This particular window is built with two layers of glass and it has a 16mm space in between. The layers of glass then are sealed at the ends of a spacer.
Regulations for Buiding & Double Glazed Windows
Home improvement is not an exact science however, and nothing is cast in stone. As such, consider it a wise advice to do some research and check with local planning authorities before you proceed with your double glazing project. (replacing bay windows? www.upvcwindowsfitted.co.uk/how-much-do-bay-windows-cost)