It doesn't matter whether it is a house or an apartment, the principle is the same. If you don't get it right, the house will not feel comfortable and it won't look good. There are a lot of things that come into play when designing the interior of a house.
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You need to consider the size and shape of the space, the design of the house, the furniture, the lighting and the color scheme. And while interior design is an art, it is also a science.
Some people like modern touches, while others like the classic look. Whatever your desires may be, you will need to communicate those preferences effectively to your interior designer. 