Well now, if ya hadn't spent the $$ for the concrete pad and 12x12 out-building in the backyard with the ventilator, garden hose fed sink with drain attachment to a large roll away motor home septic tank, etc. paying for all that dual time...

Best bet in your situation if you have the exterior space is to acquire and old (you can find them free) travel trailer. Gut it down like a mobile office trailer and put counters and cabinets in as needed. Most will have the plumbing for hose hookup, septic holding tank and electric.

If ya do it right, it can even remain mobile so you can take the darkroom with you on weekends to the mountains or ???