There is a version of the reuse argument that gets made in front of clients and does the cause active harm. It goes: keeping the building is cheaper, because the structure is already there.
Sometimes it is. Often it is not, and when a cost plan comes back proving otherwise, the whole case for retention loses credibility along with the false claim. The defensible argument is different and much stronger, but it requires giving up the easy one.
Where reuse genuinely costs more
Retention introduces cost in places new build does not have.
Survey and unknowns. You are designing against an existing condition that is only partly documented. Opening up is expensive, disruptive, and never finished until demolition starts. Every project of this type carries a contingency that a new build does not need, and the contingency is real money whether or not it is spent.
Sequencing. You cannot work in parallel the way you can on a clear site. Temporary works, propping, and phased access add weeks, and weeks add preliminaries.
Performance upgrades. Bringing a solid-wall building to a current energy standard means internal insulation, which means condensation risk analysis, which means either a vapour control layer detailed properly or a slow-moving moisture problem. This is not optional and it is not cheap.
Compliance. Fire strategy, accessibility, and means of escape in an existing plan will require compromises that cost either money or floor area, sometimes both.
Anyone arguing for retention should be able to list these before the cost consultant does.
Where it genuinely costs less
Against that, the savings are concentrated and can be large.
Substructure and frame are the obvious ones: a retained structure removes demolition, disposal, excavation, and a new frame, which on a mid-rise building is a substantial share of the build cost. Programme can be shorter where the existing envelope allows early weather-tightness. And on constrained urban sites, retention often avoids the party wall, underpinning, and neighbour negotiation that a new basement would trigger — a cost that is difficult to forecast and occasionally ruinous.
The variance is the point. Reuse is not systematically cheaper or dearer; it is more sensitive to the specific building. Which is why the honest position is that cost is a project-by-project question and carbon is not.
The carbon case does not wobble
Embodied carbon is where retention wins consistently, and by margins large enough to survive methodological arguments.
Roughly half the lifetime emissions of a new building are spent before anyone occupies it, in materials and construction. A retained frame and substructure preserves the majority of that investment. Studies comparing retrofit against equivalent new build on the same site routinely find retention saving 50 to 70 per cent of upfront emissions, and no serious study finds new build ahead once the existing structure is sound.
The operational counter-argument — that a new building performs better in use — is weaker than it sounds. A well-retrofitted building can reach comparable operational performance, and the payback period on the demolition carbon is measured in decades, frequently longer than the intended life of the replacement.
The line worth using “The greenest building is the one that is already built” is a slogan. The number behind it is that you cannot operate your way out of the emissions you spend on day one.
How to make the case properly
Lead with carbon, because that is where the evidence is unambiguous. Present cost honestly, including the contingency, because a cost plan that hides the unknowns will be found out at tender. And be specific about what retention delivers that new build cannot: the floor-to-floor heights, the structural grid, the window pattern, the address, the fact that it is standing now and a new building is four years away.
That last one is undersold. A retained building can be occupied in phases. On a mixed-use scheme, income starting two years earlier changes the financial model more than most of the construction line items being argued over.
Reuse is a good idea with real costs. It does not need to be sold as a bargain, and it stops being believed when it is.
