Half bathroom addition cost

Adding a half bathroom costs $5,000 to $25,000, typically around $12,000. A powder room built next to existing plumbing in an unused closet is at the bottom. One in a basement, or on the far side of the house from any drain, is at the top.

A half bath is a toilet and a basin. No shower, no tub, no waterproofing project. The cost is almost entirely about how far the drain has to travel.

What it costs

Space, and the minimums that matter

Building code minimums are smaller than people expect and smaller than is comfortable.

Thirty inches wide by five feet deep is a legal powder room. It is also a room you turn sideways in. Thirty-six by five is noticeably better and costs nothing extra in plumbing.

Under a staircase, check the headroom over the toilet specifically; the code minimum applies where you stand and where you sit.

The drain decides the budget

Waste needs fall, usually a quarter inch per foot, and it needs venting. Both are what make a bathroom expensive far from an existing stack.

Near an existing stack, on a floor with accessible joists below, is the cheap case: tie in and go.

Far from the stack means running new waste in the floor structure, which may mean notching joists in ways that are not permitted, or dropping a soffit in the room below.

Below the sewer line, which is most basements, means gravity does not help. The options are a sewage ejector pit with a pump, or an upflush macerating toilet that grinds and pumps waste up to the existing line. The ejector is the more permanent answer; the upflush is cheaper and easier to retrofit and has a motor that will eventually fail.

Is it worth it

A second toilet in a house with one is one of the most valuable additions there is, in daily life and at sale. Estate agents consistently rate going from one bathroom to one and a half as one of the higher-return changes to a house.

It is worth less if you already have two or three. And it is worth nothing if it comes out of a room the house needed: taking a bedroom's worth of space for a powder room reduces the bedroom count, which is the number buyers filter on.

Frequently asked questions

Where is the cheapest place to put one?

Backing onto an existing bathroom wall, sharing the same stack, on a floor where the joists are accessible from below. A closet on that wall is the ideal candidate.

Do I need a window?

No, but you do need ventilation. A fan ducted to the outside satisfies code everywhere and is better than a window anyway. Ducting into an attic is not ventilation and is how attics get mold.

How long does it take?

Two to four weeks near existing plumbing, four to six if the drain has to travel or the slab is being cut. The inspections at rough-in and final add waiting days.

Is an upflush toilet a compromise?

It works, it is noticeably louder than gravity, and it has a macerator that fails eventually. For a basement or a garden room where the alternative is cutting concrete and fitting an ejector pit, it is a reasonable trade. As the only toilet in a house, it is not.

Do I need a permit?

Yes, essentially always, because it adds fixtures to the drainage system and circuits to the panel. An unpermitted bathroom is one of the clearest things to surface during a sale and one of the hardest to retrofit approval for.