The rapid increasing of urbanization all over the world, then large and mega cities are growing in number, resulting in increase of: population, pollution, and traffic Complexity. Thus incorporating energy efficiency and sustainable design features into all contemporary buildings has become a high priority for owners, designers, and governments. The research methodology follows: Literature, Analytical and Comparative/analytical reviews respectively; examines the concept of “low-energy” building, investigates how urban density might affect building energy design in high density cities. Using Cairo as a case-study, the characteristics and factors affecting “low-energy” building design techniques are evaluated. The current energy situation in Egypt is explained and considerations for energy efficiency in high density conditions are discussed. The research aims to draw-out a frame-work for best strategies and passive design techniques of “low-energy” buildings appropriate for application in dense environments. through using the "Base-case" model vs. “Low-energy” model using “design builder” simulation tool; to compare the impact of applying different strategies: [Orientation- Façade design - glazing materials...]. The research find-out that the “low-energy” designs techniques may reduce energy consumption by about (35:50 % or more). Such results may help designers and owners to make their decisions promoting energy efficiency in: new and retrofit buildings.