What is the impact of the built environment on social relationships?

By "built environment," I am assuming that you are referring to all the ways in which man builds or otherwise imposes order on the natural world.  This impact is important, and far more powerful than most people realize.  I will go over this impact in the city and in the world of work.
I first became aware of how the built environment affected cities when I read The Death and Life of Great American Cities by Jane Jacobs.  In this book, she discussed all the ways in which cities were configured that enhanced or discouraged human interaction.  For example, large blocks composed of very large buildings discourage people from lingering and talking to one another, while shorter blocks with many small storefronts make people more likely to be approachable and friendly.  This is why one can find more people chatting on the street in SoHo than in the Capital area of Washington, D.C.  Playgrounds are created to enhance the social relationships of children, encouraging them to do things together on the same equipment.  Two different children on the beach might build their sandcastles in isolation, but give them a sandbox, and they will be working together.  How we build our parks, our neighborhoods, and our cities really does have an effect on how we interact. 
In the world of work, the built environment fosters or breaks down hierarchies. The corner office is a prime example of this. It is isolating, and it is elevating, sending the message that this person is in a superior position in the organization. Contrast the corner office with cubicles.  People in cubicles will socialize at least to some degree, while people in corner offices usually have to invite others into their offices or go out and mingle.  In organizations that value creativity, the trend today is to design a work place in which people must all encounter one another in casual ways, so that they can generate ideas in ways that are not all that effective in isolation. 
Once you become aware of the influence of the built environment upon social relationships, it is easy to take note of this phenomenon.  You might want to consider how the house you live in affects the relationships with the people you live with or how a gym you work out in is laid out to encourage or discourage people from interacting. It is a very interesting area for study. 
https://archive.nytimes.com/www.nytimes.com/books/97/08/17/reviews/jacobs.html