Two unusual features distinguish the new, £130m Hylo building on London’s Bunhill Row from others of its ilk. Brick clad, it breaks with the high-rise office tradition of an all‑glass facade. This softens its appearance and helps it to blend in with neighbouring residential towers, which include the Barbican. The second feature is less overt but more significant: rather than demolishing the 16-storey incumbent tower, the new scheme extends it upwards by 70% and sideways by another 24%, which more than doubles the net lettable space while saving 35% of the carbon footprint of a new-build scheme.

Vertically extending the existing Finsbury Tower, which was completed in 1967, has precedent. The team behind this project – developer CIT, structural engineer AKTII and contractor Mace – collaborated on an earlier project called Southbank Tower, which coincidentally was designed by the same architect, Richard Siefert.

There are some important differences, however. Southbank Tower was a residential scheme with a correspondingly smaller footprint and the extension was less, adding 11 storeys to the 30-storey tower. Hylo was more ambitious because the tower footprint is bigger and the extension is higher, which in turn demanded a greater degree of intervention to provide the standards of vertical transportation expected of a modern high-rise office building.

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