Built in the early 15th century by Winchester College, it is the largest timber-framed building in England and is regarded as an outstanding example of medieval carpentry.

The barn was briefly in royal ownership but passed into the hands of three families who continued to use it for agricultural purposes until as late as the 1970s.

After the company went bankrupt in 2006, the barn was bought by property speculators betting on its compensation value if the nearby Heathrow Airport was expanded.

English Heritage stepped in, using a rare legal procedure to carry out repairs without the owner's consent, and eventually purchased the barn in January 2012.

It is now open to the public from April to October on the second and fourth Sunday of each month under the management of the Friends of the Great Barn group.

[5] It has been described by English Heritage as "a supreme example of late-medieval craftsmanship – a masterpiece of carpentry containing one of the best and most intact interiors of its age and type in all of Europe.

Each is about 14 inches (36 cm) square and sits on a block of Reigate sandstone, a common building material in medieval London.

The builders cut and fitted the timberwork together on the ground and scratched Roman numerals, called assembly marks, on the joints to indicate where pieces of timber were to be combined.

This was because the bottom of a tree is always wider than the top; the greater width was needed to accommodate the joints with the beams that support the roof.

Roof purlins run the length of the barn and are tenoned into the principal rafters, with additional support from curved wind braces.

A number of features in the barn's carpentry are described by English Heritage as "experimental, precocious and regionally unusual," which is attributed to the very high level of skill of the master carpenters who built it.

[2] The use of aisles enabled the barn's architects to increase its width and by doing so, provided the maximum space for threshing floors.

The wheat barn at Harmondsworth was damaged in a storm in 1398 and records from Winchester College show that two carpenters were sent to make repairs, for which a large quantity of tiles, nails and other roofing materials was purchased.

The college's records indicate that in 1426–7 it commissioned two men, William Kypping (or Kipping) and John atte Oke, to obtain timbers from Kingston upon Thames to use for a new barn at Harmondsworth.

[15] In 1544, the manor was taken by King Henry VIII to add to his hunting estate around Hampton Court but he does not appear to have used it, and shortly afterwards he granted it to the Paget family.

[16] It was already a Scheduled Monument and was given Grade I listed building status in March 1950 when new heritage protection legislation was brought into force.

[14] It underwent a detailed eighteen-month survey in the late 1980s by the craftsman Peter McCurdy (who later went on to rebuild Shakespeare's Globe theatre), supported by the Museum of London.

Planning permission was granted on the basis that income from the new offices was supposed to pay for ongoing repairs to the barn, following an initial renovation carried out in 1989.

"[20] Following the publication of the Cornerstone article, English Heritage stepped in to begin legal proceedings that would lead to the compulsory purchase of the barn.

[8] The agency carried out an unusual legal manoeuvre to speed up the works, declassifying the barn from a scheduled monument to a Grade I listed building.

[19] The intervention by English Heritage led to a protracted dispute over the £30,000 cost of the repairs that was scheduled to come to trial at the High Court of Justice in April 2012.

"[21] The SPAB also welcomed the decision, calling the barn one of the "symbols of the dominance of the rural economy in the past, and the immense investment in craftsmanship and materials that agriculture deserved.