Interior architecture can be seen as different from architecture in that the interior rooms, halls, doorways + stairs can be re designed once the initial design for the architecture of the building as a whole is in place. Interior architecture can be used to completely re design, for example a room such as a kitchen, bathroom in a family home which is quite common or a meeting room or office layout in a business environment or modify the existing design based on original architecture.
The interior architecture of a building can be changed for many reasons, either practical, where the interior of a building is going to be used for a new purpose or due to changing fashions or updated technologies. Examples of this are very common in many towns and cities where the exterior of a building is in a style of when it was built, say in the late 19th century but it's interior has been completely re vamped incorporating modern design fashions and the latest technology.
The use of interior architecture means that for many buildings over a hundred years old the development of the look and design of their interior and exterior could often be the work of many different architects or teams of architects. This has meant for many of our towns and cities the architecture on view can be widely varied and reflect many different periods of time, design fashions and architecture styles. Some of which can be seen as timeless whilst others from certain eras can be seen as garish and bad taste 20 - 30 years down the line.
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However no matter what a building’s exterior may look like with the help of interior architects, refurbishment and fit out specialists the interior look and feel of many buildings can be totally transformed with enough time and money bringing them up to date, that is until the latest style, fashion and technologies need integrating a few years down the line.
The main thing to differentiate architecture + interior architecture is that an architect is concerned with the concept of designing a new building the interior architect is concerned with the modification of existing buildings, some of which the design can be seen as timeless and look good for centuries whilst other will be an eye saw and look dated within 10 - 30 years.