One reason lawyers have difficulty charging flat fees is they don’t know how to deal with the uncertainty. Uncertainty that arises because they don’t really know how much time they will spend because of the unknown actions and extra work that might result because of this particular client’s needs or demands.
Use Change Orders, similar to those used in the construction industry, to help with some of the uncertainty. You’ll have to spend time planning the scope, structure, and systems that your practice will need to accommodate both the flat fees and the change orders. But you’ll find the effort more than worth it in the long run.
Start with a Fee Engagement Agreement that clearly sets out the scope of your work with sufficient detail to earn the flat fee. Include your Change Order form for additional work as an exhibit to your Fee Engagement Agreement. Then have systems in place to remind you to have your clients agree to the additional work at the additional fee before you do the additional work.