This doesn't account that software is (relatively) easy to change, vs say building a house (to use a cliche). The client will want to see it as it progresses, a

This doesn't account that software is (relatively) easy to change, vs say building a house (to use a cliche). The client will want to see it as it progresses, and then change based on what they have learned is possible. This is the agile way. How would one account for a fixed cost then? Do you say ok it's a fixed cost, and you get 6months base on our estimate (but if the 6months has finished the contract ends, and isn't that just a time based project). Or do you tell the client no changes after the initial requirements (which doesn't sound flexible and you end up building something the client doesn't want)?