Surbiton Build Blog 001 - Design

This blog series documents the construction of a one-storey airspace development in Surbiton, where we are delivering two new residential flats above an existing building.

Next Level Airspace is both a Developer and Contractor specialising in airspace projects, with a fully integrated in-house design and build team. To our knowledge, this fully integrated model is unique within the airspace sector, allowing us to control projects from early feasibility and planning through to construction and final delivery, with the same team responsible throughout

In this build blog we share how the project progresses from early design through to practical completion and handover, highlighting some of the engineering and construction decisions that make airspace developments possible.

Design Phase

Before construction began on site, the Surbiton Project underwent an extensive detailed design and coordination phase.

Acting as Principal Contractor and Project Manager, Next Level Airspace procures and leads the full design team, coordinating the interfaces between disciplines to ensure buildability is embedded into the design from the outset.

The design team included:

• Principal Contractor

• Architect

• Structural engineer

• Hot Rolled Steel Fabricator

• Light Gauge Steel Frame Fabricator

• Fire Engineer

  
  

Coordinated Structural Design

During the design phase the team developed a coordinated 3D model of the proposed rooftop extension.

This model allowed the existing roof deck, transfer deck and light gauge steel frame to be reviewed together before fabrication began.

3D coordination model showing the proposed hot rolled transfer deck and light gauge steel frame structure above the existing building as part of the Surbiton airspace development.

Structural Layout and Transfer Deck

A key structural component of the Surbiton Project is the transfer deck, which redistributes loads from the new storey into the loadbearing walls of the host building .

The design introduced a series of hot-rolled steel beams spanning across the roof, transferring loads to appropriate structural positions within the building.

Structural steel layout showing beam spans and positions forming the transfer deck structure.

These beams form the primary structural support for the new storey and provide the interface between the existing building and the lightweight LGSF structural system above.

Design Coordination

Airspace developments require a high level of coordination between multiple designers to ensure that the various building systems can be be integrated efficiently.

Small adjustments in one part of the design could have knock-on effects elsewhere.

One example involved coordination around the stair opening, where hot rolled steel positions needed to be carefully adjusted to maintain required headroom clearances while preserving the structural layout.

Section drawing used during the design phase to coordinate structural steel positions and maintain headroom over the staircase.

Through design reviews and drawing updates, the design team refined the layout to ensure compliance with building regulations while maintaining structural efficiency.

Resolving these issues during the design stage helps prevent costly modifications once construction begins.

Steel Fabrication & Transport Strategy

Another important consideration during the design stage was how the structural steel frame would be transported and installed on site.

Because several beams were relatively long, splice connections were introduced to allow steel members to be delivered in sections and assembled once on site.

Structural model used to design splice connections allowing beams to be delivered to site in sections.

These connections use high-strength friction grip (HSFG) bolts, providing reliable structural performance while allowing rapid assembly during installation.

Designing the steel frame in this way ensured that members could be safely transported and lifted into position within the constrained residential site.

Prefabricated Connections & Modern Methods of Construction

A key design decision was the use of Modern Methods of Construction (MMC) through the adoption of light gauge steel framing (LGSF).

This approach allows structural components to be manufactured off-site with high precision before being assembled rapidly on site.

To support this strategy, the hot-rolled steel beams were fabricated with pre-welded fin plates, allowing the LGSF floor joists to connect directly to the structural steel frame.

Fin plate detail welded to the hot rolled transfer deck allowing the LGSF floor joists to be installed during construction.

By incorporating these details into the fabrication drawings, the LGSF joists could be cut precisely to length and installed efficiently once the steel frame was in place.

The structural steel frame, LGSF floor system, external wall panels and roof joists were all coordinated and resolved within the project’s 3D model prior to fabrication.

With the structural design coordinated and fabrication drawings completed, the project was ready to move from the design stage into construction.

However, before any structural steel could be installed, a carefully engineered scaffold and access system needed to be erected around the building. This temporary works structure would support the installation of the new steel frame while allowing construction to take place safely above the existing occupied building.

In the next update, we’ll look at the scaffold and access strategy used on the Surbiton Project, including the engineering considerations behind supporting cranes, lifting structural steel, and creating a safe working platform for the construction team.