What It Can Cost To Build A Detached ARU In London, Ontario

The Build Price Is Only One Part Of The ARU Budget

Many homeowners start by asking what a garden suite or backyard home costs to build. That is the right question, but it needs more than a model price. A detached ARU budget should include the unit, utility connections, design and engineering, permits, rental licensing, site work, financing, insurance, possible development charge review, and property tax reassessment.

A property can look simple from the backyard and still become expensive if services are far away, access is tight, trees are in the way, grading is difficult, or zoning forces a smaller custom layout.

Use Model Prices As A Starting Point, Not A Final Budget

Local competitors often publish model or starting prices for backyard homes and garden suites. These can be useful for early planning, but they may not include HST, utility connections, trenching, servicing upgrades, site access, drawings, engineering, permits, rental licensing, insurance, development charges, or property-specific work.

A starting price is not the same thing as a finished, legal, rentable ARU on your property.

The Main Cost Buckets To Plan For

A realistic budget gets clearer once you separate the project into its main cost buckets.

• Unit construction - The shell, foundation, framing, exterior, interior finishes, windows, doors, HVAC, plumbing, electrical, and final building work.
• Utility connections - Water, sanitary, hydro, gas, internet, trenching, service capacity checks, meters, panels, and final connection work.
• Site conditions - Access, grading, slope, trees, fences, drainage, soil conditions, crane or delivery requirements, and yard restoration.
• Design and professional fees - Survey, drawings, designer, engineering, energy/code review, permit documents, and revisions.
• City and compliance costs - Building permits, rental licence fees, inspections, possible development charge review, and other municipal requirements.
• Ownership costs - Insurance changes, financing costs, contingency, maintenance, vacancy, utilities, and possible property tax reassessment.

Utility Hookups Can Be The Hidden Budget Shock

Detached ARUs usually need a practical route for water, sanitary, hydro, gas, and internet. The distance from the main house, trenching path, capacity of existing services, grading, obstructions, and restoration work can materially change the project cost.

This is why two similar-looking backyard units can have very different final budgets. One site may have a simple service route, while another may require expensive trenching, upgrades, or access work.

Development Charges: Do Not Assume Automatic Exemption

London’s ARU development charge rules can be favourable in some situations, but eligibility is not something homeowners should assume. The City reviews development charge treatment through the permit process, and detached ARU exemptions can depend on the exact project and property facts. Review the development charges guide before you underwrite the project as exempt.

A development charge exemption can be worth a lot, but the wrong assumption can distort the budget. Treat DCs as a review item until confirmed.

How The $45,000 ARU Loan Fits Into The Cost Picture

The London ARU loan or affordable detached ARU forgivable loan can help eligible homeowners, but it does not remove the need for upfront budgeting. The program has eligibility, timing, affordability, rent-cap, owner-occupancy, rental licensing, and compliance rules.

In practical terms, the loan may reduce the long-term funding gap, but homeowners should still plan for deposits, drawings, permits, servicing, construction progress payments, contingency, and cash flow before the funding is received.

The Cost Only Makes Sense If The Rent Math Works

If the project relies on the affordable detached ARU forgivable loan, rent is capped at 100% of CMHC Average Market Rent during the affordability period. That means the cost plan should be tested against capped rent, not unrestricted market rent. Use the rent cap calculator before you rely on a builder number.

• Current planning value used on this site: 1-bedroom ARU at $1,371/month
• Current planning value used on this site: 2-bedroom ARU at $1,645/month

Verify current CMHC Average Market Rent values before applying, because rent-cap numbers can change over time.

Budget For Possible Property Tax Reassessment

Adding a detached dwelling or making major improvements can affect property assessment. Building permits and new secondary structures may trigger review, and homeowners should plan for the possibility of a future property tax change.

This page cannot estimate your tax change. Treat property tax reassessment as a budget risk to confirm before relying on the final return.

Insurance And Financing Should Be Checked Early

A detached ARU can change the risk profile of the property. Homeowners should ask their insurer how a legal rental unit may affect coverage and premiums. Financing should also be reviewed early, especially if the homeowner is relying on home equity, refinancing, or construction-stage cash flow.

The City loan is only one part of the funding stack. The rest of the project still needs a realistic financing plan.

A Simple ARU Budget Framework

Use this framework before comparing builder quotes:

• Base unit or model price
• HST, if applicable
• Foundation or site prep
• Utility trenching and hookups
• Service upgrades, if needed
• Survey, drawings, design, and engineering
• Building permit and inspections
• Rental licence costs
• Development charge review or risk
• Tree, access, grading, drainage, and restoration work
• Insurance changes
• Financing costs
• Property tax reassessment risk
• Contingency

If a quote does not clearly address these items, ask what is included, excluded, estimated, or still subject to site review.

Common Cost Mistakes Homeowners Make

The biggest budget misses usually happen before construction starts.

• Only comparing model prices - A lower starting price may not include servicing, site work, drawings, permits, licensing, or property-specific requirements.
• Ignoring utility routes - Trenching, service capacity, meters, panels, and restoration work can materially change the final budget.
• Assuming the loan solves the cash-flow gap - Funding may be issued after the ARU is created and paid for, so upfront planning still matters.
• Forgetting capped rent - If the affordable detached ARU loan is part of the plan, test the budget against CMHC capped rent.
• Treating DC exemptions as guaranteed - Development charge treatment should be reviewed through the City process before relying on an exemption.