Claverton Manor was built between 1819 and 1820 to a design by Jeffry Wyatville, the architect who would later remodel Windsor Castle. The Vivian family commissioned it: John Vivian, a barrister, had bought the estate in 1816 from owners that included Ralph Allen, the quarry magnate who supplied the Bath stone that built Georgian Bath itself. Wyatville set the house on the steep west slope of the Avon valley, three storeys of pale ashlar with full-height bows on the east front and an Ionic-columned porch, the kind of disciplined Regency design that took advantage of every angle of the view. Grade I listed since 1956, the house has had a string of owners and one strange wartime turn - it served as the headquarters of an RAF barrage balloon group during the Second World War. In 1897, while the Skrines owned it, the lawn outside hosted the first public speech ever given by a 22-year-old Winston Churchill.

Dallas Pratt was an American psychiatrist with a Standard Oil inheritance, a sharp eye, and a long partnership with the antiques dealer John Judkyn. In the late 1950s the two began assembling American decorative arts on a scale no European institution had attempted. They bought Claverton Manor in 1958 and opened it to the public on 1 July 1961. Pratt's idea was simple and pointed: postwar Britain knew almost nothing about America beyond Hollywood, and the cultural traffic ran heavily one way. He wanted to show, in real rooms with real objects, the textures of how Americans had actually lived. Judkyn died in a car accident in 1963. Pratt kept the museum running for decades afterward, and it remains the only one of its kind.

The collection works by reconstruction. A late seventeenth-century Puritan keeping room with its dark oak furniture sits a few doors from a Shaker workshop with its hung tools. An eighteenth-century tavern stands beside a Pennsylvania German kitchen. The grandest is the New Orleans bedroom, transplanted from a Louisiana plantation house and dating from around 1860 - heavy carved bed, mosquito netting, the cusp of the Civil War in every detail. The quilts collection is exceptional: more than two hundred examples spanning two centuries, from spare Amish geometries to the riotous album quilts of Baltimore. Folk art, Navajo and Pueblo weavings, scrimshaw, decoys, and a Conestoga wagon round out the period rooms. Everything in the museum had to cross the Atlantic to get here, and there is a quiet wonder in that - a New England parlour, accurate down to the floorboards, reassembled within walls of Somerset limestone.

The grounds run to thirty acres on the Avon valley slope. Lanning Roper designed an early mixed border. Then, on 26 June 1962, a replica of part of George Washington's garden at Mount Vernon was opened on the site of a former Italianate planting - and the British public could walk through a slice of Virginia colonial horticulture without leaving Somerset. A Lewis and Clark trail threads through an arboretum of American trees. There is a small grotto with a water spout, and a string of views over the valley toward Limpley Stoke and the silver line of the Kennet and Avon Canal. In high summer the planting is genuinely Atlantic - American natives in English light - and the effect is gently disorienting, which is what the museum has always done best.