The cost of building an addition, including simply enclosing a garage, in Austin is between $70 to $120 per Square Foot. This all depends on the contractor you hire, the finish out options, and other items such as electrical.
The cost to install a bathroom can be upwards to $12,000 for a simple bathroom. Depending on the size of the bathroom and finish out for a very simple bath space, it would be between $5,000 to $8,000.
Most homes in the Crestview area have concrete foundations, and could be a Post-Tensioned Concrete Foundation. There is additional cost for cutting into the concrete to reach and connect with the existing plumbing.
Since a good portion of the homes in Crestview have enclosed garages, then you wouldn't loose value and would be able to compete when you decide to sell your home. However, Crestview is an area experiencing some growth due to its ease of access, so it might be worthwhile to keep the single garage (as garage spaces are desired by most buyers) and invest the money into an addition. It may be more cost affective to add an addition to your attic area or off the back of the house, especially if you want an edge when you ever decide to sell the house.
A good contractor or Architect can assist you in determining the best approach. I encourage you to make sure you hire a good contractor.
I hope this information has helped.
Shannon T. Schmitz,
Representing Distinctive Properties