In two earlier papers we proposed a process assumption-based design method, one aim of which is the determination of the thermal requirement of a building by investigating the building functioning as a dynamic thermal system. The principal constraint of that determination is the building indoor temperature range to be no more than 2 degrees C. In this paper we focus on the thermal requirement of maximum WWR (window-to-wall ratio) allowed by the constraint as a function of envelope U-value and ambient temperature amplitude. Seven US cities are studied to represent a range of ambient temperature amplitudes. As the window part of a building's envelope is a prominent architectural feature of the building, WWR and its allowed maximum in terms of thermal autonomy are the signature/reflection of local ambient temperature amplitude and the variety of envelopes of building stock in each locality. Such signal characteristics are otherwise referred to as regional architecture. (C) 2015 Elsevier Ltd. All rights reserved.