Adding a bathroom where there isn't one: conversion vs a new room

You already have a house. You do not have a toilet on the floor where you need one, or you have one bath for four people and the line starts at 7 a.m. Adding a bathroom is not a nicer vanity in a room that already has a drain. It is a new wet room: water in, waste out, a vent to the roof, an exhaust fan, and a door you can close. Those five things are the job. Tile is the last 10 percent.

This is a hire. You are not running a 3-inch soil line through a joist bay from a Saturday kit, and you are not guessing whether the closet under the stairs can take a toilet. The useful work this week is deciding whether you are converting space you already heat, or building a new room that happens to hold a bath, then putting that choice on three comparable bids.

Stay here when the problem is a missing bathroom. If you already have a bath and the issue is grab bars, a taller toilet, or a no-step pan, that is the aging-in-place bathroom budget, not this page. If you are converting a den so someone can sleep downstairs, start with a bedroom on the main floor and price the toilet in the same week so you do not buy a closet door and then discover the powder room is still upstairs.

What are you actually buying?

A half bath in a room that already exists. Toilet and sink. No shower, no tub. Closet, under-stair void, end of a hallway, or a slice of a laundry room. Angi prices a half-bath addition at $4,000 to $12,000 for a 15- to 25-square-foot space, with a dedicated half-bath page averaging $6,500 (most jobs $4,500 to $12,000, as low as $3,800, as high as $25,000). HomeGuide’s half bath with standard features is $3,000 to $10,000. Their closet or under-stair conversion is $3,000 to $7,500 for 15 to 30 square feet.

Angi: cost to add a bathroom, 2026

Angi: cost to add a half bathroom, 2026

HomeGuide: cost to add a bathroom

A three-quarter or full bath in space you already own. Toilet, sink, and a shower (three-quarter) or those plus a tub (full). You need a bigger room, a bigger drain, and a floor that can take a pan. Angi: three-quarter $6,000 to $22,000, full $10,000 to $50,000. Their conversion table for a medium full bath (40 to 50 square feet) is $10,000 to $15,000. HomeGuide conversion of existing space is $5,000 to $25,000, or $200 to $250 per square foot. A laundry-room conversion on their location table is $7,500 to $16,000 for 40 to 70 square feet, because you often already have water and a drain in that wall.

A new bathroom that is also a new room. Foundation or bump-out, new walls, new roof, then the bath. Angi prices bathroom additions at $400 to $550 per square foot, most jobs $5,000 to $75,000, national average $35,000. Their addition column for a medium full bath is $18,000 to $28,000. HomeGuide’s new bathroom addition is $20,000 to $75,000, or $400 to $500+ per square foot. That is the bump-out vs room addition conversation with plumbing on every line, not a closet with a toilet.

If Bid A is $5,500 and Bid B is $42,000, they are usually not arguing markup. A is a powder room next to the stack. B is a full bath that also builds the room.

Why does the stack decide the bid?

The stack is the vertical drain-and-vent pipe the rest of the house already uses. A toilet wants a 3-inch drain that slopes, a vent that actually pulls air, and a tie-in that the inspector will sign. A sink is a 1.5-inch line and a trap. A shower is a pan, a drain, and a waterproof floor. Distance to the existing stack is the cost driver you can see before anyone opens a wall.

HomeGuide puts rough-in plumbing at $450 to $1,800 when the run is ordinary, and says you pay more when the new bath is far from existing drainage and venting. Angi flags location and distance to existing lines as the thing that moves the whole job, and puts electrical, permits, and structural work at $550 to $14,500 on top of fixtures. Their half-bath page budgets permits at $150 to $600.

Do not let a carpenter pick the closet because it is empty. Walk the proposed room to the nearest bathroom, kitchen, or laundry, then ask the plumber whether that wall is the stack or just a supply. A closet sharing a wall with a powder room is a conversion. A closet on the far side of the living room is a new soil line, a new vent through the roof, and often a different bid class.

Basements add a second problem: the drain may sit below the sewer. HomeGuide’s basement bath conversion is $10,000 to $25,000. Their basement-bath page puts a sewage ejector (the pump that lifts waste up to the sewer) at $2,000 to $5,500 extra. “We’ll rough it in” without naming an ejector is incomplete if the floor is below the main.

HomeGuide: cost to put a bathroom in the basement

Attics and second-floor voids have the opposite problem: you can often drop into a first-floor bath, but you still have to get a vent out and a pan that will not leak into the ceiling below. HomeGuide’s attic conversion band is $10,000 to $38,000 for 50 to 150 square feet. Price leak protection as a line, not as hope.

Half, three-quarter, or full?

A half bath is two fixtures: toilet and sink. That is the guest powder room, the main-floor toilet after you convert the den, the thing that keeps people off the stairs at 2 a.m. Angi sizes it at 15 to 25 square feet. Their size table: $3,000 to $3,900 at 15 square feet, $4,100 to $4,800 at 20, $6,200 to $7,500 at 30, $8,100 to $10,300 at 35. Conversion of a small half bath (15 to 20 square feet) is $4,000 to $8,000; building it as an addition is $8,000 to $12,000. Per-square-foot on the half-bath page is about $230 to $550, depending on conversion versus addition.

A three-quarter bath adds a shower and no tub. Angi $6,000 to $22,000. That is the usual first-floor “I can actually bathe down here” product, and it is the one that turns a den conversion into a suite. A full bath adds a tub. Angi $10,000 to $50,000, 40 to 100 square feet. A primary bath attached to a bedroom is $22,000 to $90,000 on that page, which is a different building: two rooms, a door between them, and usually a layout change.

If you already have a half bath and you want a shower in it, you are not “finishing” the powder room. You are converting a half bath to a full or three-quarter, which means a pan, a valve, a bigger drain, and often a bigger room. Angi’s small-bath remodel page puts a tub-and-shower combo at $4,500 to $12,000 and a half-to-full conversion around $2,800 to $10,000 as a remodel line. That only holds if the room can take the pan. A 15-square-foot powder room usually cannot.

Angi: small bathroom remodel cost, 2026

Pick the fixtures from the day you will actually use, not from the listing photo. A main-floor toilet next to the den is a half bath. A parent who will live on this floor needs a shower on this floor. A guest suite you will rent or sell as a bedroom wants the shower in the same conversion, because adding it later means opening the floor twice.

Closet, laundry, basement, or bump-out?

Closet or under the stairs. HomeGuide $3,000 to $7,500. Headroom, a door that swings, and a toilet that actually fits with the lid up are the constraints. Many under-stair voids fail the first two. If the carpenter has to steal hallway or steal a coat closet you still need, you are in a layout change, not a $4,000 powder room.

Laundry room. HomeGuide $7,500 to $16,000. Water and a drain are often already there. You still need a toilet drain, a vent, an exhaust fan, GFCI, and a door. Do not assume the washer standpipe is a soil stack. Ask. If the laundry is the only one in the house, carving a bath out of it can leave you folding clothes in the kitchen. Price replacing the laundry space in the same week.

Bedroom or office. HomeGuide $20,000 to $38,000 for 100 to 150 square feet. That is usually a three-quarter or full bath taking part of a room, not a toilet in a closet. You lose floor in the room you cut. If the point was a first-floor bedroom, do not steal the closet you just told the appraiser was required.

Basement. HomeGuide conversion $10,000 to $25,000, or $3,000 to $12,500 on the basement-specific page for 15 to 50 square feet at $200 to $250 per square foot. Ejector $2,000 to $5,500 when the drain is below the sewer. Moisture and a legal bedroom are extra, and they belong on the finished basement and basement bedroom and egress bids, not hidden inside “bath included.”

Garage. HomeGuide $20,000 to $30,000 for 100 to 120 square feet. That is a conversion of a bay, with insulation, heat, and a floor that is no longer a slab you park on. It is not a cheap half bath.

New room or bump-out. HomeGuide extension $20,000 to $75,000+. Angi addition for a small half bath $8,000 to $12,000, small bath $12,000 to $22,000, medium full $18,000 to $28,000. If you also need the square footage for a desk or a bed, buy the room and put the bath in it. Do not pay addition prices for a 20-square-foot bump that still leaves the bedroom on the other side of the house.

What has to be on the quote?

Three bids, same written scope. Same room, same fixtures, same stack story, same finish. A closet conversion and a bump-out are not comparable.

Ask for, in writing:

1. Name the building. Conversion of existing space, or a new room. Give the square footage and the fixture count: half, three-quarter, or full.
2. Show the stack. Distance to existing drain and vent, pipe sizes, whether a new vent through the roof is included, and whether an ejector is required. “Tie into existing” without a drawing is a shrug.
3. Show supply. Hot and cold runs, shutoffs, and whether the water heater can take another bath. A second shower on a 40-gallon tank is a different conversation than a powder-room sink.
4. Show the pan or write “none.” Shower or tub, waterproofing method, drain location, and whether the floor has to drop. A half-bath bid that includes a “future shower” without a drain is a second demolition later.
5. Show electrical. Dedicated circuits, GFCI, lighting, and an exhaust fan that vents outside, not into the attic. Angi folds electrical, permits, and structural work into $550 to $14,500 on the add-a-bath page, so make the electrician a named line, not a vibe.
6. Show the envelope if you are adding volume. Foundation or cantilever, roof tie-in, insulation, siding match. That is the bump-out quote, not a vanity allowance.
7. Show the permit path. Who pulls it, what it costs (Angi half-bath permits $150 to $600; electrical, permits, and structure on the add-a-bath page $550 to $14,500), and whether the inspector wants a new vent, a tempered window, or a bigger fan.
8. Show the interior. Flooring, moisture-resistant board, tile or not, vanity, toilet, mirror, door, paint. HomeGuide fixture bands: toilet $350 to $800, sink and faucet $580 to $2,000, vanity and counter $400 to $4,000, exhaust fan $250 to $950, shower $1,000 to $15,000.
9. Show demo and put-back. What wall comes out, what floor comes up, how you get back to a finished hallway. “Owner to finish” is a different job.
10. Show the calendar. How many days you have no toilet on that floor, and whether the existing bath stays live.

If two bids match on those ten and the third is half the money, the third left the vent or the ejector off.

When the conversion is the honest job

You have a closet or a laundry slice that shares a wall with an existing bath or kitchen. The plumber can see the stack. You need a toilet on this floor, not a spa.

The room is 15 to 25 square feet and you are buying a half bath. Angi’s $6,500 average, HomeGuide’s $3,000 to $10,000 half-bath band, is the money for that job when the quote includes the drain, the vent, the fan, and the permit.

You are not trying to list a primary suite. You are trying to keep people off the stairs, or to stop a four-person house from sharing one toilet.

When you should buy a room instead

You need a shower on this floor and the closet cannot take a pan. A three-quarter bath in a 25- to 40-square-foot conversion is Angi’s $6,000 to $13,000 conversion band, or $12,000 to $22,000 as an addition. That is still cheaper than a primary suite, and it is the line that makes a main-floor bedroom actually livable.

The only place a bath fits is outside the existing footprint. Then you are in HomeGuide’s $20,000 to $75,000 addition, not a $6,500 powder room. Price the bump-out and the room addition in the same week, because a 4-by-8 bath bump with a foundation is often the same building as a small room with a bath in the corner.

The basement is dry enough to finish and the sewer is above the floor. Then the ejector is a line, not a surprise, and the bath belongs on the basement finish bid so you do not waterproof twice.

How do I decide this week?

1. Write the use in one sentence. Toilet on this floor, shower on this floor, or a listed primary bath. Those are different buildings.
2. Walk from the proposed room to the nearest existing bath, kitchen, or laundry, and ask a plumber whether that wall is the stack.
3. Measure the room with a toilet and a door swing, then again with a shower pan. If the pan does not fit, stop calling it a full bath.
4. Put the same scope on three bids: conversion or addition, fixture count, stack distance, ejector yes or no, permit, interior finish.
5. Compare the conversion number to a bump-out and to finishing a basement in the same week, because those keep the lot coverage you already have.
6. If the cheap bid is under Angi’s $4,000 half-bath floor and includes a new soil line across the house, treat it as incomplete until the missing lines show up.

A conversion is how you steal a closet and get a toilet. A new room is how you get a bathroom that did not have walls yet. Price them as what they are, then hire the one you actually need.