Pioneering architects Alison and Peter Smithson’s no-frills glass box near the ruins of a grand 18th-century folly was an experiment, a second home and a ‘fairy story’ – all of which awaits whoever buys it next…

Upper Lawn is a weekend retreat in Wiltshire built by the late architects Alison and Peter Smithson for themselves and their family and used by them from 1959 to 1982. It’s a place of obvious delight, thanks to a garden enclosed by old stone walls in which it stands, a clump of grand old beech trees just outside, and broad views of sweeping countryside beyond. The house itself is a well-proportioned, thoughtfully detailed, somewhat rustic glass box that makes good use of the transparency and openness that modernist building methods made possible. It’s also a work of less obvious riches, a material diary of building and dwelling, a three-dimensional essay on the passage of time. Now it is being put up for sale by its owners for the past 23 years, the graphic designer Ian Cartlidge and his wife, Jo.

The Smithsons, acknowledged founders of brutalism, never saw themselves as practising a style, but applying an attitude – one that makes evident the ways buildings are made. Upper Lawn is possibly the purest expression of their ideals. Having to satisfy no clients but themselves, it was a “device”, as Peter (1923-2003) called it, “for trying things out on oneself” and for generating ideas they could use on larger projects, such as their headquarters for the Economist in St James’s, London.