The UK Built Environment sector urgently needs a sustainability transition in order to exploit the very high potential for emission reductions in, for example, existing residential buildings. According to the Climate Change Committee (CCC)—the UK Government’s own advisory body, this potential in existing buildings is more than twice that in new buildings. However, a transition, at scale, across the whole of the UK Built Environment sector is yet to materialise. The sector remains the UK’s second highest emitting sector. It should accelerate its rate of emission reduction to two or three times the current rate. This chapter makes an original contribution to existing knowledge by drawing on the multi-level perspective (MLP) in the sustainability transition (ST) literature to explain the whole sector’s lack of progress, rather than one of its sub-sectors, e.g. housing. First, the ST literature is critically reviewed to identify the key characteristics of such transitions: normative directionality, which attributes public policy a central role in STs; and values, contestation, and disagreement. Second, the methodology, which is a rigorous desktop study of the UK Government policy to reduce the GHG emissions from buildings as a key enabler of sustainability transitions and of the literature on the impact of the UK policy on practice, is outlined. This study establishes that some transition has taken place with regards to reducing emissions from new buildings, but its scale and pace is limited. Moreover, there have not been radical enough changes to practices to accelerate the refurbishment of existing buildings to reduce emissions. The firms’ heavy reliance on normative directionality in order to create a stable context for delivering any transformation emerges as the main barrier to transition. Novel business models that will reduce this reliance are identified as being integral to the pathway to transition, rendering this chapter’s contribution significant.