I am writing a business plan for a restaurant and I have a name in mind that is similar, but not the same as a restaurant that already exists. I'm not going to say the real names but, for the sake of an example, it would be the same situation as if there were a "Maple Restaurant" and I came up with "Maple Ridge Restaurant". Would this be considered an intellectual property infringement? Could the previous company that has been established for many years sue or cause problems for me?

In addition to the points raised in the other answers, you should consider whether you want to (a) potentially have a legal fight right at the opening of your restaurant and (b) whether you want the potential long term consumer confusion that could arise even if the owner of the other restaurant does not complaint. It's usually a good idea to pick something distinctive that you can own without interference with other people's trademarks. In the end that will be more valuable to your company as an asset that a restaurant with a name similar to another restaurant in the area.

Trademark rights are "earned" through use in a particular location over time.  The longer a trademark is used, the stronger it is.  The prior restaurant you mention in your question has a strong mark since you said it has been established under its trademark for many years.  The owner of the existing restaurant can sue you for trademark infringement if your restaurant name is likely to cause confusion in the minds of the public.  If your proposed restaurant name makes the ordinary consumer think that the two restaurants are related, then trademark infringement exists and the current restaurant owner can stop you from using this similar name. An example showing restaurant names that could be likely to be confused is "McCormick and Schmick's" (an existing seafood and grill restaurant) when compared to "McCormick and Mick's" or "McCormac and Mack's."   In your example, "Maple Ridge Restaurant" would definitely be likely to cause confusion with 'Maple Restaurant' if the two restaurants are located in the same geographic area and draw from the same customer base.