Neighborhood-scale projects often commence with the conceptualization of several massing-schemes as potential design solutions. There is growing interest in using building performance simulation (BPS) to evaluate and rank such conceptual stage schemes in order to choose ones that best support performance goals related to energy and indoor comfort. However, such evaluations are typically made at a time of deficiency in information on building level attributes that influence performance, raising questions regarding their usefulness for reliable decision-making. In this thesis, a new method, that builds upon existing risk assessment methods, is introduced to calculate the risk of performance loss faced by a conceptual stage BPS user/decision maker (DM). The proposed method considers three sources of risk of performance loss (1) reversal in ranks of design proposals (2) latency effect or a delayed discovery of performance gain (3) insufficient performance gain or loss of expected performance gain. These losses result from a design choice made between competing design proposals based on conceptual design stage BPS results that would be rendered invalid under future design development possibilities. To observe these losses and estimate the risk, a virtual progression of the design process is done through incremental facade levels of detail (fLOD) resulting in several future design scenarios. The resulting risk value combines the overall chance and magnitude of loss in the future design scenarios. It is further categorized as high' or 'low' risk based on the number of design paths that lead to future design scenarios with unacceptable loss. This risk assessment method was tested by running a number of relative performance comparisons between pairs of competing neighborhood design proposals (N=780), based on three commonly used indoor environment related performance metrics: spatial daylight autonomy (sDA), annual heating and annual cooling demand. The results led to several important findings. First, while many performance evaluations lead to risk-free decisions, the number of high-risk cases was large enough (e.g. 1 in 5 comparisons on sDA ) to suggest reconsideration of conceptual stage decision-making practices for projects where several design alternatives need to be ranked. Second, the likelihood of high-risk cases regarding performance loss depends on the metric, and becomes irrelevant only when design alternatives differ significantly in their evaluation outcome already at a low level of detail (LOD). Third, rank assignments based on either daylight (sDA) or annual cooling demand were found to be afflicted by all three sources of risk - rank reversal, latency effect, and insufficient performance gain, in annual heating demand evaluations, almost all cases of high-risk were due to latency effect. The aim of this risk assessment method is to link the reliability in decision making to fLOD present in the BPS models used for the performance evaluation. An original visualization aid is proposed to facilitate the DMâs understanding of risk, and identify the appropriate BPS model fLOD for making reliable conceptual stage decisions. A design competition case study was used to test usefulness of this approach in assessing risk, when going through an actual design process. This thesis also presents a novel approach for evaluating decision-making practices in an experimental manner in the BPS domain that can inform future policy making.