When determining a fee, I have always found it is best to have these questions answered first.

1. Do you have an idea of project size? Project size could have a determining factor in the amount of detail and time it would take to complete the project.
2. Do you have a project budget in mind? Budget factor must be considered as it may be required that we design accordingly, without limiting the function of the design to suit the project budget.
3. Schedule? construction start date or occupancy date. 
4. Do you have a builder / contractor in mind. If so, have you discussed a type of contract you want to pursue (fixed price vs. construction management) This will determine the amount of detail that is required in the construction document package. Fixed tender contracts usually require more information and more decisions to be made up front in order for a contractor to provide an accurate fixed price cost. Changes to the design in a fixed price contract can prove to be more costly. Construction management contracts allow for a bit more flexibility and a lot more client control in selections. Also, finished selection / millwork design decisions don't always have to be made as early on in the process as a fixed price contract. You can typically wait to walk through the space and get a feel prior to making these selection. In a construction management contract the construction manager is working for you, providing multiple quotes from sub-trades at each phase of the project. 

At the same time as I get answers to those question, it is always a good idea to outline & generate a clear scope of work so that all parties are clear on the tasks ahead.