A small but significant house in the U.K. is substantially complete and set to be occupied next month.

The 225-sq-meter cottage, located on a flood-prone island 10 m from the edge of the River Thames in Marlow, Buckinghamshire, is the U.K.'s first amphibious house—but likely not its last. Called Formosa, the house, which sits on the ground, has a concrete tub-like basement foundation that floats during floods.

The house is designed by BACA Architects with structural engineer Techniker and hydrological engineer HR Wallingfords, to handle up to 2.5 m of floodwater, which is well above predicted levels for the area.

The idea for the floating house grew from the site. Options included a houseboat or an elevated property. A houseboat was not allowed by the environmental agency because it would have been in the river's course, says Richard Coutts, a BACA project director. The architect rejected an elevated building because, though it would have been high enough to avoid an extreme flood, the living space would have been almost a story away from the garden. An amphibious house solves both issues, says Coutts.

The structure is a free-floating pontoon that combines a lightweight superstructure supported on a concrete base with "sufficient" ballast to ensure stability and adequate freeboard, says the engineer. It is secured by four permanent vertical posts arranged close to the sidewalls. The assembly is sited within a wet dock consisting of retaining walls and a base slab. In a flood, the dock fills with water and the house rises.

Planning for the project, including the permit application process with the local authority, took more time than is typical for a conventional house, says Coutts.

NRJ Project Management is managing construction. Greenfords built the "can-float" foundation. The building team gained approval from the U.K.'s Environmental Agency to construct a chain ferry to deliver materials across the river. Work began in the middle of last year and was suspended over the Christmas holiday season during the U.K.'s winter floods.

A full flood test, conducted once the wet dock and the foundation were constructed, proved the concept. Crews filled the wet dock with water and the floatable base lifted 400 mm to 500 mm through natural buoyancy.

The terraced garden will participate in the house's resilience—acting as a natural early warning flood system. The terraces are designed to flood incrementally to alert occupants before the water reaches a threatening level. The lowest landscape is planted with reeds, the next with shrubs and plants. A higher-level lawn follows, and finally, the highest elevation is a patio with direct access to the living space. The levels also help manage runoff from the house as floodwaters subside.

The owner of the house, is not revealing the cost. Coutts says the basement cost "slightly" more than a fixed basement. "This is balanced against the ability to secure building insurance,"  he adds.

BACA has two more amphibious house projects under way. Also, the architect, in collaboration with the U.K. Building Research Establishment and flood-risk management company Aquobex, has just completed plans for a flood resilient home to be sited on BRE's campus.

The project, supported by nearly $80,000 from the Dept. for Environment, Flood and Rural Affairs, is intended to facilitate construction in flood-prone areas, which would help overcome a land shortage in the U.K.