Both alteration and variation refers to some type of change.
Alteration is very often a physical change in a property item.In lease contracts, it refers to the right of the tenant to make changes in the physical layout of the leased premises.
Variation is used in contracts for works, meaning a change in the scope of the works to be completed. It must be initated by one of the parties to the contract. It is necessiated by a technical requirement or simply the intention of the parties. In practice, this requires a modification of the contract, too.
