Convert existing space vs build an addition
The cheapest way to add a bathroom is to carve it out of existing space near current plumbing, such as a closet, hallway, or corner of a large room. That keeps the structure intact and shortens plumbing runs.
Building a bathroom as part of a new addition is far more expensive because you pay for the addition shell (foundation, framing, roof) on top of the bathroom itself.
What drives the cost
Beyond location, the price depends on:
- Distance to existing water supply and drain lines
- Half bath vs full bath with a tub or shower
- Finish level of fixtures, tile, and vanity
- Whether you need a new addition or are using existing space
