You should have a budget in mind.
Gathering quotes will help you refine your plans.
You'll start with a Cadillac of a garage/workshop/gabled roof with storage/attached to your lovely home.
As you see the prices roll in, you'll keep your 'to have ideas' and the frivolous ones will be discarded.
One thing I will advise you on....and listen carefully....
Make a wish list
Draw a floor plan to the best of your abilities
Include only critical dimensions for Design Intent (i.e.) 24x24 garage, 12'x24' workshop, etc
You don't need to bog this down with a hundred detail dimensions.
Have some photos of garage/workshops that YOU want it to look like.
When you have ALL this put together make several copies and give each contractor one to give you a quote.
If you do not do this, you will get 3 guys that will design 3 different garage/workshop ideas and you won't have ANY idea if one is higher or lower than the other.
That is how I like to see it done.
But that's just me as a contractor/home owner