All our first floor windows have sills at 450mm meaning they are considered "critical locations" for building regulations.  Also, because these are on the first-floor, they are supposed to confirm to "guarding" requirements too.   Our BCO is asking for "The design loadings that conform to BS 6180", but so far I haven't been able to get hold of anything that shows this.  Have others had this requirement, and if so, what type of documentation have you provided in response? - Loading