Accessory Dwelling Units (ADUs), sometimes called additional residential units (ARUs), granny flats, secondary suites, or laneway homes are self-contained living units on the same lot as a primary house. They can be inside the home (a basement ADU or garage ADU), or detached (a backyard cottage or “garden suite”). ADUs are a great way to add rental income or house additional family without needing to purchase an entirely new property saving you on time and money.
What is an ADU?
An Accessory Dwelling Unit (ADU) is a self-contained secondary home built on the same property as an existing primary residence. These units add valuable living space without the need to buy another piece of land. In Windsor-Essex, ADUs have become an increasingly popular way for homeowners to generate extra income, house family members, or make better use of their property.
Here are some of the most common types of ADUs you’ll find across the region:
- Laneway Houses/Suites – Small, fully equipped homes built along a rear laneway or alley. These are often tucked behind the main house and provide an efficient, private living option.
- Detached ADUs – Standalone units located in the backyard of a main residence. They function as independent dwellings and are commonly used for rental income or multigenerational living.
- Basement ADUs – Finished basement suites that typically feature a private entrance, full kitchen, and bathroom. They’re one of the most affordable and straightforward ADU types to create.
- Garage ADUs – Conversions of existing or new garages into self-contained living spaces such as bachelor apartments, small studios, or one-bedroom suites.
- In-Law Suites (Attached ADUs) – Separate living areas located within or attached to a single-family home. These are designed for extended family members—like aging parents—who need their own space while staying close to loved ones.
Each ADU is equipped with the essentials—sleeping area, bathroom, kitchen, and living space—making it a true independent home suitable for both long-term tenants and family use.
Quick note about Windsor-Essex ADU rules
Before we dive into the guide on building an ADU it’s important to note that municipal rules differ across Windsor-Essex. The City of Windsor permits ADUs in many residential zones and is actively updating its ADU framework. Tecumseh and Lakeshore each have their own ARU/ADU guidelines and zoning rules that are similar to Windsor’s but include some local variations (setbacks, lot size minimums, unit types allowed, licensing or registration steps). Always check the specific municipality’s planning pages early in the process.
Step-by-Step Guide for Building an ADU in Windsor-Essex
1) Feasibility & Initial Research
Begin by checking whether your property qualifies for an ADU in the Windsor-Essex area. As an experienced ADU builder in Windsor, an experienced ADU general contractor like Sovereign Developments can help you evaluate. In Windsor the zoning by-law (Section 5.99.80 of Zoning By-law 8600) allows an Additional Dwelling Unit (ADU) in a detached accessory structure as long as certain criteria are met. Some examples of criteria surrounding ADU’s in Windsor include:
- Properties are subject to a maximum of two additional dwelling units in addition to the primary residence
- ADU’s must have a minimum setback of 1.2 meters from the rear and side yards of the property
- The maximum GFA (Gross Floor Area) of a detached ADU is 100m2 or 1000 Sq Ft. *This GFA requirement changes when the ADU is attached*
- If your lot is in a flood-plain zone marked by ERCA, basement ADUs are not allowed.
These are just a few examples of different criteria to be aware of which is why we highly recommend getting a professional involved from the very start who is aware of all the different zoning bylaws and regulations in regards to ADU’s.
How Much do ADU’s Cost?
For Windsor expect a broad ballpark of roughly $150,000 to $300,000 for a typical detached ADU, or roughly $300 to $450/Sq Ft. This is a general range only since final cost depends on size, finishes, foundation work, utility upgrades, site access and many more factors that can greatly increase or decrease the overall cost. At Sovereign Developments we’ll give our clients a free on site consultation to evaluate the estimated cost of construction for their ADU. After reviewing the quote, if our client agrees to move forwards we’ll then sit down with the client and go over a detailed line by line contract that spells out everything included in the build. Our past clients have expressed their frustrations when dealing with other contractors that they had to pay hidden fees which weren’t shown in their initial quote price which is something we don’t do at Sovereign Developments.
2) Contractor Selection, Contracts & Construction Planning
If you haven’t already it’s recommended you start getting detailed quotes from builders experienced with ADUs. The reason we recommend you contact a ADU builder as one of the first steps is because they will be able to guide you through the entire process when it comes to permitting and designing your ADU. At Sovereign Developments we include these services free of charge as we know they can be difficult tasks if you haven’t dealt with the city and permits before. When choosing the right General Contractor you should compare proposals that separate design, permits, trades, finishes and contingency. Look for a GC who has done similar ADU projects in Windsor Essex, who communicates clearly, and who offers a transparent contract. When signing a written contract, it should have a clear scope, timeline and payment milestones.
3) Concept Design & Choosing Your ADU Type
Next you’ll need to decide the type of ADU you want which could be a basement unit, internal unit within the main home, an above garage suite, or a detached backyard unit. Each option has a different permitting path, cost profile and potential rental income. You can either hire a BCIN designer or find existing floor plans that outlines bedrooms, bathroom count, kitchen needs, and a rough footprint. Some ADU builders like Sovereign Developments have an in-house design team who will cover the cost of designing custom ADU floor plans within there build price.
4) Pre-Consultation & Zoning Review with the Municipality
Next you’ll need to contact planning staff at your municipality for a pre-consultation. Windsor offers helpful pre consultation processes and some municipalities provide ARU guidance documents that describe submission requirements. The city will confirm whether you need zoning relief, site plan control or special permits. Early contact reduces surprises during permit review.
5) Utility Connections & Servicing
If you’re using an experienced builder they will be able to confirm the utility servicing capacity with the city. Detached ADUs sometimes need separate services or upgrades for sewer, water or electrical capacity so it’s important to coordinate with utility providers early in the process. Address stormwater, backflow prevention and lot grading to ensure compliance with local stormwater management rules and to avoid basement flooding. These items often create unanticipated costs if not considered early and good ADU builders will be able to recognize these potential costs before they even start building.
6) Construction & Municipal Inspections
Construction stages typically follow this order: site preparation and utilities, foundations or footings for detached units, framing, rough mechanicals and electrical, insulation and vapour or thermal barrier work, interior finishes, fixtures and final commissioning. Here is a more in depth look at what to expect and what you should be asking your ADU builder during construction:
• Site preparation and temporary services
Expect contractors to set up site access, protect existing landscaping and provide temporary power and water as needed. If the ADU is detached, coordinate deliveries and storage for materials.
• Foundation and structure
For detached ADUs the foundation or slab work is critical. Confirm soil conditions and drainage around foundations. For above garage or internal units the structural work may involve reinforcing existing framing and ensuring proper load paths.
• Framing and envelope work
Once framing is complete the building envelope is created. This is when windows, doors and the roofing material are installed. Quality control at this stage prevents water penetration and heat loss later.
• Mechanical, electrical and plumbing roughs
HVAC ducts or split systems, electrical circuits and plumbing rough ins are installed next. Ask your contractor for the mechanical design so you know where heaters, hot water units and condensate drains will be placed. Detached units may require separate meters or service upgrades which should be coordinated with the utility company.
• Insulation, vapour control and air barrier
Insulation and proper air barrier installation affects comfort and operating costs. In Windsor Essex cold weather performance matters so ensure insulation assemblies meet or exceed code and that the contractor follows best practice for continuous air barriers.
• Interior finishes and fixtures
Once inspections for rough trade work are complete, finishes such as drywall, trim, flooring, cabinets and fixtures are installed. Make sure selected finishes match your allowances and that any upgrades are tracked as change orders.
• Inspections and municipal sign offs
Municipal inspectors will visit at key stages such as footing, framing, ect. The building permit will list required inspections. Your contractor or you will coordinate timing so inspections are booked in advance to avoid delays. Keep copies of inspection records and approvals in your project file.
• Coordination and keeping timelines realistic
Construction timelines vary based on complexity, weather, material availability and trade scheduling. Expect the average detached ADU to take roughly 4-6 months to build. A reliable builder will provide a realistic schedule, update you when things change and manage trade coordination.
• Quality control and snag lists
Before final occupancy request a walk through and a snag list for any potential deficiencies. Good builders will promptly correct issues and provide a warranty or maintenance handover.
If you want a local ADU builder to manage construction and inspections in Windsor ON, Sovereign Developments can provide project management, trade coordination and help with municipal inspections for a smooth build.
7) Final Occupancy, Documentation & Move-In
The very last step is obtaining final occupancy or a certificate of completion from the building department. Update your homeowner insurance and inform your insurer about the new unit. Review property assessment and tax implications and check whether your municipality requires a registry or license for rental ADUs. If you intend to rent the ADU, prepare a tenant ready handover and confirm any rental licensing requirements with your municipality.
Conclusion
ADUs are transforming the housing landscape in Windsor-Essex by offering affordable, flexible, and sustainable living solutions. Whether you’re looking to create additional rental income, accommodate family members, or enhance your property’s value, ADUs present a compelling opportunity. Choose a builder who has completed ADU projects locally, demonstrates familiarity with local bylaws and permitting, can provide references and shows a clear understanding of costs and scheduling. Ask for a detailed quote that separates work packages and includes contingencies.
Ready to start your ADU project? Contact Sovereign Developments today for a consultation, and let us help you bring your vision to life.