As important as the needs of the client in determining an interior design program are the physical characteristics of the space itself. In interior design there is always, by definition, an architectural framework that provides a starting point for design development—even when conception of the interior design plan is concurrent with the construction of a new building.
If the space under consideration possesses particular character or distinction, or a unique feature that may become a focal point—a wall of arched windows, an attractive fireplace, or a fine view—this may suggest a way to handle the space and place furniture within it.
In the case of an older building, original architectural details, including plasterwork, woodwork, doors, and hardware, should be noted and careful consideration given to preserving them. Prior to the 1970s, renovations often stripped period buildings of their moldings and other details in the name of modernization. Since the late 20th century, the trend has been toward preserving what remains, if not restoring.
The physical character of an interior space is determined by the planes that enclose it—the walls, floors, and ceilings that form the container describing the space and imparting to it properties such as proportion, scale, and balance. The ceiling in particular, be it low or high, slanted or domed, beamed or coffered, has much to do with the way people experience interior space. Designers of the modern era have the additional challenges of smoke alarms, air-conditioning ducts, the ceiling's frequent role as conveyor of lighting, and other contemporary technology.
Walls are conspicuous, integral components of an interior, and the size and placement of doors and windows have significant consequences for the flow of people, the availability of natural light, the space's orientation and view, and options for furniture placement. Floors, too, are important, since people come into constant direct contact with them. They require treatment that is decorative as well as hard-wearing. These are the elements a designer "inherits." Sometimes they can be considerably altered to improve a space's ambience (a useful French word meaning that which surrounds, or the mood, character, quality, tone, or atmosphere of an environment), with or without construction. Often, undesirable conditions of interior space can be addressed by using color, furniture placement, or artificial lighting solutions.
If too much space is the problem, as in a large, high-ceilinged room that makes people feel lost, keeping furniture, art, and lighting fixtures low to the ground creates more intimacy. Conversely, the removal of a dropped ceiling to add 2 feet (0.6 meter) or so to a room's height can relieve a claustrophobic feeling.
The desire to maximize natural light is a 20th-century phenomenon, which reached its peak with the floor-to-ceiling glass window-walls of mid-20th-century modernist architecture. But natural light is not always a plus, as when sunlight flooding a south-facing loft creates uncomfortable conditions and high air-conditioning bills. In this case, window treatments designed to shield the space and its occupants from harsh sunlight are the designer's likely solution.
The absolute size of a space is less important than scale and proportion in interior design. Good scale is achieved when all components, including built-in architectural elements and freestanding furnishings, are appropriate to the size of the space as a whole and relate well to each other as well as to human dimensions.
Proportion has to do with the relation of one part of an object, space, or volume to its other parts or to the whole. Through the ages mathematical systems have been proposed to quantify pleasing proportions. The ancient Greeks held that a "golden mean" or "golden section" of approximately 1:1.6 was the most visually pleasing ratio (they constructed the Parthenon around that principle). The Renaissance architect Andrea Palladio employed whole-number ratios, such as 3:5 and 4:7, that his culture deemed most harmonious. Charles-Édouard Jeanneret, the master 20th-century architect known as Le Corbusier, created his own system based on the human anatomy.
Qualities such as scale, proportion, and balance (in which elements are arranged to convey a feeling of equilibrium) are subjective; they are experienced in different ways by different people. But in general, out-of-scale or off-balance conditions, like a window too small, or awkwardly placed, for its wall, are universally experienced as jarring, while appropriately scaled and balanced compositions give most people a favorable visual and emotional impression.