Alasdair Glass is quite right to point out that the recessing of window frames dates to the 1709 Building Act and that the building acts cannot be used as accurate date indicators (AJ 17.2.05). He might have noted that the act also applied to a wider area than the City of London, which the 1667 Rebuilding Act had covered.
What is most interesting about the clause for the recessing of windows is that it was added as a House of Lords afterthought to an act that was not for the most part concerned with structure. The addition came after Vanbrugh had given evidence to the Lords; it is said to be intended to prevent rot rather than fire. I believe Vanbrugh realised the architectural effect that the increased depth and modelling would have on the character of the London terraced house.
Vanbrugh was not the first to promote an architectural programme in the guise of building regulation. The two proclamations of 1615 and 1618, the first after Inigo Jones became James I's Surveyor of the Royal Works, were intended to make London safe from fire, beautiful and magnificent; they banned jetties and required windows to be of greater height than breadth with sufficient brick piers between. CABE are not the first architects to try to establish a government design policy.
Frank Kelsall, Architectural History Practice