The study of environment – behaviour theories indicate a two way relationship between people’s behaviour and the built environment. This relationship, with regards to privacy, takes different forms among people from diverse social and cultural backgrounds, in the context of modern Iranian housing. This paper investigates the significance of traditional cultural patterns in the design of modern apartments. Application of a mixed-method case study research in a multi-story apartment complex in Tehran indicates that while lack of privacy is unsatisfactory for a group of people, another group demands the more fashionable, open interior plans. Both groups, however, maintain its privacy by either permanent alterations or temporary remedies or change in behavioural patterns. The extent of this two way relationship is dependent to the people’s behavioural patterns in the use of space and its relationship with cultural values and attitudes. For centuries, house form, living patterns, and building techniques stayed unchanged in many parts of Iran. By entering the 20th century Iran, like many other countries, involved with radical political, economic, and social changes. The urbanisation and modernisation process of the country, left profound influence on built environment generally, and house form particularly. Building form changed the shape from one- or two-storey, inward looking, courtyard buildings to higher, extroverted buildings, to high-rise apartments. The change was not limited to the exterior, but the interior plan of the house was also transformed from a closed, hierarchical arrangement to an open plan space. Distinguished patterns of privacy - a cultural institution within Iranian-Islamic cultural context, which concerns two core elements of women and family started to be faded first in Tehran and later larger urban areas as many western patterns of life and architectural plans spread out. Many social changes also helped the transformation. Increasingly participation of women in social activities blurred the sharp boundaries of this section of society from male section. The process of modernisation, which easily became the dominant culture of intellectuals and upper class of the society, transferred to the middle class too. The urbanisation process and rural-urban migration, rapid population growth, and demand for housing was simultaneous with development of modern architecture and construction methods. Hence, construction of high rise buildings and apartment blocks as mass housing became the country’s housing policy. The boundaries within the house removed one after another and the pattern pulled into the houses of all sections of society. The contemporary Tehran is a composition of both the recent forms of modern apartments and people from all backgrounds adapted to modern patterns and plans.