Whatever you decide to do to improve your home needs to be practical. Enlarging the master bedroom is a reasonable consideration but considering the space above the garage an additional bedroom space with access through the master, in our opinion, would not be beneficial. This arrangement could result in a 300 sf. closet area....a bit excessive. Extending the existing master to include this additional space is a more practical approach.
Take caution.....most 300 sf areas above the garage are not constructed to handle the weight of a normal living space...you should consult your builder on this.
Our recommendation for making the best decision is to view it less from the perspective of what it is going to cost and more from the angle of how is this project going to impact the quality of life for our family.
Remax Country Properties
Amherst, NH 603-732-1834