In 1996 the Danish government presented their plan (Energi21) formulating how Denmark could fulfill the demands for CO2-reduction recommended by United Nations. The major issues in the plan, regarding new and existing buildings, is that heat demand for new buildings in year 2005 should be reduced by 33% compared to current level and that the CO2 emission should be halved. This calls for sustainable development in the building sector, but at the same time, it has to be economically efficient. People are conscious about savings in energy, but consideration to economic aspects are their primary concern which can be seen during the construction of new buildings. People want energy-friendly solutions, but they should be economical optimized. An exonomical optimized building design with respect to energy consumption is the design with the lowest total cost (investment plus operational cost over its life time). The design and construction of buildings should take into account both energy, environment and economical aspects. The design of a building is very complex and the work on optimizing the design raises several questions. Which criteria are the decisive when choosing a solution? How to evaluate different separate solutions when they interact in the building.When trying to optimize several parameters there is a need for a method, which will show the correct price-performance of each part of a building under design. The problem with not having such a method will first be showed by an example. After the example, requirements for the method will be stated.