Grab a copy of "The conditions of contract", definately need this to study. Anyway,

1. The principle wants to change the character or quality of any material or work.
2. The principle wants to change the line, level, postion or dimension of any part of the contracts work.
3. The principle wants additional work to be carried out by the contractor.
4. The principle wants additional demolition carried or removal of any material or work.

Grab a copy from NZAA. Helped me a lot!!

thanks . big help!!