What is the smallest island with a building on it, according to "Guinness Book of Records"?

The Bishop Rock is the westernmost part of the Isles of Scilly, an archipelago belonging to the United Kingdom. Islet's only building is the lighthouse completed in 1858 and first lit on 1 September that year. The "Guinness Book of Records" lists it as the world's smallest island with a building on it.

An 1818 report on the dangers of the rock to ships by the surveyor of the Duchy of Cornwall, proposed that the government should construct a lighthouse on the island. The government disapproved of the idea.The construction work began in 1847 after the Trinity-house did a survey. Engineer James Walker built a 120 ft structure with accommodation and a lighthouse at the top.

Walker started constructing the second structure in 1851. Construction of the stone structure was quite challenging since they laid the foundation stone below the water level. The engineer-in-charge was Nicholas Douglass who was assisted by his sons William and James. Despite the challenges, the lighthouse was finished without a loss of life. The height of the structure was increased by 40 ft in 1882 under the supervision of William Douglass. It's the second tallest lighthouse in the United Kingdom right after Eddystone Lighthouse.

The Island is also not permanently inhabited. At higher tides, the lighthouse covers the entire rock, therefore, constructing it was quite a challenge for the engineers. The initial keepers moved into the island in 1858, but in 1992 the light was automated.