If the task is completely separate from the task that you have contractor A doing, and outside his scope of work, such as landscaping, then obviously you can engage the services of contractor B with only a "heads up" to contractor A. If the services are something that contractor A provides, and are separate from the job he is currently engaged in doing at your home, such as replacing a bath tub in a teen bathroom while he is doing a kichen remodel, then you can get a bid from A for the services, and engage B who has a lower bid to perform the service as long as it does not impact the planning and space of contractor A. It may lead to hard feelings and poor workmanship of A to work side by side with B, so your best bet is to wait until the project with A is over before beginning the project with B.

Now, if A is in the middle of a kitchen remodel and you decide that you want to add a tile backsplash to the project, that is a change order and A adds it to the scope of work. Or, you decide that you can get it done cheaper by B, and you don't even approach A for the job and wait until the project is over to engage the services of B. What you do NOT do is decide on a change to the sope of the current project and then take that business away from A and have him work along side B. You can ask him if that's his best price, but you either suck it up and have him do the change, or wait until the project is over, and have your change bid out by another contractor at that time. Many times, if the project involves tieing in with the current project, like adding the window to the kitchen remodel, the bid you got from B is contingent on the walls being open and accessible from the current project being done, and the price will go up once it becomes a complete separate job. In that case, because of the tied together nature of the project, A should be the one to perform the work.

But, your contract will control how change orders are handled when initiated by your contractor or by you. Read your contract, and then make nice with your current contractor. It's usually easier in the long run unless they're totally incompetent. If that's the case, suck it up and do the legal steps to sever the relationship. No second contractor wants to be in the middle of a conflict between a homeowner and a contractor unless everything has been legally settled. And, even then, they are wary of taking any jobs from someone who has had to resort to legal means to terminate a previous relationship.

OK, but why can't I nail down a flat price to change the window out with Contractor A? Is that usual? He's just giving a vague estimate based on time and materials, but says it can change based on unforseen circumstances even though the drywall has been ripped out and the jambs and header are visible. Thanks.

If the window you want to install is a different size than the one you are considering replacing, he may run into issues matching exterior siding, or replacing the header with a larger one, etc.You will also get different prices depending on the brand of window and whether or not you source it or he sources it. Part of his profit is the markup on materials as well as the labor of his subs, as well as his own role as supervisor. You've gotten an estimate, right? Did you actually tell him, "OK, I do want to proceed with the project, let's add this to the contract with some hard numbers." He may be under the impression that you're shopping and second guessing him to death on this project. It doesn't sound as though you have the best communication going on. Take steps to rectify that. Why not ask him what he's talking about in his "unforseen circumstances" and a time line for adding the change order to the project.

You're right about that, today he said he can get hard numbers for me. Thanks.