RSHP transforms 20 Carlton House Terrace

RSHP has transformed 20 Carlton House Terrace – a tired 1970s office building and basement car park – into a high-quality commercial development in the St James Conservation Area. Retaining 55% of the original building, the complex transformation delivers an additional four floors and offers a total 162,000 sq. ft. of commercial space across 11 floors.

Surrounded by Grade I and II listed buildings including 10-18 Carlton House Terrace by John Nash, the scheme creates an unexpected contemporary intervention within a highly sensitive heritage context. At its heart sits a repositioned double-height reception, a fully glazed lift group connecting all floors and a new atrium flooding the lower levels with natural light. Generous rooftop terraces afford panoramic views, and a rationalised floorplan offers flexibility in tenure supported by a forward-looking servicing strategy.

RSHP’s original competition scheme was conceived as a fully new-build redevelopment. However, further careful analysis showed how the concept could be achieved while retaining most of the existing superstructure and basement. The client's brief – to substantially increase area and enhance tenure flexibility – is delivered through a new linear core that reconciles the 90s and 70s portions of the building, fundamentally revising the plan. This linear ‘insert’ re-orders the massing, yielding the most generous roof terraces in the local area, with spectacular views over St James's Park, The Palace of Westminster, Trafalgar Square and the City of London.

Interior space is maximised via strategic column removal at lower levels and column-free spans on the new upper floors, all served by an all-electric servicing strategy. New and enlarged glazing in the partly retained and restored façade improves natural daylight and occupant wellbeing. The centrally placed lift group rationalises the office space into easily splitable tenancies with shared access to terraces.

The basement car park has been transformed into well-lit, flexible-use accommodation providing for canteen and presentation rooms, with natural light admitted at the perimeter and via the new atrium rooflight. The light-weight roof extensions are clearly read as contemporary additions whilst sensitively integrating with the existing building below, respecting relationships to nearby residences and the statutory viewing plane from Parliament Hill to the Palace of Westminster. The largely retained Portland Stone façade respects and acknowledges the tripartite Georgian orders of the Nash terrace.

The discovery of archive drawings from the original scheme, unearthed by structural engineers at Elliott Wood, was vital to early-stage design development. Elliott Wood's forensic analysis proved the feasibility of RSHP's proposals with a high degree of certainty.  The findings also informed the restoration of previously lost 1970s design elements, including reinstating the missing column in the street colonnade.

Graham Stirk, Senior Design Director, RSHP said: “Through careful yet bold interventions, RSHP has radically reinvented 20 Carlton House Terrace, reorganising decades of unstructured accretion which had diminished the original. Our strategy was to create organisational clarity within the original building; this was key to us being able to create a composition that allowed clear extensions that could sit harmoniously within the historic context. The result is a robust, elegant, and flexible commercial development that lifts the building into the 21st century.”

Daniel Wright, Associate Director, RSHP said: “We set out to preserve the qualities of merit in the original building whilst fundamentally rethinking its organisation. Our approach balances sensitivity to the surrounding historic heritage with contemporary interventions where they enhance function and enrich experience. We were able to retain much of the original fabric whilst bringing much needed clarity. Flexible and highly efficient floorplates respond to modern workplace needs, and sustainable servicing extends the building's life well into the future.”