Defining the brief
When you first approach an architect or designer, they will want to know how you use your home and what problems you are looking to solve by taking on building work. All of this will go towards the brief, which is then referred to throughout the design process. One of the most important questions you need to answer to help define the brief is: why?
In my experience it's better to put aside the idea of "this is what I want", and focus on "this is why I want it". An architect or designer may be able to see a solution to your problem that you've never considered, and that could be much simpler or cost effective than what you originally had in mind.
There's a story I've heard that sums this up perfectly: A school approaches an architect with a design brief requesting a major extension. They say that there is currently not enough room, specifically circulation space, and when everyone is heading to lessons there are so many people that bottlenecks form. The architect looks at this for a while and comes back to them with a solution. Why not change the timetable? Staggering the time everyone is using the corridor will solve the problem, and the cost and scope of the work has been hugely reduced.
Now this is an extreme example, and I'm not suggesting problems can always be solved this simply! However it does highlight the need to address the problems directly and simply. So if you are considering an extension, think about what is driving it. Do you want another room entirely, or just more space generally? Could the extra space you want be delivered by going up into the roof, instead of extending out to the back?
You don't need an answer to all these questions before you meet your architect or design professional, but it will smooth out the process if you give it a bit of thought. Try not to get fixated on a single solution too early on, and above all be open to new ideas and ways of thinking that they might suggest. Equally, you should challenge your consultants when you don't agree or feel the project is going down a path that wasn't what you intended. Collaborative thinking and working is key to great design!