Decide on equipment. You will likely already be recording prior to the decision to build a studio. Make a list of the equipment you have and the equipment you will need. Later steps in your plan may cause you to revise this list, and after you've completed your studio, you will identify new needs and wants.
Evaluate your space and compare it to your needs. If you are planning to work with loops and beats, with few instruments recorded, you may be fine with a single room with small square footage. If you expect to be recording others, two physical spaces with some degree of sound proofing between them makes for best results. Don't try to put too much in too small a space however, as many instruments need room for sound to develop.
Consider acoustics and noise in your planned space. Sharing a room with a noisy furnace is of course less than ideal. Windows are another potential source of noise leakage, both in and out. Parallel walls can be a problem acoustically, creating standing waves and comb filtering, though relief can be had with acoustic treatment. If you have the luxury of an unframed space, research methods and dimensions for construction to reduce sound transmission and acoustic problems.