What if the sum your construction estimates total is greater than you had budgeted? Sit down with your designer, especially if he took part in the creation of that original budget. Review the estimates with him or her and see where the unexpected overages are.
Have separate meetings with both the architect and GC. Ask each for strategies for cost reduction. The first approach should be to work within the design you have. Can certain materials be changed to less expensive ones? Is there a portion of the job that can be postponed a year, such as leaving one of the new rooms as raw space until that next baby you're thinking about having is more than simply a thought?
If none of you can devise a way to trim the existing plan, is there a logical portion of the design that can be dropped? The second bay in the garage? Do you need that extra deck off the second bedroom? When it comes to design changes, your designer is essential.
If you get contradictory advice from the contractor and architect, ask both of them to meet with you at once. Get a brainstorming session going. They can make their cases to each other, as well as you. Yours is the deciding vote.
A word of warning: Don't be penny-wise and pound-foolish, as the saying goes. Don't eliminate something very important to you just to trim the budget. That could defeat the purpose of the whole project. If you have a number of potential cuts from the project, make a priority list, and do your cutting from the bottom up.
Try to take a big-picture approach. Don't end up compromising the whole just to save the hot tub. Consider adding later what you can't afford now.