What kind of daily functions will it serve? Will it be in the TV room, family room or formal living room? A formal sofa with rolled arms and firm cushions or a casual sofa with deep seats? Will you be mostly sitting on the sofa or do you plan to lay down and take naps too? If so, you might want to consider a sofa with an add-a-chaise. Do you have pets and/or kids? Knowing all of this beforehand will help you decide which style of sofa bests suits you and your lifestyle.
Have photos of the space handy. Take pictures of the space where you intend to put the sofa. It’s good to have at least 4 pictures that give a sense of the room. This helps the designer better assist you in choosing the right color and style of your sofa. Is there a rug in the space? If so take a detailed picture of the rug so you can decide which color fabric or leather will compliment your space.