Each year, around 50,000 UK buildings are demolished, releasing large volumes of high-quality structural steel. Today, over 90% of this steel is recycled through energy-intensive re-melting, much of it overseas. Direct **reuse**---from buildins to buildings---can cut embodied carbon by ~90% compared with recycling and ~98% compared with "new" steel, while keeping value and emissions savings in the UK. **HARNESS** is a five-month demonstration project that makes steel reuse simpler, faster and more economical for the supply chain. The project brings together an award-winning UK tech SME (**Efestos Hub**), a leading steel contractor (**Caunton Engineering**) and a not-for-profit industry network (**Alliance for Sustainable Building Products, ASBP**). Working in a live fabrication environment, the team will validate a practical, end-to-end workflow that helps fabricators and designers identify, prepare and integrate reclaimed steel into real projects with greater certainty on cost, programme, and quality. HARNESS focuses on the real-world barriers that currently limit reuse: manual records and traceability, subjective inspections, late matching of available sections to design needs, and the loss of shorter lengths to scrap. By streamlining these steps into one coherent process, the project aims to increase reuse rates, reduce lead times and minimise waste. An independent whole-life carbon assessment will verify the environmental benefits against current practice. The demonstrator is expected to enable a 25--40% improvement in reuse rates and materially faster stock-to-design decision-making, contributing to an estimated ~630 tCO₂e reduction (versus using new steel) within the project. Activities are UK-based, supporting territorial emissions reductions and aligning with national resource-efficiency goals. The outcome will be a validated, replicable approach that helps fabricators, designers, contractors and developers adopt steel reuse with confidence. Findings will be shared with the industry through a stakeholder showcase and summary report to support broader market adoption. HARNESS directly supports the UK's shift to a circular, lower-carbon construction sector by turning steel reuse from ad-hoc into standard practice---delivering carbon savings, cost efficiency and stronger supply-chain resilience.