1st August 2025

Reuse vs new build: A structural engineer’s guide to carbon-conscious construction

By Monika Rogers, Associate and Net Zero Carbon Lead

The built environment contributes roughly 40% of global carbon emissions[1], so the choice between reusing existing structures and building new in the current climate crisis is more critical than ever.

Structural engineers play a pivotal role in the transition towards more sustainable designs and low carbon structures. Their expertise in structural systems, material selection, subsequent design optimisation, and innovative solutions can play a crucial role in reducing embodied carbon. Through collaborating with other consultants, we can also influence designs related to operational carbon. This means deciding whether to retrofit an existing structure or build a new is one of the most significant choices in sustainable construction.

Understanding the carbon impact of construction choices
When a project’s environmental footprint is evaluated, we must consider both types of emissions. Embodied carbon is often ‘front-loaded’ by materials like steel and concrete, that can account for up to 50% of a building’s lifecycle emissions[2]. This upfront impact is crucial, especially given the urgent need to reduce emissions in the coming decades to meet climate targets of 2030 and 2050.

New construction typically generates substantial upfront carbon through raw material extractions and manufacturing of products, however transportation, construction processes and construction waste play significant roles too. In contrast, reusing existing structures significantly cuts these initial high upfront emissions and the already locked-in carbon is repurposed. Despite these easy embodied carbon wins, retrofit projects still require careful considerations so that the structural performance is not compromised and high carbon interventions are minimised.

As architect Carl Elefante famously said, “the greenest building is… one that is already built.” By adapting and retaining existing structures, we preserve the embodied carbon already ‘invested’ in the building, aligning perfectly with circular economy principles. At Perega, we have been working on existing buildings for decades, giving us a wealth of experience and deep expertise across the company. Our structural assessments of existing buildings cover those from early 20^th century and beyond to modern reinforced concrete frames and steel sheds. On many occasions we have worked closely with our clients on office-to-residential conversions and vice versa, recognising great opportunities in embodied carbon savings. Similarly, we believe that vertical extensions within our portfolio have helped us to enrich our holistic approach to reuse existing buildings. We challenge, where feasible, the current generous live loads within the codes and look for ample opportunities in adaptation of existing structures.

Technical challenges and opportunities
Retrofitting existing buildings presents unique challenges that often require experience but also innovative engineering solutions. This includes assessing structural capacity of different building elements with limited original documentation, adapting structures designed to old codes to meet contemporary building codes and performance standards, and integrating modern services and sustainability features. We tackle these with advanced analysis and creative design solutions.

New construction, however, offers different opportunities for sustainability. This involves optimising structural forms for material reduction or material efficiency, and implementing the latest innovations in sustainable materials and technologies.  It’s important to actively explore low-carbon alternatives during early stages of design, starting with the structural framing and extending to considerations like low-carbon concrete mixes, mass timber solutions, recycled steel and aluminium, and hybrid structural systems. New buildings can also benefit from being explicitly designed for adaptability, disassembly, and material reuse at end of life, representing a fundamental shift toward circularity in structural engineering. As a profession we have capability to provide expertise and work closely with all stakeholders to deliver more sustainable structures.

A balanced approach to sustainable construction
The reuse versus new build decision is rarely straightforward for investors, but structural engineers are well-equipped to consider the pros and cons, not only in terms of embodied carbon but also in relation to wider impacts. Often, the most sustainable solution combines elements of both, retaining valuable structural elements while replacing others that are no longer suitable. This selective approach maximises carbon savings without compromising on the client’s functional needs. To make informed decisions, it’s important to understand an asset’s embodied carbon value and the material saving opportunities, while conducting whole life carbon assessments to compare different design scenarios that account for both embodied and operational carbon across the building’s lifespan. The greatest opportunities for carbon reduction occur in the earliest project stages, highlighting the importance of engaging structural engineers from project inception.

One significant challenge in advancing sustainable construction is the perception of increased upfront costs. However, this perspective often overlooks the bigger picture: reducing material quantities lowers costs, smart use of MMC minimises waste and shortens on-site programmes, well-coordinated and collaborative design delivers long-term operational savings and early engagement with contractors provides more opportunities for carbon savings. These approaches collectively enhance asset value as they future-proof, reduce obsolescence risk, and grow market demand for sustainable buildings. Ultimately, these benefits provide a far greater return on investment in the long run.

Achieving a net-zero built environment requires collaboration across the entire construction sector with strong support from investors and clients. As engineers, we have a responsibility to advocate for practising the hierarchy of net zero design which puts at its core; retention and reuse, circular design, lean design, innovation in low-carbon design, sharing knowledge, and challenging conventional approaches when they conflict with sustainability goals.

The path forward
The choice between reuse and new build is a strategic one with significant environmental implications. It’s important that we leverage our expertise and recognise our responsibility to guide this decision, balancing immediate needs with long-term sustainability goals. By thoroughly understanding each project’s context and applying rigorous carbon assessment, we can identify the optimal path forward to a more sustainably-built future.

[1] https://assets.publishing.service.gov.uk/media/631222898fa8f54234c6a508/20220901-Carbon-Net-Zero-Guidance-Note.pdf

[2] https://circularecology.com/embodied-carbon-footprint-database.html