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Serving St. Clair Township — Corunna, Mooretown, Courtright, Sombra & Port Lambton (Lambton County)
Modular Homes & Garden Suites in St. Clair Township, Ontario
Factory-direct modular homes and backyard garden suites for St. Clair Township — Corunna, Mooretown, Courtright, Brigden, Sombra and Port Lambton along the St. Clair River. St. Clair's Zoning By-law 17 of 2003 calls a garden suite a "second dwelling unit" (the province's term is additional residential unit, or ADU), and it allows one in a separate one-storey building in the yard of a house. Here's what the by-law allows, what the province adds, what it costs, and the rules that apply.
What you can build in St. Clair Township
Full modular homes — from $175,696. 29 models from 560 to 1,405 sq ft, 1–3 bedrooms — factory-built to the Ontario Building Code and set on a permanent foundation (a real, permanent house, not a “trailer”).
Backyard suites (ADUs) — from $96,244. 7 compact models built for garden-suite and in-law-suite use, sized to fit what St. Clair Township's by-law allows.
All homes are CSA-certified (A277 modular / Z240MH manufactured), finished in the factory, and delivered ready for foundation and hook-up. What's the difference between modular, manufactured, and mobile? →
Find your backyard suite — 3 quick questions
We'll narrow the models that fit a typical St. Clair Township backyard allowance to the ones that fit your life. No email needed.
1 · Who's it for?
2 · What layout?
3 · What size feels right? (St. Clair Township lots typically allow up to up to ~721 sq ft)
A detached suite can be up to 67 m² (≈721 sq ft), one storey and 5 m (≈16 ft) tall. On a residential lot, though, every accessory building together — the suite, a detached garage, a shed and even an attached garage — may cover no more than 10% of the lot, so a 670 m² lot with nothing else on it is what it takes to reach 67 m². The by-law's 50 m² minimum doesn't apply on Township water and sewer (the Planning Act overrides it), so the compact models below work there; on a well and septic plan for at least 50 m² (≈538 sq ft). Enter your address for the specifics. Your exact lot may allow more or less — get the real number for your address in seconds.
Check your exact St. Clair Township address →Can you put a garden suite (ADU) in your St. Clair Township backyard?
Yes, beside a single-detached, semi-detached or duplex house with street access — the by-law allows one second unit, inside the house or in a separate building in the yard, in the R1, R2, R4, Suburban Residential and Agricultural zones. The specifics, from Township of St. Clair Comprehensive Zoning By-law 17 of 2003, §4.33 Second Dwelling Unit Provisions (By-law 11 of 2013) and §4.3.4 accessory coverage (January 2024 consolidation):
| Where it goes | Beside a single-detached, semi-detached or duplex house with direct street access, in the Residential R1, R2 and R4 zones, Suburban Residential (RS) and the Agricultural A1 and A1-R zones. Not in a required front yard, not in a yard that abuts the St. Clair River, not in the Environmental Protection – Hazard zone and not within 15 m (≈49 ft) of a watercourse. On a farm or Suburban Residential lot it must sit within 10 m (≈33 ft) of the house. |
|---|---|
| Max size | Up to 67 m² (≈721 sq ft) of floor area, on one floor — no second storey or basement. On a residential lot all accessory buildings together, the suite and any attached or detached garage and shed included, may cover no more than 10% of the lot and 67 m² in total: 50 m² (≈538 sq ft) on the smallest 500 m² R1 lot, 67 m² on a 670 m² lot with nothing else on it. On Suburban Residential and rural residential lots the combined limit is 10% up to 186 m², so the 67 m² cap governs. |
| Minimum size | The by-law asks for at least 50 m² (≈538 sq ft). On a lot with Township water and sewer that minimum has no effect — the Planning Act bars a by-law from setting a minimum floor area for an additional unit — so a smaller suite is fine there. On a well and septic it applies. |
| Height | 5 m (≈16 ft), measured from finished grade to the highest point of the roof — the accessory-building limit. |
| Setbacks | The same side and rear yards the house itself needs: in R1 and R2, 7.5 m (≈25 ft) rear and 1.2 m (≈4 ft) sides where the house has an attached garage (otherwise 1.2 m on one side and 2.5 m on the other); in R4, 6 m rear; in Suburban Residential, 12 m rear and 2 m / 4 m sides; in the Agricultural zones, 7.5 m rear and 3 m sides. At least 1.2 m from the house. |
| Parking | One extra space for the suite, on the existing driveway — no new driveway, and the house can't lose a required space. The rear yard must keep 100 m² (≈1,076 sq ft) of landscaped open space. |
| How many | The by-law allows one second unit per house — inside or detached. On Township water and sewer the Planning Act lets you go further: up to two units in the house plus one in the yard. |
| The garage wording | The by-law was written for a unit inside a private garage. On Township water and sewer the Planning Act guarantees one unit in a building ancillary to the house whatever the wording, so a purpose-built garden suite qualifies there; on a well and septic, confirm with the Township's Coordinator of Planning before you design. |
| Servicing | Corunna, Courtright, Brigden, Sombra, Wilkesport and Port Lambton are on Township sewers (check that your street is), and the suite must connect to Township water and sewer where they're available. On a well and septic it can connect "subject to approval" once the water quality and septic capacity are reviewed; the County of Lambton issues septic permits. |
| Conservation | St. Clair Region Conservation Authority (SCRCA) covers the whole Township. Lots with a Shoreline Constraint (-SC) zone need a slope-stability study and SCRCA consultation before a permit within 15 m of the top of bank. |
Check your specific St. Clair Township address →
Garden suite, laneway suite, in-law suite, ARU, ADU — different names for the same thing: a small, self-contained home in the backyard. New to the idea? Start with prefab garden suites for Ontario — what one costs in St. Clair Township, and the models that fit →
St. Clair Township's by-law is verified in our checker, so you can check if a garden suite is allowed at your St. Clair Township address in about a minute.
Getting your building permit in St. Clair Township
An eligible lot still needs a building permit before anything is delivered. Here's how St. Clair Township's process actually works — verified against the St. Clair Township building department's own pages.
How do you apply for an ADU building permit in St. Clair Township?
Through Cloudpermit (County of Lambton Building Services, on behalf of St. Clair Township) (application portal ↗). The County of Lambton's Building Services Department issues building, plumbing and septic permits for St. Clair Township, and takes applications online through Cloudpermit (choose St. Clair). Building Services: 789 Broadway Street, Wyoming, 519-845-5420 or 1-866-324-6912. Zoning questions go to the Township's Coordinator of Planning at the Civic Centre, 1155 Emily Street, Mooretown, 519-867-2021. St. Clair Township's building-permit page ↗
- Confirm where you stand: a detached garden suite (the by-law's "second dwelling unit") is allowed beside a single-detached, semi-detached or duplex house with street access in the R1, R2, R4, Suburban Residential and Agricultural zones (Zoning By-law 17 of 2003 §4.33). Check your zone on the Township's Schedule A maps, and ask the Coordinator of Planning about the by-law's private-garage wording if your lot is on a well and septic.
- Size the building: up to 67 m² (≈721 sq ft), one storey, 5 m to the highest point — and on a residential lot every accessory building together, attached garage included, within 10% of the lot and 67 m². Keep the house's own side and rear yards, stay out of the front yard and any yard abutting the St. Clair River, and plan one extra parking space on the existing driveway.
- Line up the outside approvals: St. Clair Region Conservation Authority permission if the lot is in its regulated area (shoreline, floodplain, near a watercourse); a County septic permit or sewage-system assessment on a well and septic; Township service-connection permits if you need new water or sewer connections.
- Have a designer with a BCIN prepare the drawings — site plan, foundation, floor plans, elevations and sections, plus the manufacturer's CSA A277 certified drawings for a modular unit — and apply through Cloudpermit.
- Pay the Township's building-permit fee and refundable completion deposit, then book inspections with County Building Services as the work goes; the Electrical Safety Authority inspects the wiring separately.
What does an ADU permit application in St. Clair Township need?
- Building and plumbing permit application (County of Lambton residential form, or the Cloudpermit equivalent) with the designer's BCIN information
- Site plan showing lot dimensions and area, the house and its attached garage, every existing accessory building (for the 10%-of-lot combined cap), setbacks to each lot line, the driveway and the extra parking space
- Construction drawings — foundation, floor plans, elevations and cross sections; the manufacturer's certified drawings for a CSA A277 modular unit
- On a well and septic: a septic permit application or a sewage-system assessment of the existing system
- St. Clair Region Conservation Authority permission where the lot is in its regulated area
How much does an ADU building permit cost in St. Clair Township?
Estimate for a ~600 sq ft (≈56 m²) detached suite under St. Clair's Fees By-law 11 of 2025: the residential building-permit fee is $100 plus $0.60 per sq ft of gross area — about $460 — plus a $1,000 completion deposit that's refunded when the work is finished. A septic system adds the County's permit fee. These are estimates: the fees are re-set by by-law each year, so confirm current figures with the Township (519-867-2021).
| Building permit — residential | $100 + $0.60 per sq ft of gross area (Fees By-law 11 of 2025). A 600 sq ft suite ≈ $460; the 721 sq ft maximum ≈ $533. |
|---|---|
| Completion deposit | $1,000, refundable; a further $2,000 refundable deposit applies where the work crosses the St. Clair River Trail |
| Septic (County of Lambton, 2025) | $500 for a new Class 4 leaching-bed system; $250 for a tank or bed replacement; $100 for an assessment to reuse an existing system with a new connection |
| Water and sewer | $50 per service-connection permit (water, storm, sanitary); $2,500 to install a new water service on an existing residential lot (up to 25 mm), where one is needed |
| Second Dwelling Agreement | $200 (Township fees by-law) — ask the Clerk's Department whether your unit needs one |
A qualifying additional residential unit is exempt from development charges under the Development Charges Act (s. 2(3.1)–(3.3)), which since June 8, 2023 covers one unit in a building ancillary to a house on any parcel. For scale, St. Clair's 2026 schedule charges a new single or semi-detached house $9,815.79 where it connects to the sewer and $6,747.15 elsewhere. Check any quoted charge against the Act before you budget.
How long does an ADU building permit take in St. Clair Township?
Ontario's Building Code requires a decision on a complete house-class application within 10 business days. The Ontario Building Code gives a house-class permit 10 business days from a complete application. A conservation-authority permission, a septic permit or a Second Dwelling Agreement comes before the building permit, so start those first.
Registration: St. Clair's Fees By-law 11 of 2025 lists a $200 Second Dwelling Agreement. Ask the Clerk's Department (519-867-2021) whether your unit needs one before you apply for the permit.
Worth knowing before you apply
- The 10%-of-lot cap counts every accessory building on a residential lot, attached garage included — a 600 m² lot with a 25 m² attached garage leaves 35 m² for the suite.
- Height is measured to the highest point of the roof, not the midpoint, and the limit is 5 m — check the foundation and roof pitch on the drawings.
- Permits are issued by the County of Lambton, not the Township: the Township sets the fees and answers zoning questions, the County reviews drawings and inspects.
- Electrical is a separate permit and inspection through the Electrical Safety Authority (1-877-372-7233).
- Starting construction without a permit doubles the permit fee under the Township's fees by-law.
Fees and timelines are estimates from St. Clair Township's published schedules, verified October 20, 2026. Rates and processes change — always confirm current requirements with the building department before you apply.
Grants & financing in St. Clair Township
Development charges — exempt for a qualifying garden suite — St. Clair charges a new single or semi-detached house $9,815.79 in 2026 where it connects to the sewer ($6,747.15 elsewhere). The Development Charges Act exempts one unit in a building ancillary to a house, so a qualifying garden suite should owe none of it — check any quoted charge against the Act before you budget.
See every program → Ontario ADU Grants Directory
A modular or prefab home on a permanent foundation is financed like any house. CMHC Prefab Plus allows an insured mortgage with as little as 5% down on the first $500,000, with construction funds released in stages. How modular home financing works →
Thinking of the suite as a rental? Run the numbers →
St. Clair Township modular homes — FAQ
Can you build a garden suite in St. Clair Township?
Yes. St. Clair's Zoning By-law 17 of 2003 §4.33 allows one second dwelling unit beside a single-detached, semi-detached or duplex house with street access in the R1, R2, R4, Suburban Residential and Agricultural zones — inside the house or in a separate one-storey building in the side or back yard. In the sewered villages (Corunna, Courtright, Brigden, Sombra, Wilkesport, Port Lambton) the Planning Act also guarantees a unit in a building ancillary to the house.
How big can a garden suite be in St. Clair Township?
Up to 67 m² (≈721 sq ft), one storey and 5 m (≈16 ft) tall. On a residential lot all accessory buildings together — the suite and any attached or detached garage or shed — may cover no more than 10% of the lot and 67 m² in total, so a 600 m² lot with a 25 m² attached garage leaves 35 m² (≈377 sq ft). On Township water and sewer there's no minimum size; on a well and septic the by-law asks for at least 50 m².
Can you build a garden suite on a well and septic in St. Clair Township?
Yes, "subject to approval": the Township reviews the water quality and septic capacity first, and the County of Lambton's Building Services issues the septic permit ($500 for a new leaching-bed system in 2025, or a $100 assessment to reuse the existing system). On a farm or Suburban Residential lot the suite must be within 10 m of the house and meet the province's Minimum Distance Separation from barns.
Can you put a modular home in St. Clair Township, Ontario?
Yes. A CSA A277 modular home on a permanent foundation is legal residential housing under the Ontario Building Code, and the County of Lambton issues the building permit for St. Clair through Cloudpermit. As a garden suite it has to stay one storey and within 5 m (≈16 ft) of height to its highest point.
How much does a modular home or backyard suite cost in St. Clair Township?
Modular home models start at $175,696 and run to about $338,000 for the largest layouts; backyard ADU models start at $96,244. Site work, foundation, delivery, and permits are additional.
Can I get a mortgage on a modular home in St. Clair Township?
Yes — on a permanent foundation it is financed as real property, including via CMHC Prefab Plus (5% down on the first $500,000).
Local rules summarized from Township of St. Clair Comprehensive Zoning By-law 17 of 2003, §4.33 Second Dwelling Unit Provisions (By-law 11 of 2013) and §4.3.4 accessory coverage (January 2024 consolidation); verified by Modular Homes 400 and reviewed by James Clarke, REALTOR®. Always confirm current requirements with the municipality before you build.