Seattle-Tacoma International Airport (SEA) has completed a four-storey expansion that more than doubles the size of its existing Concourse C and D buildings, marking one of the largest capacity interventions at the airport since the Port of Seattle launched its Upgrade SEA modernisation initiative in 2023.

Architecture studios The Miller Hull Partnership and Woods Bagot designed the C Concourse Expansion (CCE) in collaboration with the Port of Seattle. The wedge-shaped building spans Concourse C and parts of Concourse D, carries a slightly sloping roof lined with photovoltaic panels, and closes the envelope with a crinkled glass facade. Turner Construction Company served as general contractor and construction manager.

The photovoltaic array is a first for SEA. So are the building's all-electric systems, and the project will be the airport's first to target LEED Platinum certification — a sustainability benchmark that, if achieved, would set the standard for subsequent phases of the airport's capital programme.

An envelope that responds to daylight

The alternating south- and west-facing glass panels use electrochromic glazing to minimise glare and optimise daylight, according to the design team. Rich Whealan, principal at The Miller Hull Partnership, describes the facade's behaviour in operational terms.

"As the day progresses, the concourse responds to the changing light and weather of the Pacific Northwest," Whealan said. "On overcast, cloud-rich days, visibility is maximised to highlight the views of the airfield and Olympic Range, while soft, even daylight fills the interior, enhancing comfort and connecting travellers to the surrounding landscape."

The claim is architecturally specific rather than performance-quantified: the project team has not published energy or daylighting figures alongside the completion announcement. The LEED Platinum target remains a goal, not a certified outcome.

The Tree at C as passenger anchor

Inside, the building is organised around a double-height "marketplace" containing stadium seating, food and shopping kiosks, and a large structural column clad in timber panelling that extends across the ceiling. The design team calls this element the Tree at C. It references the forests of the Pacific Northwest and uses natural materials sourced from the region.

The central staircase and the Tree were conceived as a "point of orientation and a place to linger" — a wayfinding device in a terminal where dwell time and concession revenue increasingly drive the economics of concourse design. Terrazzo floors run through the public areas.

The upper levels include an open-air lookout terrace facing the airstrip, enclosed in glass along its walls and sheltered by the photovoltaic panels overhead.

Broader programme context

The CCE is one of several major projects under Upgrade SEA, the capital programme the Port of Seattle launched in 2023. Other completed works include widened roadways and a more efficient baggage system. An update to the airport's international terminal remains in the design phase, leaving further capacity growth at SEA still on paper rather than in construction.

The delivery strengthens Woods Bagot's aviation portfolio: the studio has recently completed projects at Los Angeles International Airport — a modular terminal extension with aluminium fins — and at Western Sydney International Airport, both featuring timber panelling. Miller Hull Partnership's more recent work includes a renovation of a Brutalist university library in California.

The project's engineering and consulting bench was extensive. Magnusson Klemencic Associates handled structural and civil engineering; PAE Engineers covered mechanical, plumbing and sustainability; Arup delivered electrical engineering, smoke control, facade engineering and acoustics among other scopes; Jensen Hughes covered code compliance and fire protection; and Cumming Group provided cost advice.

With Concourses C and D now more than doubled in floor area and SEA's first on-site photovoltaic generation and all-electric building systems in service, the airport has shifted the next capacity question to its international terminal, where design work is still underway.