#1 Go as big as you can.
#2 Do a mock-up of the darkroom (sheets for walls, boxes for sink and counters etc.)set up the enlarger if you can and role play developing and printing processes. This will give you an idea if the size and layout are ok.
#3 Run comercial grade wireing in conduit on the outside of the walls. This will let you change things later without cutting into the walls.
#4 Place one light switch by the door and another by the sink (unless sink is by the door) with a light over the sink to view test prints. Running from the sink to the light over and over gets old fast.
#5 Good ventelation.
#6 Use an exterior prehung door instead of an interior door. It will be more light and air (dust) tight than an interior door. Mount it so it opens out. They run about $120.00 from home depot but find one with a dent and have a manager give you a good discount to take it off his hands. Damaged doors are hard to sell.
#7 Good stereo. Fiber prints dry better to jazz. I have no proof of this but that's my story.
#8 When you have the mock-up done invite other local photographers to see it and give you their opinion. It is easyier to change tape and cardboard than stud and drywall. When we did this in my last darkroom we changed the door, location of the enlarger, height of the sink, several outlets and several other small things.
#9 Visit as many darkrooms as you can. Ask amatures, pro photographers and labs if you can see their darkrooms. You might even pickup some gear.
Good luck