What is the budget you have to work with? My art studios have been everything from the spare bedroom to a tent out in the backyard.

This is not a very tough call. Build the studio under two circumstances: (1) your art generates enough profit already to cover the cost of building the studio, or (2) you have that money from another source, can responsibly spend it on your art rather than something else, and don't have any other hobbies you'd rather spend it on. If neither of those circumstances is true, either go paint in a studio outside your home or buy some good artificial light bulbs with a 90+ CRI rating and 5000K- 5500K color temperature. It will give you more consistent light than any north window anyway.