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The HYLO building in Islington saved a reported 35% of upfront embodied carbon by retaining the reinforced-concrete frame and foundations of a vacant 1960s office tower, then building on top of them. The project did not achieve that result by preserving a façade alone: engineers investigated the original structure, redirected loads, limited strengthening, reused the basement and foundations, and added 13 new storeys with a lightweight steel-and-composite system.
That 35% figure comes from New London Architecture’s project profile and is a comparison with equivalent new construction, not a universal saving for every retrofit. The published sources do not provide the calculation workbook, assessment boundary or independent audit, so the percentage should be read alongside the project’s separate tonnage and lease-period estimates.
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What is the HYLO building?
HYLO is the redevelopment and vertical extension of the former Finsbury Tower at 105 Bunhill Row, Islington, London. The original 1960s office building had become outdated and vacant. Rather than demolishing it, the project team retained its structural frame, basement and foundations, enlarged the podiums and added 13 storeys to the existing 16-storey tower.
The scheme increased leasable area from 12,000 to 25,800 square metres, according to New London Architecture’s project profile. Its profile describes the overall development as doubling the site’s leasable area while keeping the original structure in service.
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| Project fact | Reported detail |
|---|---|
| Location | 105 Bunhill Row, Islington, London |
| Original building | 1960s, 16-storey office tower |
| Vertical extension | 13 additional storeys |
| Structure retained | Reinforced-concrete frame, basement and foundations |
| Leasable area | 12,000 m² increased to 25,800 m² |
| Headline carbon comparison | 35% less upfront embodied carbon than equivalent new construction, as reported by New London Architecture |
How did retaining the old structure save carbon?
Demolishing a tower and constructing an equivalent replacement would have required new concrete, reinforcement, excavation, foundations and a new frame. HYLO avoided much of that initial material demand by keeping the load-bearing elements already in the ground and in the tower.
The retained frame is the central carbon strategy. Concrete and steel carry substantial upfront emissions because of extraction, manufacture, transport and construction. Reusing those elements avoids the emissions associated with making an equivalent quantity of new structural material, although it does not make the extension carbon-free: new steel, slabs, finishes, services and construction activities still have impacts.
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Keeping the basement and foundations also reduced the need for new excavation and substructure work. That can avoid additional concrete and reinforcement, soil removal, vehicle movements and temporary works. The sources describe this as part of the structural-reuse approach, rather than as a separately measured percentage saving.
The project listing’s 35% comparison therefore represents a whole-scheme decision: preserve the existing structure, add capacity where it is needed and avoid the material-heavy parts of a complete replacement. It is not a claim that every retained component delivered the same percentage reduction.
How was the existing frame shown to be capable of supporting 13 more floors?
The design team began with evidence about what was already there. They reviewed the building’s history and records, including when it was built and the codes used at the time, then checked the structure through analysis and back-analysis. Albert Williamson-Taylor, design director at AKT II, explained that a historical check gives engineers “a very good idea of what kind of substructure it has got” in a report by Building.
Back-analysis uses the observed building, available drawings and measured or inferred loads to understand how the existing members actually work. For a retrofit, that evidence can be more useful than assuming every element has only the conservative capacity implied by incomplete records. It also helps identify where additional loads can travel and where strengthening is unavoidable.
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At HYLO, the team redirected loads so that strengthening was limited to essential locations. The Tyréns AKT project account describes the retained frame, basement and foundations, together with the analysis that enabled the extension. This was a design exercise in finding usable capacity, not simply placing a new tower on top of an old one.
The approach still required detailed surveys, verification of materials and connections, checks of load paths and coordination with the architectural and services design. Existing drawings can be incomplete, and a structure that appears robust may contain alterations or deterioration not shown in the record. The carbon benefit depends on doing that investigative work early enough to guide the scheme.
What structural system did the extension use?
The 13-storey addition used lightweight steel with composite slabs on metal decking. A lighter superstructure reduces the extra gravity load imposed on the retained columns, beams, foundations and basement. It can therefore make vertical extension feasible with less strengthening than a heavier concrete addition would require.
Lightweight does not mean impact-free. The new steel, decking, concrete in the composite slabs, connections, façades and internal fit-out all contribute to embodied carbon. The advantage is that the new work was sized around an existing structural asset rather than replacing the entire asset and its substructure.
The podiums were expanded as part of the wider redevelopment. That allowed the scheme to provide more floor area and supporting accommodation while preserving the tower’s primary frame. The Institution of Structural Engineers’ 2022 HYLO award profile records the retained structure and added storeys; its judges called the result “a quite brilliant example of structural retrofit” and said it “could only have been achieved through masterful structural engineering.”
What does the reported 35% carbon saving actually mean?
New London Architecture reports a 35% saving in upfront embodied carbon compared with equivalent new construction. Upfront embodied carbon generally concerns emissions released before a building is occupied, but the HYLO listing does not publish the full calculation method, life-cycle modules, baseline specification or assessment boundaries. It is therefore important not to present the figure as a verified whole-life-carbon result.
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The comparison is also conditional on “equivalent” new construction. The result depends on what replacement scheme is assumed: its floor area, structural grid, materials, demolition treatment, construction method and specification. Changing those assumptions can change the percentage. The defensible statement is that the project listing reports 35% less upfront embodied carbon for HYLO’s retained-and-extended approach than for the equivalent new-build case used by the project.
That distinction matters when comparing projects. A percentage from one development cannot be transferred directly to another building with a different frame, condition, height, floor area or carbon baseline. The boundary and assumptions should accompany the number whenever it is quoted.
How do the project’s tonnage estimates differ from the 35% figure?
The HYLO brochure presents additional carbon figures, but they measure different things:
- More than 2,500 tonnes of CO2: the brochure attributes this saving to designing around the original superstructure.
- 500 to 750 tonnes of CO2: the brochure estimates this amount would have been wasted if the building had been demolished and rebuilt.
- Around 25% over a typical 10-year lease: the brochure estimates lower carbon than leasing a new building for that period.
These are project claims from the HYLO brochure, not alternate ways of expressing the 35% result. The tonnage estimates should not be added together, converted into a percentage without the underlying baseline, or described as upfront embodied carbon unless the relevant boundary is documented. The 10-year lease comparison is a separate operational or leasing-period narrative, not another embodied-carbon calculation.
Why is reuse usually preferable to demolition and replacement?
Reuse preserves the carbon already invested in a building and avoids emissions from producing a replacement structure. It can also reduce demolition waste, excavation and the temporary works associated with a new basement. Those advantages are strongest when the existing frame and foundations have enough capacity, can meet current requirements after adaptation and can support the desired floor area.
Reuse is not automatically the lower-carbon choice. A severely deteriorated or poorly adaptable structure may require extensive strengthening, complex temporary works or inefficient new elements. Operational energy, fire performance, accessibility, resilience, façade condition and the service life of the retained materials also affect a whole-life decision. HYLO’s reported saving is evidence for this particular retained-and-extended scheme, not a rule that every old tower should be kept.
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What process can other projects learn from HYLO?
Owners considering a vertical extension can use HYLO as a sequence of decisions rather than as a material prescription.
- Recover the building’s history. Gather original drawings, construction records, alteration details and the codes in force when the structure was designed. Missing information should be identified before the concept is fixed.
- Survey and investigate the existing asset. Confirm geometry, materials, reinforcement, connections, deterioration and previous modifications. The aim is to replace assumptions with evidence.
- Model actual load paths. Use structural analysis and back-analysis to understand how the frame, basement and foundations behave under existing and proposed loads. Test where loads can be redirected and where strengthening is essential.
- Set a lightweight extension brief. Compare structural systems by added load as well as cost, programme and carbon. HYLO’s steel-and-composite approach limited the burden placed on the retained structure.
- Reuse the substructure where justified. Retaining foundations and basement construction can avoid new excavation and large quantities of concrete, but only after capacity and condition are demonstrated.
- Define the carbon baseline and boundary. State whether the comparison covers upfront embodied carbon, demolition, construction, use, a lease period or another scope. Keep percentage, tonnage and lease-period results separate.
- Check the design against current requirements. A retained frame must satisfy the regulations and performance criteria that apply to the new project, not merely the standards used in the 1960s.
- Document uncertainty. Publish the assumptions, data sources and exclusions behind any carbon number so that clients and reviewers can understand what is—and is not—being compared.
This workflow is why early structural investigation has a carbon value. It can prevent a project from discarding a usable frame before anyone has quantified the consequences of demolition and replacement.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What are the main risks in a retained-frame extension?
The first risk is incomplete information. If records do not match the constructed building, design teams may need additional surveys, opening-up or conservative assumptions, all of which can affect programme and cost. The second is hidden condition: corrosion, cracking, water ingress or undocumented alterations can reduce available capacity.
Load transfer is another constraint. Adding floors changes vertical loads and can affect stability, foundations and the interfaces between old and new construction. A lighter steel extension can reduce those effects, but it still needs a complete load-path assessment and carefully designed connections.
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What is the clearest takeaway from HYLO?
HYLO’s carbon saving came from treating the existing building as a structural resource. Historical investigation and back-analysis allowed the team to retain the 1960s frame, basement and foundations; load redirection and limited strengthening made room for 13 additional storeys; and a lightweight steel-and-composite extension supplied new area without rebuilding the entire tower.
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New London Architecture’s reported 35% reduction is therefore best understood as a project-specific upfront embodied-carbon comparison with an equivalent new build. The project’s brochure adds larger tonnage and lease-period estimates, but those figures have different scopes. The transferable lesson is the method: investigate first, design around verified capacity, and disclose exactly what each carbon comparison includes.
Frequently Asked Questions
Does HYLO’s 35% figure include operational energy over the building’s life?
The published 35% statement is specifically an upfront embodied-carbon comparison. The brochure’s estimate of about 25% lower carbon over a typical 10-year lease is a separate comparison and should not be treated as an operational-energy result or merged with the embodied figure.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesWhy is the saving reported as a percentage rather than one definitive tonnage?
A percentage expresses the difference against the project’s equivalent-new-build baseline, while a tonnage depends on the assumed floor area, materials and assessment boundary. HYLO’s brochure supplies tonnage estimates, but they describe different scopes and cannot be converted reliably without the underlying calculation.
Did HYLO avoid using new concrete altogether?
No. The retained frame and foundations avoided replacement structural concrete, but the extension used composite slabs on metal decking and the redevelopment required new construction materials. The carbon advantage came from avoiding a complete replacement structure, not from eliminating concrete from the project.
What information should a client request before approving a retained-frame scheme?
Request the existing drawings and investigation records, a capacity and load-path assessment, the proposed strengthening schedule, and a carbon model that identifies its baseline and life-cycle boundaries. Those documents show whether the claimed benefit comes from genuine structural reuse or from an unclear comparison.
