As an Interior designer that works with a lot of independents I get a varied range design briefs from clients that haven’t had to write one before. Sometimes they focus on the look of a project and sometimes the function. I have also had design briefs that are all about the food or the wine / coffee a client wants to sell / serve. The most important ingredient is that the client is passionate and knowledgable about what they want to do.
It is also a good idea to write your design brief down, this starts to bring the projects to life just by confirming everything on the page. You can then show it to colleagues / friends and get their opinion. Once your happy with it you can show it to all of the other people that will help you get your project off the ground, this will range from a prospective landlord to an Interior designer. Along with your business plan the design brief is one of the most important elements of a new project to consider.