History of the Box Sash Window
Building Act
Two Building Acts affected the appearance of sash windows in London. They were imposed because exposed sash boxes were
seen as a fire risk.
The 1709 Act stated that sash windows had to be recessed 4" back from the outer brick-work or masonry. The 1774 Act required the sash windows box frame to be set behind the brickwork, so that only about an inch of the sash box was visible from the outside.
This, however, is not a dating guide. These Acts applied to London, but were not generally followed, as can be seen in many 18th and 19th Century towns. The speed with which changes took place also varied between areas.
