Stanton Williams has reworked the grade II*-listed headquarters of the Rhodes Trust educational charity in Oxford, improving its environmental performance and doubling the usable area largely by reimagining previously vacated spaces beneath the existing structure.
Basements and archives have been transformed into light-filled foyers, offices and a 300-seat conference hall beneath a concrete vault. New elements include a sculptural glass pavilion and an earth-sheltered residential courtyard, sunken into the landscaped gardens. The reuse-led, material-efficient approach means that the upfront embodied carbon of the structure was calculated at 179kgCO2e/m2 for the overall project (rather than simply the new-build elements), largely in line with the LETI 2030 target.
The original Rhodes House, a 1929 building by Sir Herbert Baker, combines classical elements, such as a portico and rotunda, with rubble stone walls and arts and crafts motifs. What is less apparent is that behind those walls is an early example of a reinforced-concrete structure. This has played a quietly significant role in the reimagining of the subterranean space. “That was part of the reason why we could put so many new openings into the existing building,” says Alex Lynes, associate director at structural engineer Webb Yates Engineers. “We could remove parts of walls and rely on the slab above to be able to span.”
The largest space – a former archive added in the 1950s – was earmarked for the conference hall early on in the design process. This area, below the southern courtyard, is defined by the Baker building’s metre-thick concrete retaining walls on three sides. Stanton Williams and Webb Yates have removed the internal columns and pushed out the fourth side, beneath the open end of the courtyard. Crucially, the space has also expanded upwards, with a vaulted concrete soffit that rises a metre beyond the original ground level.
The vault offers a number of advantages. It introduces space for a clerestory window along the southern wall, following the arc of the slab, which brings daylight into the basement and gives views out to the landscaped garden. It is also extremely material-efficient. Spanning the 9m x 12m central section of the hall, the vault is 200mm thick – far slimmer than a flat slab, or even a coffered alternative, says Lynes. This brings architectural benefits too, adds Tom Fotheringham, associate at Stanton Williams. “In addition to the expressive qualities of the structure, this intervention was key to make a previously very introverted building more permeable, bringing connections between inside and outside.”
The conference hall reads as three sections, with two side wings that can be partitioned off with a sliding wall system, allowing various configurations. These areas have a coffered soffit, based on a 1.5m x 1.5m grid of 600mm-deep beams with a 150mm-thick slab in between. Overall, the vaulted and coffered structure means that the volume of concrete is reduced by 50% compared to a more standard structural design.
Above ground, the landscaping has been raised to accommodate and conceal the vault. “The structure is working with the landscape,” says Fotheringham. “There are deep areas with 700-800mm of soil on the two sides above the coffers, where planting can really take root. Then in the centre, where it’s shallower, we've worked with the landscape architects BSHLA and the head gardener at The Rhodes Trust, Neil Wigfield, to introduce more drought-resistant species.” He adds that the raised courtyard also brings the foliage up to the same level as the dining hall’s mighty two-storey bay window, improving the connection between interior and exterior.
The ground in this area hides more than a vaulted slab. Earth ducts have been buried into the landscape, drawing in and passively cooling fresh air before distributing it at low speed behind low-level timber slats. The vault plays a part in this natural ventilation strategy too, developed in collaboration with services engineer Skelly & Couch: exhaust air is drawn up through a series of holes cast into the slab – “a bit like Emmenthal cheese”, says Fotheringham. It is then fed into a plenum before being dispersed into the north side of the courtyard.
The east side of the site reveals more subterranean intrigue – here, driven by the need to maintain sightlines between the house and the city’s Civil War-era ramparts, which form the eastern boundary of the site. A residential courtyard has been cut into the ground, providing 16 guest bedrooms in two facing rows of highly insulated, thermally massive earth-sheltered structures. Again, the concrete structure has been designed to be as efficient as possible, with blade columns in each separating wall to minimise the slab depth. And as with the conference hall, landscaping has been reinstated, with up to 800mm of soil placed on top of the roofs to enable large plants to thrive.
The trick here was to balance the necessary soil depth with the need to make sure the courtyard wasn’t too deep to restrict natural light to the rooms. “We did quite a lot of tests on the width of the courtyard and the size of the brick columns between the bedrooms. You get really beautiful views out to the trees,” says Fotheringham. The position and size of the glazing also maximises daylight, while ensuring privacy and preventing overheating.
Throughout the project, the concrete used has high levels of GGBS: 65% for visual and 50% for non-visual elements. It is another quietly sustainable aspect of a project that hides many of its accomplishments deep below the surface.