If you don’t like the look of the washed fabric, the finished costume will have to be dry-cleaned later.
Pro tip: most fabrics can be washed if you do it in a sink / bathtub, with lukewarm water and a mild detergent, and no spinning or tumbling. Test it on a fabric swatch first and compare the result with the unwashed fabric if you’re nervous (I always test-wash expensive silks). If you don’t like the look of the washed fabric, the finished costume will have to be dry-cleaned later.