Before comitting to a project I would offer a discovery session where we will discuss the project in depth to better understand your requirements.

Depending on the complexity of the project, I would then purpose a rough estimate from the information understood in the Discovery session.

Once we are both happy we'd like to continue, project planning would begin on an hour rate where we will discover the full extent of the WebApp together.

A full project plan and quote will then be delivered.

Once both parties have accepted, the WebApp would be developed according to the project plan (with oppertunities to review and make changes where nessecary).