Accessory dwelling units — coach houses, basement apartments, attic conversions, garage suites — are having a moment. Cities across the country are loosening the rules that kept them off the table for decades, and homeowners are discovering that the square footage they need may already be sitting on their property.
Chicago reopened the door in 2021 with its Additional Dwelling Unit Ordinance, and suburbs across the collar counties have been quietly following suit. For homeowners in the Chicagoland area, an ADU can mean rental income, a place for aging parents, a landing spot for an adult child, or simply a way to make an underused basement or backyard structure earn its keep.
At Century City Baths, we spend our days in the room that makes or breaks an ADU: the bathroom. It’s the smallest space in the unit, the most heavily regulated, the most expensive per square foot, and the one that inspectors scrutinize hardest. Below is a practical guide to how ADUs work in our region — and how to plan the bath so the whole project doesn’t stall.
What Counts as an ADU
An accessory dwelling unit is a self-contained living space on the same lot as a primary home. To qualify, it generally needs its own entrance, sleeping area, kitchen, and full bathroom. That last requirement is not optional — a space without a legal bathroom is a bonus room, not a dwelling unit.
In Chicagoland, ADUs typically take one of four forms:
Basement conversion units. The most common path in Chicago’s bungalow belt and two-flat neighborhoods. You’re working with existing foundation walls, existing utilities, and existing ceiling height — which is both the advantage and the constraint.
Attic conversions. Less common because of headroom and stair requirements, but viable in larger frame homes and Victorians on the North and Northwest Sides.
Coach houses. Chicago has thousands of historic coach houses sitting behind three-flats and single-family homes, many of them uninhabited for decades. Rehabbing one is often more straightforward than building new, though systems usually need a full replacement.
Garage or backyard new construction. Building fresh gives you design freedom but brings the highest cost per square foot and the most zoning scrutiny — lot coverage, setbacks, and height limits all come into play.
The Regulatory Landscape
Chicago’s ADU ordinance operates through designated pilot zones covering portions of the North, Northwest, West, South, and Southeast sides. Rules differ by zone, and some areas cap the number of conversion units permitted per year or restrict them in certain building types. Affordability requirements can attach to larger projects. The city has continued to revisit and adjust the program, so the first phone call on any ADU project should be to the Department of Buildings or a permit expediter who works in your zone.
Outside city limits, it’s a patchwork. Evanston has permitted coach houses for years. Communities like Oak Park, Skokie, and Arlington Heights have their own frameworks, some permissive, some restrictive, some requiring owner-occupancy of the primary residence. Unincorporated Cook County follows county zoning. Naperville, Aurora, and other DuPage and Kane County municipalities each set their own terms.
The takeaway: never assume. Two homes six blocks apart can fall under completely different rules.
The Non-Negotiables for Basement and Attic Units
Before you fall in love with a floor plan, confirm the fundamentals.
Ceiling height. Chicago generally requires a minimum finished ceiling height for habitable basement space. Many older basements come up short, and lowering a floor means underpinning the foundation — a serious structural project that can add tens of thousands of dollars.
Egress. Every sleeping room needs a code-compliant means of escape. In a basement, that usually means an egress window with a properly sized window well, or a second exterior door. This is one of the most common reasons a conversion gets rejected.
Light and ventilation. Habitable rooms need window area proportional to floor area. Bathrooms are the exception — they can be windowless if mechanically ventilated, which is why the bath often ends up in the interior of the plan.
Moisture control. Chicagoland basements are wet. Between clay soil, aging drain tile, high water tables in river-adjacent neighborhoods, and combined sewer backups, water intrusion is the rule rather than the exception. Address drainage, sump systems, backflow prevention, and vapor barriers before a single stud goes up.
Sewer capacity and ejector pumps. A basement bathroom sits below the sewer line in most Chicago homes. That means a sealed ejector pit and pump — properly vented, properly sized, and accessible for service.
Designing the ADU Bathroom
Here’s where our work begins. An ADU bath is a small-footprint, high-performance space, and small doesn’t mean simple.
Plan the plumbing before the walls
The single biggest cost variable in an ADU bathroom is how far the fixtures sit from existing drain lines. Stacking the new bath under or near an existing bath, kitchen, or main stack can save thousands. In a coach house or new-build ADU, you’re trenching a new sewer lateral and water service regardless — but even then, grouping the bath and kitchen plumbing on a shared wall keeps costs down.
Respect the clearances
Code sets minimum clearances that don’t bend: space in front of the toilet, clearance to the side of the centerline, door swing that doesn’t collide with fixtures, and a minimum shower footprint. A tight bath drawn without these in mind will fail plan review or, worse, fail inspection after it’s built. We lay out ADU baths to clearance first, aesthetics second — and there’s still plenty of room for a beautiful result.
Choose the right shower approach
Most ADU baths work best with a shower rather than a tub. A curbless or low-threshold shower makes a small room feel dramatically larger, eliminates a trip hazard, and future-proofs the unit for an aging parent or a tenant with mobility needs. In a basement, curbless requires recessing the drain assembly into the slab — plan it during rough-in, not after.
Where a tub makes sense — say, an ADU intended for a young family — a compact soaking tub with a shower over it delivers both functions in a single footprint.
Waterproof like the room will be tested
Because it will be. A below-grade bathroom deals with humidity from every direction. We build with proper substrate, full waterproofing membranes at wet walls and pans, sealed penetrations, and thoughtful transitions. A shower that leaks into a finished basement isn’t a callback — it’s a demolition.
Ventilate aggressively
Undersized exhaust fans are the most common shortcut in ADU construction and the most expensive to fix. In a below-grade or attic bath, ventilation isn’t just about comfort — it’s the primary defense against mold in a space with limited natural air exchange. Size the fan to the room, duct it to the exterior with rigid or smooth-wall duct, insulate the run, and put it on a humidity sensor or timer so it actually runs long enough to do its job.
Build for durability, not just for photos
An ADU bathroom serving a rental tenant will see harder use than a primary bath. Porcelain tile over vinyl. Solid-surface or quartz over laminate. Quality valves from a brand with parts availability. Wall-mounted or floating vanities that make floor cleaning easy and give a small room visual breathing room. Good lighting layered between the vanity and the shower area, because a single ceiling fixture makes a small bath feel like a closet.
Consider accessibility from day one
A large share of Chicagoland ADUs are built for multigenerational living. Even when they’re not, accessibility features widen your tenant pool and add resale value. Blocking in the walls for future grab bars costs almost nothing during framing and is a wall-opening project later. A 34- to 36-inch door, a comfort-height toilet, a handheld shower on a slide bar, and a curbless entry cover most of what matters.
Budgeting Realistically
ADU costs in Chicagoland vary enormously by type. A straightforward basement conversion in a home with adequate ceiling height, dry conditions, and nearby plumbing sits at the low end. Add underpinning, foundation waterproofing, a new sewer connection, an egress window cut through a foundation wall, or full electrical service upgrades, and the number climbs fast.
The bathroom alone typically represents a disproportionate share of the total — it’s the densest concentration of trades in the unit. Plumbing rough-in, electrical, HVAC, waterproofing, tile, and finish work all converge in eighty square feet or less.
Budget for the unglamorous line items early: permits, architectural drawings, structural review if required, utility separations or sub-metering, and a contingency for what you find behind the walls of a hundred-year-old Chicago building. Something will surprise you.
Sequence and Timeline
A typical Chicagoland ADU project moves through: feasibility and zoning confirmation, design and construction documents, permit submission and review, rough-in construction with inspections at each stage, finish work, and final inspection and certificate of occupancy.
Permit review is the least predictable phase. Plan for it to take longer than you’re told, especially in the city. Meanwhile, decisions on tile, fixtures, and vanities have long lead times — order finish materials early so a backordered shower valve doesn’t idle your crew for three weeks.
Where Century City Baths Fits In
We’re not a general contractor for whole-ADU construction, and we won’t pretend to be. What we do is design and build the bathroom — the room that determines whether your ADU is a legal dwelling unit or an expensive storage area.
We work alongside homeowners, architects, and general contractors across the Chicagoland area to get ADU baths right the first time: laid out to code, plumbed efficiently, waterproofed properly, ventilated correctly, and finished so the space looks like a bathroom someone would happily pay to live in — not an afterthought crammed under a staircase.
If you’re weighing a coach house rehab, a basement conversion, or a new backyard unit, bring us in during the planning stage. A one-hour conversation about drain locations, ceiling height, and clearances at the start of a project saves far more than it costs at the end.
Ready to talk through your ADU bathroom? Reach out to Century City Baths for a consultation. We’ll walk your space, review your plans, and give you a clear picture of what’s possible — and what it will actually take to get there.
