Plans

By this stage you will have a set of plans and ideally already have planning permission. Your architect should provide you with at least a couple fo sets and you should keep one safe and as a master, and use the other set to get more sets copied.

These plan will show your extension in a format that planners, and builders should understand and be able to use to approve planning permission and your builder should be able to price the job from them, combined with other information in the tender package.