The new eight-sided tower on the Birmingham skyline is a geometric paradox: architect Howells shrunk it on the outside while creating more interior space. Nick Jones finds out how it was done.

“It shouldn’t work,” says Dav Bansal, partner at Howells. “If someone told you they were reducing the area of a building’s facade, but the apartments inside would get larger, those two ideas don’t go together in a sentence.” It seems hard to argue with this logic. Indeed, the only thing undermining Bansal’s premise is the fact that he and Howells have just built an apartment tower where they did just that.

The Octagon, which sits at the northern end of the £1.2bn Paradise regeneration scheme in central Birmingham, lays claim to being the first purely octagonal residential tower in the world. From an eight-sided central core, eight apartments radiate outwards on each floor. Each quarter contains a one- and two-bed flat, arranged so that – despite their different floor areas – both have a frontage that extends across one full face of the octagon. In all, the 49-storey tower contains 370 apartments.

Compared with a rectilinear plan for the same site with the same number of apartments, the facade is 7m shorter, equating to about 20m² less surface area per floor. “If you take that over 49 storeys, that’s nearly 1,000m2 less facade, which is a huge saving both in carbon and build costs,” points out Bansal. At the same time, the net-to-gross floor area has increased slightly, from 75% to 76%, with the result that apartments exceed national space standards by up to 10%.

And each home actually has a more generous facade than the rectilinear equivalent, with 12.5m of wall-to-wall perimeter glazing compared with 6.5m and 10m for typical one and two-bed flats respectively. The idea for an octagonal tower emerged from the constraints of the site, which is squeezed between two roads at the very northern tip of the Paradise scheme.

“We were really struggling. Because it’s a commercially led masterplan, we were left with a very small footprint for the residential element in a very awkward part of the site,” says Bansal. He quickly realised that moving away from a rectangle had several benefits – not just maximising internal space and daylight, but doing so with a symmetrical, easily divisible structural form that could be manufactured off site and delivered on a just-in-time basis.

Service runs are also shorter, coordinated in a concentric circle around the core, and wind loadings are reduced. “There’s a reason why the Victorians built cylindrical factory chimneys,” says Bansal. “Wind sweeps around the building rather than hitting into it, which meant we could optimise the thickness and strength of the concrete and the overall diameter of the core.”

The efficiency of the floorplan led to cost and carbon savings at every stage. Compared with the RICS’s 2022 benchmarks for residential construction (A1-A5, cradle to completion), upfront carbon is 44% lower on the substructure, 21% on the superstructure and 43% on the facade. Overall, A1-A5 embodied carbon weighed in at 516kgCO2e/ m2 compared with the benchmark of 850kgCO2e/m2.

The structure was almost entirely precast and repeated from floor to floor. The core walls were installed first, followed by triangular lattice planks, and then the columns. These comprised a primary diamond-shaped perimeter column at each of the eight corners, two secondary perimeter columns between the corners and a blade column halfway along each radial axis. The 200mm-thick party walls and 100mm-thick internal walls are all made from insulated concrete panels, providing all spaces with two hours’ fire separation and high levels of acoustic protection.

The aluminium facade panels – 600mm deep and faceted to draw in daylight from below while maximising shading from above – were also manufactured off site. These lightweight elements were pre-glazed and insulated, and fixed to the slab edge, with no additional internal structure required.

There is a sense of spectacle to the construction of precast structures of this scale, and the pace at which it takes place. “Everyone was just following the building as it went up,” says Bansal. “It became a social media event: people were saying, I was only in Birmingham two weeks ago, and it’s gone up five storeys.”

So why has no one ever built an octagonal residential tower before? The answer probably lies with the perceived difficulties of wedge-shaped layouts radiating from the core –furniture tends to fit better in right-angled spaces. Howells was able to call on its experience redeveloping Birmingham’s grade II-listed modernist landmark, the Rotunda, 20 years ago.

There, it had turned a drum-shaped tower of outdated office space into sought-after city-centre apartments, breaking the plan into similar trapezoidal layouts. At the Octagon, there is an 800mm zone beneath the glazing, which means that furniture can be placed along the outer wall, while views out can still be enjoyed from seating.

Prefabricated bathroom and utility pods have been installed in the smaller, more awkward areas towards the core. “Everyone thinks that pods need to be rectangular,” says Bansal, “but they can be any angle, as long as they’re repetitive and there are a lot of them.” The Octagon clearly shares some DNA with the Rotunda, as well as the slender profile of Birmingham’s other mid-century icon, Richard Seiffert’s Alpha Tower.

There’s a suggestion too of the brick chimneys that dotted the skyline during the city’s industrial heyday. “It’s definitely a building with a Brummie accent,” says Bansal. “It’s a city that was defined by innovation and advanced manufacturing. It’s a bit brutal, a bit modernist, a bit futuristic.” 

Interview by Nick Jones

Photos: Hufton + Crow, Greg Holmes​