Thorne Island, a privately owned outpost situated three nautical miles off the coast of Pembrokeshire in west Wales, has been listed for sale with an asking price of offers exceeding £3 million (US$4 million).

The 2.49-acre island is home to a 19th-century fort originally constructed between 1852 and 1854 to accommodate 100 soldiers as part of Britain’s coastal defence strategy against a potential Napoleonic invasion.

The Grade II-listed fort has undergone an extensive restoration, transforming it into a self-sufficient property capable of sleeping up to 20 guests across five bedrooms.

Facilities include grand dining rooms, sheltered terraces, a sea-view office, a covered rooftop bar with games room, and a helipad.

The building retains its historic barrack-style walls while integrating modern utilities.

The current owner, British technology entrepreneur Mike Conner, purchased the island in 2017 for £500,000 (US$670,000) after discovering it through an online video.

At the time, the fort lacked windows and utilities and was in a state of significant disrepair.

Conner’s restoration project, completed this year, cost over £2 million (US$2.7 million) and required substantial logistical efforts, including 350 helicopter trips to transport construction materials, equipment, and topsoil.

The renovation included the installation of renewable energy systems such as solar panels, battery storage, and a biodigester wastewater system, enabling the property to operate entirely off-grid.

Builders lived on-site for four years during the works, addressing structural issues, moisture damage, and sandblasting stonework to restore original features.

The fort, accessible only by sea or air, is designed to withstand salt exposure and severe coastal weather.

It has previously served as both a hotel and a private residence since its sale from military use in 1932.

Conner has used the property with his family and describes its potential for use as a venue for luxury retreats, celebrations, or as a secluded private residence.