First off, interior designing is not every ones cup of tea and you need to be creative and have some designing background to design a house beautifully. Moreover, there is a difference in an interior designer and an interior decorator. The difference is, an interior designer helps you in designing your home in a way that all the space of your home is made use of and giving you more spacious homes while none to very little wastage of space. Whereas, an interior decorator will help you with decorating your homes to make it tasteful suiting your personality and theme you have chosen for decorating your house.
Now that you know the difference in between the two, you will realize that interior decorating of your house at times can be done by you without hiring a professional. However, interior designing is completely another ball game and not all will be able to handle this job with fineness, if he/she is not a pro at it. As a matter of fact, personally I think that decorating your home should be done by the people who will live in the house, so that they can put in their personal touch and effort in the making of their homes.
Therefore, hiring an interior designer for your house is a must and you should not try to do a professionals job yourself. You will only fail in many ways and later you will not be able to alter the mistakes you have made as the cost of redoing the house will pinch your pockets. However, you can always give a chance to new interior designers to save some money. At times newbie do a better job than most professionals in this field have ever done in their long 10 or more professional years.