Probably the most critical piece of the planning stage is choosing an architect. You may be asking if you need an architect for your addition project and I would say the default answer should always be yes because your product should be more valuable in the end so your money is never "wasted." If you are doing more simple changes such as building a garage or just tearing down or moving interior walls. As soon as you are adding square footage to your house, an architect is probably necessary so you can get the most asthetically pleasing results - both inside and out. It was never a question for us that we wanted an architect.
There are 2 types of architects. There are independent architects who are not directly affiliated with any builder or design/build firm. Then there are also design/build architects who generally work with a specific builder and you get a package deal when you use them. Some architects do both services, but most do one or the other.
I would recommend interviewing both independent architects and design/build firms to see which style you like. If you choose and independent architect, you'll have to then do a second step of choosing a builder. This has the advantage of conrol by the owner over who works on the project and, in most cases, more control over total costs. The design/build option generally gives you architectural services at a reduced cost or no cost if you proceed to the second phase - building - with the same firm. Fees that you paid for the design phase are often credited to you in the building phase or waived if you sign the build contract. The downside is that you may feel locked in and you may not like the builder as much as you liked the architect. You may also pay more overall for the project because you didn't bid out the build phase (more on that later). This may not always be the case, but it's something that you should be careful of as you enter into this phase of your project.
Edit: Now 5 years later, we are very happy with the route that we took. We found that our architect provided an additional check on our builder. In many cases there are several ways of doing things but we wanted the building to be done as noted in the plans. Our architect was on site at least once per week talking to the builder, answering questions, and requesting changes on our behalf. At times she played "bad cop" for us with our builder to get the changes that we wanted made. We got incredible value for our money so it was worth any additional headaches and work on our parts to get to our final product.