My parents added onto their home for her painting studio in the 1980's. It also made the kitchen bigger as well on the upper floor. It was one of the best changes to the house they ever made and have been really happy with it. Because she's a painter she included a sink (for washing up) and a built in light box on a counter. It's been the family hang-out for projects for years!!
Probably the biggest challenge with the room is heating. She has to use space heaters under the desk to keep her feet warm. If they were doing it now, there would be many more heating options available.
My husband and I built a home about 10 years ago, and I insisted on lots of built in lightning (even in the main living areas). I am so GLAD that I did. Various types of lighting: overall room ambiance and task lighting on different circuits and switches. Spending time with the lightning is really worth it!!! I also put in quite a bit of built in cabinetry and storage. If I was building a sewing room from scratch, I would include kitchen cabinets and kitchen height cutting tables with cabinetry underneath. Ooooh la la!
Enjoy! Will be fun to watch your progress!