It's important for both parties, client and agency, to have a detailed scope document to refer to. We request a brief from our clients but don't expect it to go into the detail that we need. We believe it should be our job to interpret the brief and add the detail.
Within the scope the project is broken down into phases and each phase has stages and deliverables. We explain the standard deliverables and then bespoke deliverables. Standard deliverables might include browser support back to IE9, CMS training, post launch bug fixing, Google analytics setup etc. Bespoke deliverables would be items specific to your requirements, these might include an events calendar, a press area, an image gallery etc.
Timescales with milestones are drawn up against each phase and dependencies are assigned to the agency and the client respectively.
We ask for an official sign off and a 30% deposit from new clients. This approach allows peace of mind for all, it means that the client and the agency are both inline with the project objectives, deliverables and timescales.