Plan, Permit & Build an Accessory Dwelling Unit With ADU Ontario
Your new unit is a self-contained home on the same lot as your house, built for rental income, family, and long-term value. At ADU Ontario, we provide full-service planning, design, and construction for every type of build across the province.
Ontario’s ADU Specialists
What Is an Accessory Dwelling Unit?
An ADU is a self-contained residential unit built on the same lot as a principal dwelling. Homeowners across the province use them to generate rental income, house family members, and add long-term resale value.
Our team handles zoning reviews, building permit applications, custom design work, and code compliance so your project moves forward without delays, from a compact basement conversion to a standalone backyard build.
What We Handle
- Zoning & by-law reviews
- Building permit applications
- Custom design work
- Ontario Building Code compliance
Your Options
What Types of ADUs Can You Build in Ontario?
Homeowners can choose from four main options, depending on lot size, zoning, and budget:
Basement Apartments
An apartment built within the principal dwelling. Often the most cost-effective route to a legal rental unit when ceiling height and egress allow.
Coach Houses & Backyard Buildings
A separate building placed in the backyard. These buildings must meet specific height and floor area limits set by your municipality.
Interior Conversions
Reworking an existing basement or attic into livable, code-compliant space. Typically the least disruptive option for a single lot.
Detached ADUs
A standalone accessory structure on the same lot, sometimes above a detached garage. Requires more setback space than an interior conversion.
Each dwelling unit is located differently based on your property layout and local zoning by-law. We confirm what your lot allows before any design work begins.
Provincial Rules
How Many Units Can You Add to a Residential Lot?
Provincial law sets a minimum that every municipality must allow. Local by-laws fill in the details, such as setbacks, height, and lot coverage.
Since the More Homes Built Faster Act, 2022 (Bill 23) received Royal Assent on November 28, 2022, the Planning Act lets you create up to three residential units on a lot zoned for a detached house, semi-detached house, or rowhouse, with no rezoning needed. The lot has to sit in a settlement area with full municipal water and sewage services. Two layouts qualify:
- Three units inside the house, for example the main home, a lower-level apartment, and a third apartment upstairs.
- Two units inside the house plus one unit in a separate building on the property, such as a coach house, laneway house, or converted detached garage.
Two more provincial limits shape the budget. A municipality can’t require more than one parking space for each additional unit, and it can’t impose a minimum unit size. Development charges, the fees cities collect on new homes to pay for roads, pipes, and transit, don’t apply to these second and third units under the Development Charges Act.
The first thing we check on any lot is servicing. A rural property on a private well and septic system falls outside the three-unit rule, so what you can build there depends entirely on the local official plan and zoning by-law. Floodplain and other hazard-land mapping can also rule out a lot that otherwise qualifies.
Zoning permission is only half of it. Each addition needs a building permit and must meet the Ontario Building Code. The 2024 edition (O. Reg. 163/24) took effect on January 1, 2025, and it governs ceiling height, egress windows, fire separations, and smoke and carbon monoxide alarms in a house shared by two or three households. Our basement requirements guide walks through those numbers.
How ADU Ontario Matches a Unit Type to Your Lot
The right choice usually comes down to what the house and yard already give you. Start with the option that reuses the most existing structure, then move outward:
- If the basement has workable ceiling height and room for an egress window, converting it is usually the lowest-cost route. See what makes a basement rental legal before you sign a lease.
- If the goal is housing a parent or adult child close by, an in-law apartment works, but it has to meet the same code rules as any rental apartment. Our guide to the legal rules for housing family explains why.
- If you have a sound detached garage, a garage conversion reuses the shell, although the slab, insulation, and fire separation usually need upgrading.
- If the yard is deep and has rear or lane access, a new coach house or other detached backyard building gives the most privacy, at the highest cost of the four.
For the full rule set on second and third units, read our guide to additional residential units. For plain definitions and terms, start with the ADU definitions guide.
Our Services
Full-Service Support From Concept to Inspection
We stay involved at every stage so your build stays on track, on budget, and fully code-compliant.
Planning & Zoning Consultation
We analyze your local zoning by-law, assess property lines and setbacks, and identify which additional residential units are permitted, preventing costly surprises once building starts.
Building Permit Assistance
We prepare documentation, verify compliance with building code requirements, and submit applications to municipal authorities on your behalf, reducing approval timelines.
Custom Design
We create plans that maximize floor area within allowable limits, integrate the new space with your main dwelling, and balance aesthetics with full building code compliance.
Code Compliance & Safety Reviews
We review ceiling heights, plumbing and HVAC, fire separation, and ventilation so every self-contained dwelling meets Ontario Building Code safety benchmarks.
Project Management
From concept through final inspection, we coordinate contractors, run on-site evaluations, and provide ongoing support to keep your build on schedule.
Why Build
A Versatile Way to Expand Without Buying a Second Property
Second and third units have grown in popularity since Bill 23 made them as-of-right on most serviced residential lots. A few municipalities, such as St. Catharines, also publish pre-approved designs for detached units.
Rental Income
A legal rental apartment generates steady monthly cash flow on land you already own.
Property Value
A well-built additional dwelling boosts resale appeal and appraisal numbers.
Additional Housing
ADUs help address the housing shortage by adding density to existing residential lots.
Family Flexibility
House aging parents or adult children in a private, accessible space on your own property.
The Process
Building an ADU, Step by Step
Five clear stages take you from confirming eligibility to handing over the keys.
Zoning Review
Confirm your lot is eligible and check by-law requirements for setbacks, coverage, and permitted structure types.
Design & Planning
Work with our team to create a plan that meets building code requirements and your personal goals.
Building Permit
Submit the application with all required drawings, site plans, and documentation for municipal approval.
Construction
Build the dwelling to code, with inspections at each key milestone to keep quality on track.
Final Inspection & Occupancy
Pass the final review, then move in your tenant or family member.
Service Area
Where ADU Ontario Works: Planning & Construction by City
ADU Ontario works with homeowners in every major Ontario city. Not sure if we cover your area? Send us your address.
FAQ
Frequently Asked Questions
Common questions homeowners ask before they build.
What is an ADU?
An ADU is a self-contained dwelling unit that includes a kitchen, bathroom, and sleeping area. It functions independently from the principal dwelling and can be attached or detached. These units are built to meet strict safety, size, and livability standards under the Ontario Building Code.
Are ADUs permitted across Ontario?
Yes. Provincial legislation allows ADUs in most zones, though building code rules can vary by municipality. Check your local zoning by-law before starting. Whether you plan a detached build or an addition to an existing dwelling, verify what your lot permits first.
Do I need a building permit for an ADU?
Yes. Every additional dwelling unit requires a building permit. The application process involves submitting architectural drawings, site plans, and proof of code compliance. Our team handles this paperwork so you can focus on the project itself.
What are the minimum standards for an ADU?
The building code sets out minimum standards for size, safety, structure, and systems. Each dwelling unit requires a private entrance, independent kitchen and bathroom, proper ventilation, fire separation between living areas and the rest of the house, and compliance with gross floor area limits. Plumbing, electrical, and heating systems must also meet current code.
How much does it cost to build an ADU in Ontario?
Costs depend on the type of unit, location, and scope of work. A basement apartment conversion typically costs less than a new detached structure. Factors include materials, labour, permit fees, and site preparation. Contact us for a detailed estimate based on your property.
Can I add more than one ADU to my property?
Some municipalities allow both a secondary suite inside the house and a detached garden suite on the same lot. Others limit you to one ADU per property. Review your local by-law or contact us for guidance on what your zone permits.
Does an ADU increase property value?
Yes. A well-designed additional dwelling unit adds flexible living space that appeals to buyers, renters, and multigenerational families. In most Ontario markets, ADUs deliver strong returns through both increased property value and rental income potential.
Guides & Resources
Learn the Rules Before You Build
In-depth guides on build types, zoning, permits, and costs, written for homeowners, not contractors.
What Is an ADU?
A complete guide to accessory dwelling units in Ontario.→
Additional Residential Units
ARU rules, as-of-right zoning, and the 2026 build process.→
In-Law Suites
What an in-law suite is and how it differs from a secondary suite.→
Are In-Law Suites Legal?
Rules, permits, and costs for a legal secondary suite.→
Basement Apartment Requirements
Ceiling height, egress, fire separation, and the code basics.→
Garage Conversions
Turn an existing garage into permitted, livable space.→
Ready to Start Your ADU Project?
Contact ADU Ontario today for guidance on what you can build in Ontario. We confirm your options, estimated costs, and timeline so you know exactly what your property can support.