Collins is proud to announce the completion of the refurbishment and extension of Twenty Balderton Street on behalf of Grosvenor and Stow Capital Partners, to reposition the 1930s art-deco building as Grade A office space in the heart of Mayfair.

Working alongside Trehearne Architects, the Collins team has refurbished and fit-out 27,500 sq ft of office space to modernise the building for future tenants whilst retaining its Mayfair heritage.

The refurbishment has improved the quality and presentation of the building to enhance the health and wellbeing of tenants. The team has achieved this by reconfiguring the layouts of the upper floors (levels 4-8), creating floor plans that are open and spacious, upgrading the office reception area, making the existing terraces accessible, installing terrace greenery, and constructing a stunning new rooftop terrace.

Sustainability of the scheme, both during construction and for the ongoing use of the building upon completion, was a key requirement for Grosvenor and Stow Capital Partners. The project was designed to meet challenging sustainability targets developed by Grosvenor and TFT. Grosvenor’s core purpose is to improve properties and deliver lasting commercial and social benefit, and so the Collins team implemented current industry best practice environmental measures to improve both the performance of the building and the satisfaction of its occupants. This included the use of 100% sustainable timber, low VOC materials, Freegle to minimise waste onsite, and reducing building water consumption by 50% using water efficient fixtures and fittings.

Another sustainable practice adopted for the project was the retention of existing materials. The team retained and repurposed the façade to ensure the project minimises embodied carbon compared to that of all new products. Terrace alterations included dropping the windowsills to provide doorways, adding tiling, and adding a variety of raised box planters to improve appearance and add privacy. The team also installed gloss black art-deco inspired metal balustrades that are in keeping with the existing look of the building.

Constructing a new rooftop terrace required reconfiguration of the roof plant to reduce its footprint, as the existing roof level was extensively covered in mechanical plant and so unusable. The team constructed a new roof level extension and extended the existing stair core for access. Being located within the Mayfair Conservation Area, the roof level extension and enclosures have been located and designed to work with the existing stepping of the building to minimise their visibility from street level.

Whilst forming the rooftop extension, the team discovered that the steel arrangement wasn’t as anticipated. Working with Heyne Tillett Steel, the team re-designed the new steelwork in line with the existing structure. Collins Construction’s flexible and collaborative approach meant the team were able make the necessary changes and continue to meet the client’s aspirations.

Another key enhancement to the building was the upgrade of all services. Reconfiguring the plant, as well as replacing the services on all floors, allowed the team to increase the energy efficiency of the building. This required careful coordination as the proposed scheme called for exposed services in addition to increasing the floor to ceiling heights. To achieve this, the concrete encasements were removed from the existing structural beams, which were then strengthened, and service castellations formed to run the services through. The team formed almost 200 castellations to achieve the desired result.

The team also extensively remodelled and enlarged the reception area to create an impressive and striking new entrance.

The full delivery team included: Trehearne Architects, Trident Building Consultancy, Heyne Tillett Steel, Hurley Palmer Flatt, Leslie Clark and TFT.


  • Project

    20 Balderton Street

  • Value


  • Project Manager

    Trident Building Consultancy

  • Architect

    Trehearne Architects

  • QS

    Leslie Clark Construction Consultants

  • Structural Engineers

    Heyne Tillett Steel

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