Adding a Second Bathroom to a King County Rental: Costs and ROI | Valta Homes Blog
Renovation & Construction
Adding a Second Bathroom to a King County Rental: Costs and ROI
Adding a second bathroom to a King County rental: real Seattle-area costs, permit rules, and how to tell whether the ROI actually justifies the project.
Most rental upgrades make a property nicer. A second bathroom makes it a different listing. When you turn a two-bedroom, one-bath into a two-bedroom, two-bath, you have not just improved the unit. You have moved it into a higher rent bracket and widened the pool of tenants who will even look at it.
We manage repairs and renovations for landlords across Bellevue, Issaquah, Mercer Island, Kirkland, and the broader Eastside, and adding a bathroom is one of the few projects where the math is worth running carefully before you spend a dollar. Done in the right house, it pays back. Done in the wrong one, it is the most expensive way to add a room you will ever regret. This guide walks through what it costs in King County, where to put the new bathroom, what permits you need, and how to tell whether the return justifies the work.
If your existing bathrooms are simply dated rather than too few, this is the wrong project. Read our breakdown of kitchen and bathroom remodel ROI for rentals instead. Adding a bathroom is about changing the bedroom-to-bath ratio, not refreshing what you already have.
Why a Second Bathroom Changes the Rental Math
Renters filter by bedroom and bathroom count before they read a single line of your listing. A household sharing one bathroom feels the squeeze every morning, and a unit that forces that compromise sits longer and attracts more price-sensitive applicants. Adding a second bathroom removes that objection entirely.
It also broadens who applies. Two unrelated roommates, a couple working opposite shifts, or a small family with a teenager will pass over a one-bath rental and pay a premium to avoid the daily bottleneck. The extra bathroom is not a luxury feature to most of these tenants. It is the deciding factor.
The resale side matters too, even if you plan to hold. An added bathroom raises the bathroom count an appraiser records, and bathroom projects are among the more reliable value-adds in a remodel. Industry data puts the average return on a midrange bathroom project around 60 to 70 percent of cost at resale, with one widely cited figure showing about a 72.7 percent return on a bathroom remodel (Zillow). That return climbs when the project changes the bath count rather than just updating finishes, because it moves the property into a new comparison set.
What It Actually Costs to Add a Bathroom in King County
This is where landlords get surprised. Adding a bathroom is not the same line item as remodeling one. You are building plumbing, drainage, electrical, and ventilation from scratch, and often framing a new space to hold it all.
In the Seattle and greater King County market, adding a bathroom typically runs $25,000 to $65,000, depending on how far the new fixtures sit from your existing plumbing stack, the finish level you choose, and whether the space already exists or has to be built (, ). The single largest cost driver is distance from the existing drain and supply lines. A bathroom backed up against an existing wet wall is far cheaper than one stranded on the far side of the house.
Heat Pump Conversion for King County Rentals: Costs, Rebates, and 2026 ROI
Heat pump conversion costs $7,500-$11,000 in King County, but PSE rebates of $1,500+ and the cooling demand of hotter Seattle summers can make it pay off for landlords. Here is the 2026 ROI math.
Rough-in plumbing for a bathroom averages around $8,470 nationally, and that is before finishes (Einstein Pros). Running new lines a long way, or breaking a slab, pushes it higher.
Licensed plumbers in the Seattle area charge roughly $95 to $145 per hour for residential work (Einstein Pros).
Construction labor across King County runs an estimated 25 to 40 percent above national averages, so cost guides written for the country as a whole understate what you will actually pay here.
For a clear-eyed comparison against your other options, our guide to renovations that increase rent in Seattle ranks where a bathroom addition sits next to kitchens, flooring, and in-unit laundry.
Where to Put the New Bathroom
The cheapest bathroom is the one you do not have to frame. Before you draw plans, look for space the house already gives you.
Convert existing square footage. A large closet, a wide hallway, an oversized primary bedroom, or the dead space under a staircase can often hold a half-bath or a compact full bath. You avoid the cost and permitting of an addition, and you keep the footprint the same.
Borrow from an oversized room. Carving a bathroom out of one end of a generous bedroom is common, and as long as the remaining bedroom still meets minimum size and egress requirements, you preserve the bedroom count while adding the bath.
Finish a basement. If you own a house with an unfinished basement, adding a bathroom down there is frequently part of a larger project that adds real rentable living area. The plumbing is often easier because basement floors give direct access to drain lines. We cover the full economics in our basement finishing ROI guide for rentals, and a basement bath frequently pairs with adding in-unit laundry since the rough plumbing overlaps.
Wherever the bathroom lands, position it near existing plumbing if you possibly can. Stacking a new bathroom above or below an existing one, or backing it onto an existing wet wall, is the single most effective way to control cost. Our plumbing service team scopes this proximity question first on every addition we price, because it sets the ceiling on the whole budget.
Half-Bath or Full Bath: Which Makes Sense for a Rental
A half-bath, which is a toilet and sink only, costs far less because it skips the tub or shower, the larger drain, and most of the waterproofing. It still removes the morning-bottleneck objection and still bumps the bathroom count on the listing.
For most one-bath rentals, the decision comes down to the unit type:
A second full bath makes sense in a three-bedroom or larger house, or any unit where two separate households or generations share the space. Tenants paying for a true two-bath home expect two showers.
A half-bath is often the smarter spend in a smaller two-bedroom, a townhome, or a unit where space is tight. It captures most of the convenience value at a fraction of the cost, and it converts a one-bath listing into a 1.5-bath listing, which still reads as an upgrade.
If you are weighing the bathroom against simply modernizing what you have, run both numbers. Sometimes a sharp paint and fixture refresh plus new flooring delivers a better return than a costly addition, especially in a neighborhood where rents are capped by what comparable units fetch.
Permits: What King County and Seattle Require
Adding a bathroom is permitted work, full stop. Anytime you install, relocate, or change a plumbing system, Seattle and King County codes require a plumbing permit (King County Public Health). New fixtures and new drain lines, which is exactly what a bathroom addition is, fall squarely inside that rule.
A few specifics worth knowing before you budget:
Plumbing permits run through Public Health, not the city building department. In King County, plumbing and gas permits are issued through Public Health, Seattle and King County, separate from the main building permit (King County Public Health).
The base plumbing permit fee is modest, around $165 for a typical residential job, with larger valued projects costing somewhat more (CRD Design Build).
Older drains may need upgrading to code. Many older Seattle-area homes have 1.5-inch tub drains, but current code calls for a 2-inch drain, so tying a new bathroom into old plumbing can trigger upgrades (CRD Design Build).
Small fixture swaps can be exempt. Simply replacing a vanity, sink, and toilet under roughly $6,000, with fixtures connecting to existing lines and no electrical changes, may not require a permit (CRD Design Build). An addition never qualifies for that exemption, because you are creating new connections.
Do not be tempted to skip the permit to save a few hundred dollars. Unpermitted plumbing can surface during a future sale or refinance appraisal, force you to open finished walls to inspect, and void coverage if a leak causes damage. It is the kind of deferred shortcut that costs landlords far more later.
The ROI Math: Does a Second Bathroom Pay Off
Here is how we tell a landlord whether to spend the money. Run two numbers side by side.
First, the rent lift. Pull comparable listings in your specific submarket. Find what a two-bed, one-bath rents for in your neighborhood, then what a two-bed, two-bath rents for. The monthly gap is your annual return engine. Multiply the monthly difference by twelve, then divide your project cost by that annual figure to get a rough payback period in years. A $35,000 addition that adds $300 a month pays back in under ten years on rent alone, and faster once you factor reduced vacancy.
Second, the resale and appraisal value. Even if rent alone does not pencil out quickly, the added bathroom count raises what the property appraises and sells for. Midrange bathroom projects recoup roughly 60 to 70 percent of their cost at resale on average, with bathroom work cited near a 72.7 percent return (Zillow). A project that changes the bath count tends to land at the higher end of that range, because it reclassifies the home rather than just refreshing it.
The honest answer is that the return depends entirely on your submarket. In a neighborhood where two-bath units command a clear premium and vacancies are short, the addition is one of the strongest moves you can make. In a market where renters will not pay meaningfully more for the second bath, you are over-improving, and your capital is better spent elsewhere. We always model this before recommending the work, and we are comfortable telling a landlord not to do it.
Once the bathroom is in, the upgrade also gives you a clean, defensible reason to reset rent. Our guide to raising rent without losing good tenants covers how to frame a genuine improvement so renewals go smoothly.
How We Phase the Work
A bathroom addition touches several trades in sequence, and the order matters as much as the individual quality. Rushing one stage to start the next is how leaks and failed inspections happen.
Design and permit. Lock the location, confirm the plumbing path, and pull the plumbing permit before anyone swings a hammer.
Framing and rough-in. Frame the space, then rough in the drain and supply lines, electrical, and the exhaust fan ducting. A properly vented fan is not optional in our wet climate, and it ties into the same energy efficiency thinking that protects the rest of the unit from moisture.
Inspection. The rough-in is inspected before walls close. This is the checkpoint that protects you.
Waterproofing, finishes, and fixtures. Tile or durable flooring, waterproof the wet areas, hang the vanity, set the toilet, and connect the fixtures.
Final inspection and turnover. Final sign-off, then paint and detail before the unit goes back on the market.
Coordinating those trades is the hard part for a remote or busy landlord, and dropped handoffs between trades are where timelines slip. If you are managing this yourself, our guides on vetting contractors and handling contractor delays will save you grief. Our kitchen and bathroom remodeling crews run this sequence as a single managed project so you are not chasing five separate contractors.
Mistakes We See Landlords Make
Putting the bathroom far from existing plumbing. Chasing the "perfect" location across the house can double the rough-in cost. Let the plumbing stack guide the floor plan, not the other way around.
Over-finishing for the neighborhood. A rental bathroom does not need imported stone or a designer vanity. Tenants want function, cleanliness, and durability. Spend on a quiet exhaust fan, a solid-surface floor, and fixtures that survive turnover, and skip the luxury finishes that a luxury bathroom project's lower return reflects.
Stealing too much from a bedroom. Carve a bathroom out of a bedroom and shrink it below minimum size or block its egress, and you have traded a bedroom for a bathroom rather than added one. That usually lowers value instead of raising it.
Skipping the permit. Covered above, and worth repeating. Unpermitted plumbing is a liability that follows the property.
Treating it like routine upkeep. A bathroom addition is a capital project, not a repair. Budget it as one. Our annual maintenance budgeting guide keeps capital projects from colliding with the ordinary repair-or-replace decisions on aging systems.
A Practical Budget Framework
For a landlord with one to three properties, here is how we suggest sizing the decision:
Confirm the rent gap first. If two-bath comparables in your submarket do not rent for meaningfully more, stop here.
Find the cheapest viable location. Near existing plumbing, ideally in space the house already has.
Budget $25,000 to $65,000 for a full-bath addition in King County, less for a half-bath, and add a 15 to 20 percent contingency for the surprises older homes hide behind walls.
Include soft costs. Permit fees, design, and a possible drain upgrade to code.
Compare against alternatives. If the numbers are close, a focused refresh of the existing bathroom may win.
When It Is Worth It, and When to Skip
Add the bathroom when your unit has three or more bedrooms sharing one bath, when two-bath comparables clearly out-rent one-bath units in your neighborhood, and when you can place the new bathroom near existing plumbing. In those cases it shortens vacancies, widens your applicant pool, and raises both rent and resale value.
Skip it when the rent gap is thin, when the only viable location is far from existing lines, or when shrinking a bedroom is the only way to fit it. In those cases your money works harder on flooring, paint, appliances, or simply keeping the property well maintained.
We have run this analysis for landlords on both sides of the answer, and the discipline is the same every time: prove the rent gap, control the plumbing distance, permit the work, and build for durability rather than show. Get those four right and a second bathroom is one of the most reliable ways to move a King County rental into a higher bracket.
If you want help deciding whether a bathroom addition pencils out for your specific property, our team can model the rent gap and scope the plumbing before you commit. Learn how ongoing project management works through our membership, or contact us or call (425) 800-8268 to talk through your property.
