Formosa: The Thames House That Floats When Floodwater Arrives

Formosa, a 225-square-metre, three-storey house on the River Thames in Buckinghamshire, can rise up to 2.7 meters with floodwater and settle back into place — a system its designers at Baca Architects say already passed a real-world test during the severe winter floods of 2019 and 2020, when the house rose and settled back down as intended.

From the riverbank, Formosa does not look like experimental engineering. It has the pitched roof, generous glazing, and garden relationship of a contemporary riverside house. For most of the year, that is exactly how it behaves. The unusual part sits buried beneath it: the house occupies a concrete wet dock and has been designed to leave the ground when the river rises.

Why not just build higher?

The site lies within a designated flood zone, where conventional building forced a difficult choice. A permanently elevated house could have cleared projected flood levels, but it would have pushed the main living floor almost an entire storey above the garden. Baca Architects instead kept the structure close to the landscape during ordinary conditions and allowed it to move only when water makes movement necessary. The ground floor sits roughly 1 meter above ground level, preserving an immediate relationship between the house, its terraces, and the river beyond.

How does the floating system work?

The mechanics are surprisingly straightforward:

• A concrete pontoon beneath the building acts as a buoyant base.
• During a flood, water enters the surrounding concrete dock and gradually lifts the house.
• Four vertical guideposts — sometimes described as dolphins — keep the structure aligned as it rises, preventing it from drifting or rotating.
• Flexible service connections move with the building, allowing electricity and water to remain connected as the house changes elevation.
• The river itself provides the lifting force; buoyancy does the work as the dock fills.

The concrete base carries enough weight and ballast to keep the structure stable, while the guideposts control its vertical movement. The system is tested periodically by filling the dock with water and checking the building's ability to float safely. Formosa is less a machine disguised as a house than a house designed to understand the physical behavior of water.

Can a garden act as a flood warning?

The garden is part of the system too. Terraced levels create what Baca calls an intuitive landscape, allowing rising water to become visible before it reaches the house. Reeds, shrubs, lawn, and changing ground levels form an informal warning system that makes flood behavior readable through the landscape. Instead of treating water as something that must remain outside the boundaries of domestic life, Formosa builds an expected relationship with it.

The sectional logic reveals the project most clearly. Its main components read almost like a diagram of adaptation: solar shading, the pontoon beneath the house, the terraced garden, maintenance access, flexible connections, and a servicing hub. Each element has a fairly ordinary architectural or technical role. Together, they allow the entire building to change its position without fundamentally changing how it functions.

What does amphibious construction cost?

There is a cost attached to that adaptability. Baca has estimated that the amphibious system added around 20 percent compared with a similar conventional house with a basement. Specialized foundations, a wet dock, high water table construction, flexible utilities, and flood planning make the idea considerably more complex than simply placing a house beside a river.

Yet the project makes an argument about what that additional investment buys. Instead of permanently raising the architecture in anticipation of a rare event, the building maintains an everyday connection to the ground and reacts only when conditions demand it. That distinction gives Formosa significance beyond engineering novelty: it suggests that resilience does not always have to look defensive. Architecture facing environmental uncertainty can retreat, resist, or, in this case, move. When the Thames rises, the river lifts the house, the guideposts hold its position, the services travel with it — and once the water recedes, Formosa returns to exactly where it started.