Which contract are we talking about?
Design and permit? Delivery of the modular unit? Site preparation, foundation, trenching and utilities connections? Full scope from design to move in?
Let us start at design stage. This means you are working on a custom ADU (you skip this step with most modular builders). Who is responsible for surveys, easements, boundaries? Who gives direction to the structural engineer? Who is paying for printing the permit set and who is getting it delivered to the Building department?
Are you collecting estimates yourself? Or is your architect or designer evaluating them for you?
Who is making sure the material allowances and schedules are reasonable?
Have you accounted for the process if you need a change order? Who is making sure all your tile, fixtures and appliances selection is arriving on site on time?
There is a lot of detail that goes into a construction project, event if it is a small cottage in the backyard. Make sure you think it through, and it gets reflected in the contract.
Read more on scope here.