Insights
•Retrofit-first: the new commercial baseline
Over the last two years, ‘retrofit-first’ has evolved from an aspiration to being enshrined in planning policy. Across Westminster, the City of London and a growing list of boroughs, retrofit-first policy no longer sits alongside your development strategy; it is your development strategy, tested before your brief is even finalised¹.
The new baseline
Underpinning the new retrofit-first policies is a requirement to fully appraise the existing buildings, structures and materials already on your site. Knowing exactly what you have, what condition it’s in and what you can do with it, is essential to retain its value and maximise its potential.
At Heyne Tillett Steel, in-depth research and pre-redevelopment audits are nothing new, they are the starting point for every project. We shared this expertise with Westminster City Council as part of the design working group for PACER (Planning Application Compliance Evaluation and Readiness), their tool developed with prop-tech company Preoptima to help streamline the submission and review process of Whole Life Carbon Assessments (WLCAs) required under their new policy. Alongside this, we were one of the first consultants to complete the new planning process, including a pre-redevelopment audit and WLCA, for a live pilot project in Belgravia.
Since then, we’ve taken multiple projects through Westminster’s planning process on both sides of the 50% retention threshold, both full retrofits and schemes where the evidence supported a lower retention figure.
At 1 Victoria Street, we identified significant ongoing corrosion of the existing superstructure which meant that full demolition and replacement was the most practical and sustainable solution for the building’s future life. Despite being considered a new-build under the emerging policy, the team worked hard to retain the building’s 3-storey substructure, which accounts for 52% of the existing structure by volume, which will support a new 10-storey post-tensioned concrete frame with reconfigured cores and set-back terraces. This is the skill that the new policy requires: not keeping everything by default, but the evidence and expertise to know, and justify, what a site can support.
Traditional methods aren’t enough
In today’s retrofit-first planning context, the traditional options for refurbishing and upgrading buildings often don’t go far enough to make retention a viable option.
Compress and adapt: retains the most but leaves you fighting reduced ceiling heights and services that are hard to modify again later.
Strip back internally: frees up height and space for modern services but can sacrifice enough floor area to break the business case.
Demolish and rebuild: maximum design freedom but a much harder case to evidence under current policy, and a bigger carbon liability to explain to funders.
None of these conventional approaches reliably clears a 50%-plus retention benchmark at Grade A specification, so the engineering must become more inventive.
Where the real gains are being found
- Floor jacking: Ceiling height is usually the real obstacle to reusing an older frame – most were built well below what today’s occupiers expect. Rather than demolishing for clearance, existing slabs can be propped, cut free, jacked to a new level and secured in place – unlocking servicing capacity and clear height without losing the structure underneath.
- Low-impact underpinning: Deepening a basement or extending a footprint has traditionally meant heavy disruption. Newer methods deliver the same additional capacity with far less impact on the structure and ground around it – valuable on the tight, sensitive sites the new policy is most focused on.
- Element reuse at scale: Where full retention isn’t viable, demolition doesn’t have to mean waste. Concrete slabs, walls and columns can be cut down and reused elsewhere – as infill panels, lintels, partitions, stair treads; recovering value most people don’t associate with concrete.
Woolgate, City of London
One Hotel Mayfair, City of Westminster
Arthur Stanley House, London Borough of Camden
Our in-house research team, HTS+, has been testing exactly how far this can go, including researching routes to cast large blocks of reclaimed material into new primary structural concrete, not just cosmetic applications. Early findings suggest reused concrete meaningfully displaces virgin materials and avoids the associated embodied carbon and land use impact, turning what’s usually treated as demolition waste into a structural material in its own right.
“The question for most sites isn’t whether reuse is possible. It’s how much value gets left on the table if you don’t properly test it before deciding to demolish.”
Tom Watson, Director, Heyne Tillett Steel
Why this needs to be provable, not just plausible
It’s still early days for retrofit-first policy, and interpretation varies by borough. One thing the adopted retrofit-first policies have in common is promoting the uptake of a ‘reuse hierarchy’, requiring a pre-deconstruction audit and circular economy statement that outlines how materials from the existing building will be salvaged, reused and recycled.
We’ve spent several years turning reuse from a promising idea into a specified, procurable, insurable method, including co-authoring a new NBS C20 demolition specification that formally defines deconstruction, developed through our work as a founding member of the Engineers Reuse Collective2. Where new material can’t be avoided, we’re also part of the Accelerating Concrete Decarbonisation Group, working to de-risk low-carbon cement replacements for wider industry adoption3, because the goal isn’t reuse at any cost, it’s the lowest-impact answer for each specific site.
Diagram: Reuse hierarchy based on the Ellen Macarthur Foundation’s butterfly diagram
One team, one read of the site
Ground conditions, access strategy, structural capacity and planning evidence all form part of the same conversation, not four separate ones. It’s why we work as an integrated team of structural, civil, geotechnical and highways engineers under one roof to provide the right expertise, at the right moment, for every project.
Where to start
When assessing sites in Westminster, the City, or any retrofit-first borough, early structural analysis is essential for keeping your options open. A pre-redevelopment audit isn’t just a planning requirement to check off – done early and well, it’s the single best tool for telling you what a site can offer, before you’ve committed capital to a demolition strategy that might not survive committee.
- Westminster City Council formally adopted its new City Plan 2019-2040 on 21 January 2026. At the heart of the updated City Plan is the new Retrofit First policy (Policy 43), which requires developers to fully explore all reasonable options for retrofitting and extending existing buildings before seeking permission for demolition and redevelopment. Policy 43 requires a sequential test where the feasibility of different building options is established through test 1 (what the existing structure can accommodate) and test 2 (what the proposed use needs).
The City of London’s Sustainability SPD requires schemes to take a ‘retrofit-first’ approach, prioritising the retention and retrofit of existing buildings (at least 50% of the existing buildings’ superstructure by mass), informed by an appraisal of the development options. - Developed and peer-reviewed as part of ongoing work with the Engineers Reuse Collective, a not-for-profit group of practising engineers championing, accelerating and delivering reuse in the built environment. Heyne Tillett Steel is one of the six founding practices.
- The Accelerating Concrete Decarbonisation Group (ACDG) is a developer-led group of businesses and people within the built environment looking to drive down the carbon emissions of concrete through real-time prototyping and testing.
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