@Jaron Walling and @Will Fraser are correct. I just want simplify and clarify what I believe they are talking about. Most importantly get newbies to fully understand what y'all are saying in this discussion because it is great information.
You might convert the garage and gain 300-400 sq ft to your house, but if all of the other comps in your area have garage and now you don't have a garage, then you added value with the sq ft but you also subtracted value by taking away the garage. Does the value add of the sq ft far surpass the subtracted value of the garage? The comps will tell you.
Another thing is, will the appraiser you are using count that attic addition as full value or only half value? It is an attic and I've seen appraisers give some spaces half value before.
Due some due diligence, talk to appraisers, talk to your agent and find out what other houses in the area are doing. Ask the contractor which one they've had more success with when working with other clients.
I just want to give you and everybody else out there specific things to look for and to make it as simple and clear as possible.