QuiltE

Exactly what I was going to suggest ... there's lots of ideas there, along with lots of pics!

Start with a dream list of what each of you would really like to have in there .... don't worry about whether it's realistic at this point. Just list the ultimates. At the same time, start saving pics of what you want as they're great to work with. You'll be surprised how many of them can get included with a little thought, and planning.

Then do a detailed to scale drawing with measurements and what is there now (ie doors, electrical, windows, etc). Should be a breeze for him on his computer! And then start adding in the must haves and the dreams .... and move them around as many times as you wish until you get the plan put together.

Some of the things that are must haves for others, you may not care for.
Likewise, some of your must haves, others may think there's no need.
So be careful to not be guided by what others suggest ... and come up with what you really want.
It's better to take some time and do it right ... than rush into it and possibly have HUGE regrets!