Okay, so most of you won’t have a standing structure to contend with (lucky you!) but after clearing the site you’ll be able to start imagining what your garden room might look like.
Try sticking a few bamboo sticks in the corners and connect them with string to get a better feel of the footprint of your building. Is it big enough for what you wanted and the right shape? Where will the table saw/sofa/drum kit/running machine go?
You don’t have to have a full design in your head at this stage but try to think of what the site will allow, or not allow you to do.
Now take some measurements to work out the rough size you have to play with (length x width = m2). You’ll need this for the next step which is looking at whether you’ll need planning permission or building regs.