The feasibility layers, in decision order
Drainage rules the ranking. New fixtures need drain diameter, continuous fall to the existing system, and venting that reaches air; toilets are the strictest clients. Spaces above or beside existing wet walls inherit that logic cheaply, while spaces across the house pay per foot in demolition, framing, and restoration; below-grade candidates add pumping systems with their own power and service needs.
Water, hot water, and electrical form the second layer: supply reach is usually manageable, but a new bathing room tests the water heater's real capacity, and a new bath's fan, lighting, outlet protection, and possible floor heat want circuit capacity that older panels may not spare. These are quotable facts, not mysteries; get them into the feasibility file early.
The livability layer decides among survivors: sound and sight privacy for the new room and its neighbors, a ventilation duct path that actually reaches outdoors, ceiling height including under-stair geometry, and what the borrowed space costs the house, a lost closet is a real price in a small home. Approvals ride throughout: added bathrooms are exactly the scope local review processes exist for, and the paper trail protects the investment at resale.
