What You Should Expect
Your builder should be happy to answer any questions as the work progresses, so make sure that you are well-informed and happy with each stage of the process.
Notify your builder of problems as soon as they arise as changes become more difficult once work is completed. Agree with the builder what changes this will mean to the original plans, adjustments to timescales and whether this will result in additional costs.
Keep a record of all correspondence and a diary of events, taking notes and dated photographs on progress and listing any problems.
If you do change the scope of the project after the work has commenced make sure that you detail the changes in writing and request that any changes to the cost/timeframe are confirmed, again in writing, by your builder.