Ontario’s Bill 23 fundamentally changed what you can build on a residential lot. Since 2023, municipalities are required to allow up to three residential units on most properties without rezoning. That means a garden suite, basement apartment, or above-garage unit is now a realistic option for almost every homeowner in Kitchener, Waterloo, Cambridge, and Paris.

But “allowed” doesn’t mean “cheap.” ADU costs vary enormously depending on the type, size, and site conditions. This guide gives you real numbers for every ADU type in the region.

ADU Costs by Type

Basement Apartment Conversion

Typical cost: $60,000–$120,000

Converting an existing unfinished basement into a legal, code-compliant apartment is the most affordable ADU option because the structure already exists. Costs include: separate entrance installation, full kitchen, bathroom, egress windows (required by code), fire separation between units (typically 45-minute fire rating), electrical sub-panel, plumbing connections, insulation, drywall, flooring, and finishes.

The biggest cost variables are whether you need to lower the basement floor (underpinning) to meet minimum ceiling height requirements and how much plumbing work is needed for the kitchen and bathroom locations.

Above-Garage Unit

Typical cost: $100,000–$200,000

Building a living unit above an existing detached garage is often the most cost-effective way to add a self-contained unit because you’re using existing foundation and walls. However, the existing garage structure must be assessed for its ability to support a second floor — many older garages require structural reinforcement.

Detached Garden Suite

Typical cost: $150,000–$350,000

A standalone unit built in your backyard — essentially a small custom home. Garden suites require: new foundation, full framing, roofing, siding, windows, plumbing and electrical service connections from the main house or street, HVAC, and full interior finishing. Size typically ranges from 400–800 sq ft.

The biggest cost variables are foundation type (slab vs. full basement), servicing distance from the main house, and level of finish. A basic 500 sq ft garden suite might come in at $175,000. A high-end 750 sq ft suite with a basement can exceed $300,000.

Attached Secondary Suite

Typical cost: $80,000–$180,000

An addition to your main home designed as a self-contained unit with its own entrance, kitchen, bathroom, and living space. Fire separation, sound insulation, and separate mechanical systems are required between the units.

Grant Programs

The City of Waterloo currently offers an ARU grant program: up to $20,000 for interior ARUs (basement apartments) and potentially more for exterior units. The Region of Waterloo’s Secondary Suites Program offers up to $25,000 for units rented at affordable rates for 15 years. Cambridge has offered similar programs in the past. These grants can significantly offset your construction costs — ask your contractor about applicable programs before starting.

Zoning Requirements

While Bill 23 eliminated the need for rezoning, ADUs still must comply with municipal zoning bylaws for: lot coverage, setbacks from property lines, maximum height, parking requirements, and servicing. Each municipality in the Waterloo Region has slightly different rules. A contractor experienced in ADU construction will know these requirements and design your unit to comply from the start.

ROI: Do ADUs Pay for Themselves?

In the current Kitchener-Waterloo rental market, a legal one-bedroom unit rents for $1,400–$1,800/month. A two-bedroom unit rents for $1,800–$2,400/month. At the lower end, a $100,000 basement apartment generating $1,500/month in rent produces a gross annual return of $18,000 — paying for itself in roughly 5–6 years before considering property value appreciation. Garden suites have longer payback periods but add significant property value.

Frequently Asked Questions