This article is devoted to the solution of an actual problem — assessments of level of thermal performance of the building taking into account its form. The criterion of a thermal performance of envelopes on the basis of the basic resistance to heat transfer of envelopes is defined. By results of statistical data processing dependence of basic resistance to heat transfer of envelopes from an index of compactness of the building, allowing to define minimum admissible level of a thermal performance of envelopes at the set form of the building is received. On the basis of the developed technique it is shown that level of a thermal performance of the building depends on its form. Complication of a form of the building demands higher level of its thermal performance. Design of buildings of a compact form allows to lower reasonably level of a thermal performance of the building and has the high potential of energy saving. The received results of researches can be used when updating norms on thermal protection of buildings.