Without question, the number one question we get when talking with homeowners, real estate agents or real estate investors on the topic of ARUs/ADUs is “How much does it cost to build an ARU/ADU in Ontario?”
As Ontario cities continue pushing for gentle intensification and more flexible housing options, more homeowners are exploring the idea of adding an Additional Residential Unit (ARU) or Accessory Dwelling Unit (ADU) —also known as a secondary suite, in-law suite, basement apartment, coach house, converted garage or backyard home.
While exact costs are dependant on a myriad of factors, we’ll provide some ballpark figures here to give you an idea of what to expect.
Costs vary widely depending on unit type, design complexity, existing site conditions, and municipal requirements. Below is a rough cost breakdown to help Ontario homeowners plan an ARU. At MW Drafting & Design, we specialize in architectural design, drawings and permits for ARUs/ADUs & secondary suites.
What is an ARU? (Quick Definition)
An Additional Residential Unit (ARU), Additional Residential Dwelling Unit (ARDU), or Additional Dwelling Unit (ADU) - names interchangeably used depending on your jurisdiction in Ontario - is a self-contained dwelling with its own kitchen, bathroom, living space, and entrance. ARUs include:
Basement apartments
Main-floor or upper-level secondary suites
Garage conversions
Detached backyard homes (also called coach houses, garden suites, tiny homes)
Additions that create a new dwelling unit
As per Ontario’s Bill 23, most Ontario municipalities now permit up to two ARUs on a single residential lot, depending on zoning. Some municipalities have local by-laws that permit even more.
How Much Does It Cost to Build an ARU in Ontario?
Below is a breakdown of typical 2025 construction costs across Ontario, based on current market pricing, contractor averages, and MW Drafting & Design’s real-world project experience.
1. Basement or Interior Secondary Suite Cost
Average cost: $140,000 – $220,000
Cost per sq. ft.: $180 – $280/sq. ft.
Interior ARUs are the most affordable option because the structure already exists. Costs depend on:
Ceiling height upgrades
Egress window installation
Fire separations (Ontario Building Code requirement)
Plumbing and electrical upgrades
Waterproofing or foundation work
HVAC reconfiguration
Soundproofing
Most common cost drivers:
Adding new plumbing lines
Enlarging basement windows
Correcting moisture or structural issues
2. Garage Conversion ARU Cost
Average cost: $180,000 – $260,000
Converting a garage into an ARU requires significant upgrades:
Foundation insulation
Raising floor levels
Running new utilities
Reinforcing doors/windows
Full interior construction
This option is still often cheaper than a detached build.
3. Detached Backyard Home / Coach House ARU Cost
Average cost: $250,000 – $420,000+
Cost per sq. ft.: $350 – $500/sq. ft.
Detached ARUs are essentially mini houses, so costs include:
New foundation
New services (water, hydro, sewer)
Full interior build
Roof structure
Exterior materials
Site access and grading
Tree removals (if required)
Longer permit and zoning review
In cities like Waterloo, Kitchener, London, Hamilton, and Guelph, these builds can trigger additional requirements such as:
Servicing reports
Stormwater management
Fire access routes
4. Additions That Add a New ARU
Average cost: $250,000 – $350,000
Adding square footage to create an ARU often includes structural engineering, building envelope upgrades, and OBC fire separation requirements.
Other Costs to Factor Into Your ARU Budget
Design & Drafting Costs
Typical range: $6,000 – $12,000 for a full ARU design package
MW Drafting & Design provides:
Site measurements
Zoning review
Architectural drawings
Structural coordination
Permit-ready plans
Mechanical design support
Minor variance prep if required
Detached ARUs may require additional grading or servicing drawings depending on the city.
Permit & City Fees
Costs vary by location, but typical expenses include:
Building permit fees: $2,000 – $5,000
Development charges: Often exempt for ARUs (depends on municipality)
Site plan approval fees (if required): $2,500 – $6,000
Minor variance application: $1,800 – $3,500
Utility Upgrades
Budget $5,000 – $20,000 for:
New electrical service
Sewer upgrades
Waterline upgrades
Gas extension (optional)
Contingency (very important)
ARU projects often uncover hidden issues.
Recommended contingency:
10–20% of construction budget.
Are There Grants for ARUs in Ontario?
Some municipalities now offer ARU incentive programs. Other cities periodically offer renovation or secondary suite grants tied to affordable housing or the CMHC Housing Accelerator Fund.
MW Drafting & Design can help prepare the necessary:
Preliminary drawings
Project descriptions
Permit-ready plans
Documentation required for grant applications
How MW Drafting & Design Helps Homeowners Control ARU Costs
MW Drafting & Design specializes in ARUs across Ontario and supports you through:
Complete ARU design & permit-ready drawings
Zoning review to confirm what’s possible on your lot
Structural coordination (via BCIN or engineering partners)
Minor variance preparation if needed
Cost-aware design to match budget limits
Clear plans for contractors to reduce change orders
A well-designed ARU saves money before construction even starts—by preventing delays, rejections, and contractor confusion.
So… What’s the Total Cost of an ARU in Ontario?
Basement / Interior Suite $140,000 – $220,000
Garage Conversion $180,000 – $260,000
Detached ARU / Coach House $250,000 – $420,000+
Addition-Creating ARU $250,000 – $350,000
These ranges reflect professional, legal, Ontario Building Code–compliant builds—not DIY renos or unpermitted work.
Planning an ARU? MW Drafting & Design Can Help.
If you’re thinking about a basement suite, backyard home, garage conversion, or addition, MW Drafting & Design offers:
ARU feasibility assessments
Zoning verification
Full architectural drawings
Permit submission support
Grant documentation assistance
Professional, affordable, Ontario Building Code–compliant design
Whether you’re in Waterloo, Kitchener, Cambridge, Guelph, London, Hamilton, Norfolk, Haldimand, Brantford, Niagara or anywhere in Ontario—we bring clarity and confidence to the process. Click here to book a call with us to discuss your plans and how we can help you create your ARU project to maximize purposeful design, aesthetics, cost efficiency and ROI.