For knock throughs and extensive internal alterations, you could set a specific payment stage.
For variations, make sure that the contract defines a daywork hourly rate for various trades, and a percentage uplift to apply to materials to cover contractor profit and overheads. Any variations will be based on these rates.
The contractors should include these rates on their quote - make sure to ask for them
Then you agree the time and materials for any extra work as it arises. Ideally the contractor notifies you before hand, and estimates the cost before proceeding.
For variations, make sure that the contract defines a daywork hourly rate for various trades, and a percentage uplift to apply to materials to cover contractor profit and overheads. Any variations will be based on these rates.
The contractors should include these rates on their quote - make sure to ask for them
Then you agree the time and materials for any extra work as it arises. Ideally the contractor notifies you before hand, and estimates the cost before proceeding.