It will be "the ultimate Lego experience", the toy company claims - an entire theme park dedicated to the eponymous building brick, with Lego models and activity "zones" designed to stimulate and entertain.
Lego bought the 150-acre site after Windsor Safari Park closed in 1992, having been forced into receivership with pounds 40m debts. "We selected Windsor from 1,000 possible sites in the UK. The image fitted with Lego very well," explains the project's Canadian managing director, Robert Montgomery. As part of the deal, the receivers were obliged to find homes for the park's 600 animals and Lego committed itself to regenerating the area, which had fallen into disrepair. "Much of the site was in need of environmental protection and repair," Mr Montgomery explains. Extensive work has been undertaken to restore the woodland. "Our intention was always to create a natural and scenic attraction for our guests and neighbours."
But Lego inherited more than it bargained for. The Mansion House, built in the centre of the park in 1751, was once the Kennedy residence, when Joe Kennedy served as US ambassador to the United Kingdom. When work started on the site two years ago, local contractors reported discovering "dozens" of bugging devices secreted throughout the building - whether of US or UK origin, no one knows.
The site also revealed other mementoes of former occupants. Such as the skeleton of a killer whale unceremoniously buried in an unmarked grave. And "toxic" hippo dung - the central lake was once the hippo pool: when dredged, 600 tons of the stuff were removed. The hoard has been left to decompose in a quiet corner until fit for use as fertiliser.
Forty acres were eventually landscaped. The site is now divided into separate zones. Miniland is fashioned entirely from Lego: it has taken three years, 100 model-makers and 25 million bricks to build. Many of the 800 models are animated, complete with sound effects. Then there is the Imagination Centre, with computer rooms available for school parties; and the Duplo Gardens for younger children, with Lego helicopters, boat rides and water games, and two driving schools with electric cars that kids can drive.
My Town, a group of life-sized buildings, huddles around a harbour stage for an hourly live-action stunt show. And at the far end of the site lie the Wild Woods, where children can ride a flume or play in a labyrinthine system of tree-top walkways, scramble-nets and chutes. Each "zone" is separated by a "quiet area" for children to calm down in. Between trees and early spring flowers, Lego pheasants and Lego fox cubs roam.
The first Legoland, at Billund in Denmark, opened to the public in 1968 and has drawn almost 25 million visitors since. Lego hopes to attract a similar volume of visitors to the British version. Mr Montgomery expects families will spend around six hours each visit. But to avoid queues and prevent overcrowding, the aim is to limit access to a maximum of 15,000 visitors at any time.
Sponsorship may be a commercial necessity for all theme parks, but here, Mr Montgomery claims, the Lego approach differs. The park - which has cost pounds 85m to develop - was funded entirely from cash flow; Lego prefers not to borrow from banks. "By working with only a few like-minded partners - Ford, Apple, Cow&Gate - we can add extra value," he says. Ford's sponsorship of the driving school - where commercial involvement is most blatant - is only a concession to realism, he insists.
Legoland Windsor must quickly pay its way: a target of 1.4 million visitors in the first year has been set. Despite being the only theme park catering for under-15s, Legoland has a firm eye on the opposition. Entrance fees are set just below other parks' prices - pounds l5 for an adult and pounds l2 for a child, compared with pounds 17 and pounds 13 at Alton Towers. Hardly a cheap family day out. But already 250,000 tickets have been sold on the strength of the Lego name alone. It may be child's play, but Lego takes its business very seriously indeed.