Electrical: Put white light switches six feet from the floor; safe light switches 3 feet from the floor. Use GE 24v switches to control 120V solenoids and to control 120 volt lights. The advantage is you can have many switch locations (i.e. 5+) to control the lights and lessens chance of shock. Have plenty of incandescent lights. NO FLORESCENT LIGHTS! They can glow.
Plumbing: Hot, cold, and filtered/tempered water faucets. Two sinks. One for tray processing and a slop sink for mixing and washing.
Wall and ceiling color: WHITE to bounce safelight illumination around.
Counters: separate wet and dry areas.
Ventilation: Fresh air supply and exhaust (over the sink) with a remotely located low speed fan so you don't hear it in the darkroom.
I have all this in a 6 x 12 space: 6 foot sink, 2 foot sink, and two separate 6 foot dry counters. One with a drop easel.