The Garden Suite That Almost Didn't Pencil

The file

• Simcoe County family, mid-career, two kids. Long-term owners.
• Primary residence: roughly $850,000 appraised value, $350,000 mortgage remaining at 4.79%.
• Lot size and zoning that work under Bill 23 for a detached garden suite as-of-right.
• No rental property yet. Combined household marginal tax rate around 38%.
• Walk-in question: “Should we build a garden suite, and if yes, how do we finance it?”

The default path

Most files in this seat get a builder quote, a refinance bid, and three siloed conversations: a builder talks build cost, a broker talks rate, an accountant gets a question after the fact about whether anything's deductible. The build pencils on optimistic rent and optimistic cost, the file gets a refi to 80% loan-to-value, construction starts. Six months later there's a $40,000 cost overrun and the household is bringing cash to close.

The path we took

Three conversations on the same file, in the same week:

1. Pre-construction appraisal pre-cleared with the lender. Two appraisers gave the same range on post-build value. The conservative number became the planning case. That number locks in what the lender will actually finance to, before a shovel enters the ground.
2. Build cost sized with a 12% contingency. Fixed-price contract with the builder where possible. Soft-cost line items for permits, services hookup, and grading priced separately so they don't leak into structure. The number we ran on the financing assumed the contingency was spent. If it wasn't, the cushion went straight at the personal mortgage.
3. Refinance into a readvanceable mortgage for the build, sub-account dedicated. From day one of suite occupancy, the suite's operating expenses route through a rental HELOC sub-account and the suite's rental income flows to the personal mortgage as prepayment. The cash-damming structure is built into the file before the first tenant moves in, not retrofitted later.

The 15-year math

Modelled on these inputs, the garden suite generates somewhere in the $1,500 to $1,800 a month range of net operating income (rent less the operating expenses the household covers). After debt service on the build portion of the refi, the file cash-flows positive from year one. The cash-damming layer adds roughly $25,000 to $35,000 of cumulative deductible interest over 15 years. By year 15, the household has paid off the personal mortgage roughly four years earlier than the no-build path, owns a second income stream that's appreciated alongside the primary, and is sitting on a balance sheet that looks structurally different than the “just refinance and renovate” alternative.

Why most clients miss this

Three lenses, three conversations, three blind spots. The builder quotes optimistic without contingency. The broker shops a rate without modelling the post-build LTV. The accountant flags deductibility too late to set up the account routing cleanly. None of them are wrong; they're just operating in their lane. The edge of running broker-builder-developer in one operator is that the three lenses can sit on the same kitchen table at the same time, and the file gets sized to land instead of sized to start.

What it took

• Three strategy calls (initial scoping, mid-build check-in, post-occupancy structure review).
• One pre-construction appraisal opinion in writing from the lender, before the builder contract was signed.
• A CPA conversation to confirm the cash-damming structure on a household with no prior rental file.
• About 9 months from first conversation to occupancy, including a 4-month build window.

The thing I keep coming back to is that this file almost didn't pencil on the conservative numbers. If the rent assumption had landed $200 a month lower, or the LTV ceiling had come back tighter, the right answer would have been “don't build,” and that would have been a valuable outcome too. The strategist's job isn't to make the file work. It's to know whether the file works on numbers tight enough that real life can land inside them.