In recent years, a great challenge with the building sector (Particularly designers, and decision-makers) is to prevent the waste of energy, cost and natural resources, and associated emissions (e.g. carbon etc.) and negative environmental impacts during the life cycle of buildings. Hence, Life Cycle Assessment (LCA) has provided new tools to calculate the rate of consumptions, emissions and impacts during the phases of building construction, service-life, and disposal. However, the results of these tools are not as sensitive, user-friendly and precise to benefit building designers and decision-makers in the building sector. The paper introduces a new simplified semi-quantitative methodology and attitude towards building life cycle assessment (BLCA) process by demonstrating this through a case study of an office building (Arts Tower in Sheffield, United Kingdom). The methodology has been developed to help as a building design and decision-making support tool considering total building energy including embodied and operational energy.The results will reveal the advantages and disadvantages of the existing BLCA methodologies and tools with a particular focus on BRE’s (Building Research Establishment, UK) Envest2 tool as an example of similar type of BLCA tools, in terms of sensitivity and credibility of the outputs. The outcome of the study is a flexible, micro-detailed and simplified BLCA tool (a calculation engine) with capability of benefitting from other programmes and databases. The tool can be applied by designers, investors, and stakeholders in the building and construction industries. The other outcome of this research is a new attitude that provides a more sensitive BLCA for the future.