The unseen carbon cost of late design changes

There is a particular moment in almost every project when someone says, “It’s only a small change.” A wall shifts half a metre. A grid tightens. A plant room grows. A façade is reconfigured to sharpen the elevation. On paper, it looks harmless – maybe, a few amended drawings, a revised model, a fresh issue to the team. However, what rarely enters the conversation at that point is carbon, writes John Ridgeway.

The construction industry has become fluent in the language of embodied carbon, whole-life assessments and net zero pathways. We can calculate the emissions associated with concrete mixes, steel sections and façade systems with increasing accuracy. Yet there is a quieter source of carbon cost that seldom appears in sustainability statements - late design change.

Redraws feel digital and therefore immaterial. They take place on screens, in coordination meetings and in cloud-based models. But they have physical consequences. When a design changes after structural elements are sized, orders are placed or fabrication slots are booked, the ripple effects extend far beyond the drawing board.

A modest structural adjustment can require heavier sections, additional bracing or deeper foundations. A revised façade grid can alter panel dimensions, leading to waste in materials already procured. A last-minute services reconfiguration may require larger risers, additional plant capacity or secondary steel to carry new loads. None of these changes announces itself as a carbon event. They are presented as technical refinements or aesthetic improvements. Yet each one carries embodied emissions that were not part of the original calculation.

The earlier a building’s key decisions are made, the more opportunity there is to optimise. Structural spans can be rationalised. Material quantities can be reduced. Components can be standardised. When those decisions are revisited late in the programme, optimisation tends to give way to accommodation. The priority shifts from minimising material to avoiding delay. Steelwork is upsized “to be safe”. Concrete depths increase to absorb revised loads. Plant space expands because there is no time left to reconfigure the strategy properly. However, time pressure is rarely neutral in carbon terms.

What about work lost?

There is also the question of abortive work. Drawings that are superseded do not emit carbon in themselves, but the activities they trigger often do. Fabrication may already have begun. Orders may have been placed for materials cut to specific dimensions. In some cases, elements are manufactured, delivered and then modified or replaced. Even when materials are recycled, the energy used in their initial production and transport cannot be reclaimed.

These inefficiencies are seldom visible in headline carbon reporting. Whole-life carbon assessments typically measure the emissions associated with the final built outcome. They do not always capture the iterations that occurred along the way - the steel that was fabricated twice, the concrete poured thicker than necessary because the alternative required more design time than the programme allowed.

Late design changes also disrupt one of the most effective tools for reducing embodied carbon - repetition. When structural grids are stable and façade modules consistent, manufacturers can optimise production. Transport loads are rationalised. Installation becomes predictable. Change that interrupts repetition reintroduces variability and variability tends to generate waste.

This is particularly acute in projects that rely on prefabrication or systematised approaches. Factory-based production thrives on early certainty. Components are engineered, tested and scheduled with precision. When dimensions or specifications shift late, the knock-on effects can be significant. Production slots may be lost. Elements may need redesign. Replacement components may require additional raw material because standard sizes no longer apply.

Again, none of this is dramatic enough to make headlines. It happens quietly, absorbed into contingency sums and revised programmes. But from a carbon perspective, it erodes gains that may have taken months to achieve.

There is also a cultural dimension to this problem. In many projects, sustainability is discussed enthusiastically during concept design, then gradually competes with cost, aesthetics and programme as pressures mount. When compromises are required, carbon reductions achieved early can be diluted in incremental steps. A slightly thicker slab here, a more robust section there. Each adjustment feels marginal. Collectively, they accumulate.

The difficulty lies in the fact that buildings are complex and change is inevitable. Clients refine briefs. Tenants request adjustments. Regulations evolve. Unforeseen site conditions emerge. The goal is not to freeze design at an artificially early stage, but to recognise that stability itself has environmental value.

A carbon strategy

Good coordination is not simply a matter of efficiency - it is a carbon strategy. When architects, engineers and contractors resolve structural logic, services distribution and façade systems early and collectively, they reduce the likelihood of disruptive change later. Clear spatial allowances for plant and risers prevent reactive enlargements. Rational grids limit structural recalculation. Early engagement with suppliers highlights dimensional constraints before drawings are fixed.

There is also an argument for designing with tolerance rather than precision to the millimetre. Buildings that rely on tightly optimised dimensions can be vulnerable to late shifts. A modest allowance in floor-to-floor heights or service zones can absorb change without triggering wholesale redesign. This approach may appear less materially efficient at first glance, but it can prevent more carbon-intensive alterations later.

Another overlooked factor is decision latency. Projects often reach critical junctures without firm agreement on key parameters. Structural engineers may size members based on provisional loads. Façade consultants may develop systems without confirmed interfaces. When final decisions arrive, they necessitate rework. The carbon cost of indecision is rarely acknowledged, yet it can be substantial.

Procurement routes further influence this dynamic. Traditional linear models, where design is substantially complete before contractor involvement, can separate carbon ambition from buildability insight. Contractors and manufacturers frequently identify rationalisation opportunities that reduce material use, but if they are brought in late, those opportunities shrink. Conversely, when delivery teams are engaged early, design development can proceed with fabrication realities in mind, reducing the risk of abortive effort.

Revision made easy

Digital modelling has improved our ability to visualise and coordinate, but it has also made revision deceptively easy. A line can be moved in seconds. A grid adjusted with a few keystrokes. The friction that once accompanied manual redrafting has largely disappeared. Yet the physical consequences of those digital shifts remain stubbornly real. There is an uncomfortable truth in this, where the most sustainable drawing is often the one that does not need to be changed.

This does not mean resisting improvement. Iteration is fundamental to good design. But there is a difference between purposeful development early in a project and reactive alteration late in the programme. The former refines performance - the latter often protects deadlines.

If the industry is serious about reducing embodied carbon, it may need to treat design stability as a measurable objective. Tracking the carbon implications of major late-stage changes could illuminate impacts that are currently hidden. Clients might begin to ask not only how much carbon a building contains at completion, but how much was expended in getting there.

Ultimately, sustainability is not only about materials. It is about decisions. Every redesign carries consequences beyond professional fees and programme adjustments. Steel does not forget that it was fabricated twice. Concrete does not regain the emissions released in its first pour.

The carbon cost of late design change is rarely visible, rarely dramatic and rarely discussed. But it is there, embedded in the quiet inefficiencies that accompany rework and revision. In a sector striving to align intention with impact, recognising that reality may be one of the most practical steps forward.