A Needs Assessment and a Feasibility Study are almost exactly the same when considering how it meets the "external needs". These external needs might be of a potential lessee who is looking at your rooms to rent, or a banker who is looking to fund your property. Both will tell you about the local area, the population, the general rent range and the fundamental numbers of the practice.
A Feasibility Study is usually requested by corporates and will ensure that they understand the market forces shaping demand for their services in your area, as well as specific challenges they may have to account for in their operations.
A Needs Assessment is more business owner-focused, an analytical and qualitative exercise of the dynamics influencing the operations and the business. It is shaped to answer the business owner's questions. It includes an outline of costs, analytics on property and location. Sample questions that business owners have asked are, what will need to change in the physical property for this to become a dental suite?, or what has demand for the services dropped and what can I do? I've noticed less paediatric patients in this area and this might affect my practice - what should I do?