Obviously, a half bath needs running water as well as drainage and ventilation. For budget reasons, try to locate your new half bath as close to existing water and sewer connections as possible. One of the cheapest options is to place it where it can share a wall with an existing bathroom. For older houses built on piers or over basements, new plumbing lines can be placed below the main floor fairly easily. Houses built on slab foundations, though, will require some jackhammering and patching of the slab to connect existing plumbing lines to the new space, which adds to the project's cost.
Shown: Where possible, fixtures, including sinks, should be hooked to existing plumbing lines. The good news is because powder rooms are used less frequently, you can choose more delicate (and often more visually appealing) fixtures. Here a Waterworks
vessel sink is filled by a wall-moounted brushed nickel faucet.