Bathroom Installation
A properly installed bathroom is critical for both function and longevity. The most important aspect is waterproofing - failed waterproofing can lead to mold, rot, and expensive repairs. Get this right the first time with proper materials and techniques.
Quick Summary
Timeline
1-2 weeks
Budget
$6,000-12,000
Critical step
Waterproofing
Why Proper Bathroom Installation Matters
Bathrooms are the most failure-prone rooms in any building due to constant exposure to water. A bathroom that fails can cause thousands in water damage, create health hazards from mold, and require complete tear-out and rebuild. Proper installation includes:
- Complete waterproof membrane behind all tile surfaces
- Proper slope to drains (1/4 inch per foot minimum)
- Adequate ventilation to prevent moisture buildup and mold
- Quality fixtures that will last for years of rental use
How to Install an ADU Bathroom
Plan the Layout
ADU bathrooms are typically 35-50 square feet. Plan fixture placement to maximize usable floor space and ensure code-required clearances.
- • Minimum 30 inches wide for toilet area (21 inches from center to wall)
- • 24-30 inch wide vanity is typical for ADU bathrooms
- • 32-36 inch wide shower stall or 60-inch tub/shower combo
- • Door swing should not hit fixtures (consider pocket door)
- • Locate plumbing on shared wall with kitchen if possible to reduce costs
Choose Shower or Tub Configuration
Decide between a shower-only design or tub/shower combo. Each has advantages depending on your target tenant and available space.
- • Shower only (32x32 to 36x48 inches): More floor space, easier access, modern feel
- • Tub/shower combo (60x30 inches): Appeals to families, soaking option, traditional
- • Prefab shower pan: Faster install, guaranteed waterproof ($200-600)
- • Tile shower: Custom look, requires expert waterproofing ($800-2,000+)
- • Alcove tub: Standard 60-inch fits most spaces ($150-400)
Pro tip: For rentals, a tub/shower combo appeals to the widest range of tenants. For aging-in-place or accessibility, a curbless shower is ideal.
Waterproof Everything Properly
This is the most critical step. Waterproofing membrane must cover all surfaces that could get wet, extending at least 6 inches beyond the shower/tub area.
- • RedGard: Liquid-applied membrane, rolls on like paint, dries red ($100-150 per bathroom)
- • Schluter Kerdi: Sheet membrane, adheres to cement board ($200-300 per bathroom)
- • Cement board: Use Kerdi-Board or cement board substrate, not green board drywall
- • Waterproof the shower floor, all walls, curb, and niches
- • Test waterproofing before tiling (flood test or inspection)
Critical: Never skip waterproofing. Water penetration behind tiles causes mold, structural damage, and costly repairs. This is not where to save money.
Install Tile or Surround
Tile provides the most durable and customizable finish. Prefab surrounds are faster but look more basic.
- • Ceramic/porcelain tile: Durable, many styles, $3-15/sq ft plus $8-15/sq ft install
- • Large format tiles: Fewer grout lines, easier cleaning, modern look
- • Subway tile: Classic, affordable ($3-5/sq ft), timeless appeal
- • Prefab surround: Quick install, $200-800, looks more budget
- • Use unsanded grout for joints under 1/8 inch, sanded for larger
- • Seal grout after installation to prevent staining
Select and Install Toilet
Toilets are standardized and relatively easy to install. Choose a quality, water-efficient model that will hold up to rental use.
- • Standard height: 15 inches, traditional comfort
- • Comfort height: 17-19 inches, easier for adults, ADA compliant
- • Elongated bowl: More comfortable, takes 2-3 inches more space
- • Round bowl: Saves space, good for tight bathrooms
- • Look for WaterSense label (1.28 GPF or less)
- • Budget $150-400 for quality toilet, $50-100 for installation
Install Vanity and Sink
The vanity provides storage and defines the bathroom style. Choose a size appropriate for the space with a durable countertop.
- • 24-30 inch vanity: Standard for small bathrooms ($200-800)
- • Wall-mounted vanity: Makes floor space feel larger, easier cleaning
- • Vanity top options: Cultured marble (budget), quartz (durable), solid surface
- • Integrated sink: Easier to clean, no seams, modern look
- • Include soft-close drawers and doors for quality feel
- • Add a medicine cabinet with mirror for storage and function
Install Ventilation
Proper ventilation is required by code and essential for preventing mold and moisture damage. The exhaust fan must vent to the outside.
- • Minimum 50 CFM for bathrooms under 100 sq ft
- • Quiet fans (less than 1.0 sone) are worth the extra cost
- • Must vent to exterior, not into attic or wall cavity
- • Consider fan with humidity sensor for automatic operation
- • Budget $50-200 for fan, plus installation and ductwork
- • Combination fan/light units save ceiling space
Note: Venting into the attic (not through the roof) is a code violation and causes moisture damage. Always vent to exterior.
Budget Breakdown
Common Bathroom Installation Mistakes
Skimping on Waterproofing
Using only cement board without a membrane, or not extending waterproofing far enough, leads to water damage. Waterproofing costs $100-300 but prevents thousands in repairs.
Improper Shower Slope
The shower floor must slope toward the drain at 1/4 inch per foot. Standing water leads to mold and soap scum buildup. Pre-sloped shower pans eliminate this issue.
Inadequate Ventilation
An undersized fan or one that vents into the attic creates moisture problems. Size the fan properly (1 CFM per sq ft minimum) and always vent to exterior.
Cheap Fixtures
Budget faucets and shower valves fail quickly under rental use. Spend a bit more on quality fixtures from brands like Moen, Delta, or Kohler that offer good warranties.
Frequently Asked Questions
Should I use a prefab shower pan or tile the floor?
For most ADUs, a prefab shower pan is the safer choice. It is guaranteed waterproof, properly sloped, and faster to install. Custom tile floors look great but require expert installation to avoid leaks. If you tile, use a Kerdi or similar system.
What is the best flooring for an ADU bathroom?
Luxury vinyl plank (LVP) or tile are the best options. LVP is 100% waterproof, comfortable underfoot, and affordable ($3-6/sq ft). Porcelain tile is more durable but colder and requires proper subfloor preparation. Avoid laminate flooring in bathrooms.
Do I need GFCI outlets in the bathroom?
Yes, all bathroom outlets must be GFCI protected per code. This includes outlets near the sink and any within 6 feet of water sources. Modern codes also require AFCI protection on bathroom circuits in some areas. Your electrician will ensure compliance.
Should I install a bathtub or just a shower?
For rental ADUs, a tub/shower combo appeals to the widest market including families. For aging parents or accessibility needs, a curbless shower is better. Walk-in showers feel more modern and luxurious if you are targeting higher-end renters.
Ready for the Next Step?
With your bathroom structure in place, the next step is installing all the fixtures and appliances - connecting everything and testing for proper operation.