Buildings are material- and energy-intensive engineering structures, and they are responsible for 40% of the collective energy consumption and 36% of the collective CO2 emissions in Europe. Currently, long-term priority goals for new building construction and extant-building renovations are as follows: 1) Raise building energy efficiency, approaching zero-energy-consumption levels; 2) Increase the utilization of renewable energy resources for energy consumption by buildings; 3) Reduce CO2 emissions. The reduction of energy consumption during building utilization is one of the most important factors for reducing building impact on environment. Currently, standard criteria for LEBs have not been developed. While the reviewed directive for energy efficiency defines qualified, LEB decision criteria, they are not expressed in terms of quantified, verifiable decision criteria. Latvian construction standard poses only minimal requirements for energy efficiency, and in the case for new building construction, it is neither clear what standards must be met nor what technologies are utilizable in Latvia to achieve LEB levels. In building renovation, there is a lack of clear methodology for assessing and achieving a cost-optimal energy-efficiency level. Computer models developed in Latvia are neither applicable to calculations for LEBs, nor have they been validated for cases involving LEBs. Models developed in Latvia do not allow for the consideration of the impact utilities have on a building’s energy consumption, and do not provide opportunities for integrating energy sources into the building’s energy balance. This thesis includes the development of a model for calculating and optimizing the building-energy consumption applicable for conditions in Latvia. This model is applicable for identifying measures for optimal energy efficiency and is utilizable for new-building projects and renovation work planning. Optimization includes two different optimality criteria, where one illustrates the performance of investment and the other evaluates the effectiveness of CO2 emission reduction. The following optimality criteria were selected: 1) Cost-optimal energy-efficiency level: The lowest global costs for one square meter in the selected period, LVL/m2; 2) Optimal level of energy efficiency for CO2 reduction: Largest reduction in CO2 emissions versus global costs in the selected period, kg CO2/LVL. The computer model has been approbated in creating the design and plan of energy-efficiency measures for a multi-apartment building.