Step-by-Step Guide to Building an ADU in Niagara (Permits, Costs, Timelines)

As contractors here in Niagara, one of the most common questions we get is: “What does it actually take to build an ADU?” Whether you’re planning an in-law suite, a rental unit, or extra space for family, an Accessory Dwelling Unit (ADU) can be a smart investment. But before you start swinging hammers, it’s important to understand the process — from permits to costs to realistic timelines. Here’s a step-by-step guide based on how we help our clients bring their ADU projects to life.

Step 1: Understand Zoning & Bylaws

Every municipality in Niagara has its own rules around ADUs. Some allow basement conversions, detached backyard units, or additions — while others have restrictions on size, height, and parking. Before you spend money on drawings, check your local zoning bylaws (or have your contractor do it for you). This ensures your ADU will be approved on paper before it’s ever built.

Step 2: Get a Design & Drawings

Once you know what’s allowed, the next step is to work with a designer or architect to create official plans. These drawings need to meet Ontario Building Code requirements and reflect how the space will be used (living, sleeping, cooking, bathroom facilities). At this stage, think ahead about utilities like water, hydro, and HVAC — it’s much cheaper to plan it now than to change it later.

Step 3: Apply for Permits

With stamped drawings in hand, your contractor will submit them to your local building department. Permit approval times in Niagara can range from 2–6 weeks, depending on the complexity of the project and the municipality. Inspections will also be scheduled throughout the build to ensure everything is done to code.

Step 4: Budgeting & Costs

Costs can vary widely, but here’s a rough guide for Niagara:

• Basement ADU conversion: $80,000–$120,000+ depending on finishes and existing structure.

• Attached addition: $150,000–$250,000.

• Detached garage or backyard ADU: $200,000–$300,000+ depending on size and features.

Keep in mind, site conditions (like needing a new sewer connection or electrical upgrade) can increase costs. A good contractor will give you a ballpark estimate early on and a detailed quote once drawings are finalized.

Step 5: Construction Timeline

Construction timelines depend on the type of ADU:

• Basement conversion: ~3–4 months.

• Addition: ~4–6 months.

• Detached ADU/garage suite: ~6–9 months.

Weather, permit delays, and client-requested changes can extend timelines, but a strong plan up front helps keep things on track.

Step 6: Finishing & Final Inspection

Once construction is complete, a final inspection ensures the ADU meets all code requirements. After approval, you’ll have a fully legal, functional second unit ready to move into or rent out.

Final Thoughts

Building an ADU in Niagara is a fantastic way to add value to your property and create flexible living space. But success comes down to planning: knowing your bylaws, working with the right team, and budgeting realistically for both time and money. As contractors, we’ve seen firsthand how rewarding these projects can be when they’re done right.

If you’re considering an ADU, don’t hesitate to reach out — we’d be happy to walk you through your options and help you get started on the right foot.