The relationship of modern architecture to its external physical conditions and its natural surroundings has been ambivalent: on one hand largely neglected, on the other hand producing profoundly innovative concepts. Alvar Aalto was one of the first architects who took advantage of both structural and conceptual possibilities of modern architecture to create an original approach to the above mentioned issue, conceiving the interior space of a house as a counterpart to the outside spatial situation. The paper is based on a comparative case study of two key works of modern architecture, both completed in the 1930s: the Villa Mairea by Aalto and Villa Tugendhat by Ludwig Mies van der Rohe. Both architects arrived at original formulations of the interior-exterior relationship using major modern conceptual invention, the plan libre. While Mies uses rather classical strategy and colonizes the exterior space by means of a spatial grid of load-bearing columns, projecting the architectural order onto the outside, Aalto invents truly new approach. With great respect to natural conditions on the site he lets the exterior of his buildings, their natural surroundings, enter the interior of the house in the metaphorical form of a forest of columns or complex vertical composition. The different concepts of the two modern masters are identified as two distinctive types of modern architectural space. The paper proposes to term them as „projective space“ (Mies) and „ambient space“ (Aalto).