and at the party wall, and the junctions are where the performance is lost.

Internal insulation takes floor area, which on a small flat can be material, and it moves the dew point inwards. It also creates bridging at every intermediate floor and internal partition unless returns are detailed, and those returns are disruptive in an occupied building.

Windows sit between the two. Slim double glazing, secondary glazing and draught sealing each do different things at different cost and different visual impact, and secondary glazing is frequently the better answer on a listed building because it is reversible and it also deals with noise.

Set out what was considered and rejected, with the reason. That analysis is the part a planning officer can actually weigh.

Ventilation and moisture risk in solid wall buildings

This is the section most retrofit statements skip and the one that causes the most expensive failures.

A solid wall building works by absorbing and releasing moisture. Add impermeable insulation and reduce air leakage, and the moisture has nowhere to go, which produces interstitial condensation, mould and in the worst cases decay in embedded timbers. The response is a vapour-open build-up, careful detailing at junctions and embedded timber ends, and a designed ventilation strategy rather than reliance on leakage.

Say in the statement which moisture risk assessment method was used and what it showed. Say what ventilation is proposed, whether mechanical or natural, and how it is controlled. And where the building is traditionally constructed, refer to the technical guidance on energy efficiency in traditional buildings rather than to new build standards.

There is a planning consequence too. Ventilation terminals, external grilles and any mechanical plant have an appearance, and on a principal elevation they need locating on the drawings rather than left to installation.

Where renewables are limited by roof and plant space

Renewable capacity on a refurbishment is usually set by the roof, not by the demand.

A roof already carrying a heavy covering may not take an array without strengthening. A flat roof with existing plant, lift overrun and access zones has less usable area than its footprint suggests. A pitched roof on a listed building may be visible from every approach. And a heat pump needs a location with airflow, acoustic separation from windows and a route for pipework, none of which are easy in a tight urban plot.

Where capacity is limited, say what the limit is and quantify it rather than asserting that renewables are not feasible. An officer will accept "the usable roof area after plant and access zones is limited, giving the capacity set out below" and will not accept a bare statement that there is no room.

Connect the numbers to the Energy Statement calculation and keep them consistent with the Sustainability Statement, because the two are read together.

Comparing retrofit against demolition and rebuild

Where the alternative to refurbishment is demolition, the carbon comparison is now a planning question in its own right, and in London a whole life carbon assessment is required for referable applications under the greenhouse gas policy of the London Plan.

The comparison has a shape. Set out the upfront embodied carbon of the new building, the embodied carbon of the retrofit option, the operational carbon of each over a defined study period, and the carbon released by demolition including the loss of what is already in the ground. Then state the study period, the assumptions about grid decarbonisation and the method used, because the answer is sensitive to all three.

Retention usually wins on upfront carbon and often loses on operational carbon against a very efficient new building, and the crossover point depends on assumptions the assessor chooses. An honest assessment states where the crossover falls and how sensitive it is. A Whole Life Carbon Assessment is professional recommended work, and the recognised professional methodology should be named in the report.

A Pre-redevelopment Audit covering what can be reused or recovered strengthens the retention case and is increasingly expected on larger schemes.

Evidencing the carbon case for retention

The evidence pack that persuades has four parts: a condition survey showing the building can be retained, a structural appraisal showing what the frame will take, the carbon comparison with its assumptions stated, and a costed options appraisal.

Leave out the fourth and the case looks theoretical. Include it and be candid about where retention costs more, because a planning authority weighing retention is entitled to know what it is asking for.

One honest uncertainty deserves naming. Whole life carbon results depend heavily on assumed grid emission factors over the study period, and different published trajectories give materially different answers. State which you used.

Conditions on phased improvement

Where a full retrofit cannot be done in one go, usually because the building is occupied or the funding is staged, a phased improvement plan secured by condition is a workable outcome.

Two principles make phasing acceptable. Sequence fabric before services, so that the plant installed later is sized for the improved building rather than the original one. And avoid lock-in: do not carry out works in phase one that make a later measure impossible, such as a wall finish that prevents future insulation or a boiler replacement in the same position as the eventual heat pump.

Write the phasing into the submission as a schedule with trigger points, and offer it. A condition drafted by the applicant, tied to defined works and dates, is almost always better than one drafted at the end of a committee meeting.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval, and for whole life carbon and moisture risk work qualified professional input may be required. This is general guidance rather than legal advice, so check the applicable policy with your local planning authority before relying on it.