Construction Mortgage Guide for ADUs and Garden Suites in Ontario

A HELOC or a cash-out refinance covers most secondary suite builds because the money lands in one lump sum and the homeowner manages the spending. A detached garden suite or coach house is a different animal. The build takes months, the money goes out in stages tied to physical progress on the lot, and the lender is underwriting a property that does not exist yet. That is what a construction mortgage is for. The Ontario ADU Financing Hub covers the full menu of ADU financing options at a summary level. This one goes deeper on the mechanics of the draw mortgage itself, because it is the option people understand the least and get wrong the most often.

What a construction draw mortgage actually is

A construction mortgage, sometimes called a builder's mortgage or a draw mortgage, is a facility where the lender commits to a maximum loan amount but releases the money in installments as the build reaches specific physical milestones, not all at once at closing. This is fundamentally different from a standard mortgage or a HELOC, where the full amount is available the day the deal funds. With a construction mortgage, the lender is financing a process, and every dollar released has to be justified by work that is already in the ground.

The logic is protective on both sides. The lender does not want to advance $220,000 against a garden suite that turns out to be a hole in the backyard if the builder walks off the job in month two. The borrower does not want to pay interest on money sitting idle in an account before the framing crew has even shown up. Staged draws solve both problems at once, which is why the structure has not changed much in decades even as the products around it have.

How the draw schedule works in practice

Most Ontario construction mortgages for an ADU or garden suite break the build into five draws. The first releases after the foundation is poured and inspected, typically 15% to 20% of the facility. The second follows framing, once the structure is up and enclosed, usually another 20% to 25%. The third comes at lock-up, meaning the roof, windows, and exterior doors are in place and the building is weathertight, another 20%. The fourth follows drywall and the rough-in of plumbing, electrical, and HVAC, another 15% to 20%. The final draw releases at substantial completion, when the unit has passed final inspection and, where required, an occupancy permit has been issued.

Before each draw is released, the lender sends an inspector, or accepts a progress certificate from the appraiser, confirming the work claimed actually matches the work on site. This is not a formality. Lenders have seen enough construction files go sideways that the inspection step is treated as non-negotiable, and it is the single biggest source of delay in the entire process if the paperwork or the site is not ready when the inspector shows up.

Interest-only during the build, then a real mortgage after

During construction, interest is charged only on the portion of the facility that has actually been drawn, not on the full committed amount. If the facility is $220,000 and only the first two draws totaling $90,000 have been released, the borrower is paying interest on $90,000, not $220,000. This is the main financial advantage of a construction mortgage over simply refinancing the full build cost upfront: carrying cost scales with progress instead of being maximized from day one.

Once the final draw is released and the build is complete, the construction facility converts into a standard amortizing mortgage. This is usually a set term negotiated at the start (commonly a 5-year fixed or variable, in the current rate environment), and the payment shifts from interest-only on a partially drawn balance to full principal-and-interest payments on the entire amount financed. Some lenders fold the converted balance into the existing first mortgage at renewal; others keep it as a separate registered charge. Which structure applies should be confirmed in writing before the construction mortgage is signed, not assumed at the end of the build.

How lenders size the facility

The maximum construction facility is sized against the projected as-complete appraised value of the property, not its current value. A home appraised today at $650,000 with a $300,000 first mortgage, adding a garden suite that will cost $220,000 to build, might appraise at $860,000 once the unit is finished. At a standard 80% uninsured loan-to-value ceiling, the lender is willing to lend up to $688,000 against that future value, which comfortably covers the existing $300,000 mortgage plus the $220,000 build. For borrowers who qualify for the insured refinance program for secondary suites (see the ADU Financing Hub link above for the full mechanics), the ceiling rises further, applied against the same as-complete value logic. Either way, the as-complete appraisal is the number that governs everything, and it is worth getting a written opinion of that value from the lender before committing to a build budget.

What the lender actually wants to see in the file

A construction mortgage file is underwritten differently from a standard purchase or refinance. The lender wants a signed, fixed-price builder's contract, not a verbal estimate. They want an itemized cost breakdown showing where the money goes: site prep, foundation, framing, mechanical, finishing, and a contingency line. They want confirmation of permit status, ideally an issued building permit rather than a pending application, since draw timing is tied to inspections the municipality has to sign off on. Some lenders also require the builder to be licensed and insured, and for a detached garden suite specifically, working with a builder who has done draw financing before matters more than most homeowners expect. A company like DevCom Homes in Southern Georgian Bay works directly with the financing side of a build, which keeps the draw schedule and the construction timeline moving together instead of fighting each other.

Disclosure: I co-own DevCom Homes, and I would rather state that than leave you to find it. It is also why I can speak to this side of it in detail. A draw schedule only works when the lender's inspection timing and the builder's milestones are planned against each other, and having both at the same table from day one is what keeps a project from stalling between advances. Compare quotes anyway, because a good builder will expect you to.

Why the timeline is longer than a refinance or a HELOC

A standard refinance or HELOC can close in three to four weeks. A construction mortgage routinely takes six to ten weeks to get to the first advance, and that clock does not include the build itself. The difference comes from the underwriting layer that does not exist on a standard file: an as-complete appraisal based on plans and specs rather than a finished structure, a review of the builder's contract and cost breakdown, confirmation of permit status, and in some cases a separate legal review of the draw structure itself. None of this is optional, and trying to compress the timeline by skipping a step almost always costs more time later when a draw gets held up mid-build.

Where these files go wrong

Three mistakes show up on construction mortgage files more than any others. The first is underestimating contingency. A $220,000 quote with no buffer becomes a $250,000 problem the moment the excavator hits rock or the trades quote comes in above the original estimate. A 10% to 15% contingency built into the facility from the start absorbs this without forcing a mid-build renegotiation with the lender. The second is not having the next draw ready when a milestone completes. If the builder finishes framing on a Friday but the inspection paperwork, photos, or appraiser sign-off is not submitted until two weeks later, the crew either sits idle waiting on cash or the borrower fronts money out of pocket to keep momentum, which defeats the purpose of the draw structure. The third is not confirming appraisal timing with the lender upfront. Some lenders require their own appraiser for the as-complete valuation and will not accept one commissioned independently; if that requirement surfaces after the build has started, it can add weeks to a draw that was otherwise ready to release.

A construction mortgage is the right tool when the build is large enough or the borrower's equity thin enough that a HELOC or a single-advance refinance does not cover it. It is not the simplest product on the shelf, and it demands more coordination between the borrower, the builder, and the lender than any other financing path for an ADU. Get the builder's contract, the cost breakdown, and the permit timeline lined up before the mortgage application goes in, and the draw process runs the way it is supposed to. If income from the finished unit is part of how the file qualifies, the companion piece on using rental income to qualify walks through how lenders treat projected rent on a unit that is not built yet. And before locking in a build budget, it is worth running the numbers on the ADU Financing calculator to see what facility size the as-complete value actually supports, or run the full build-vs-stay numbers to see how the finished project compares to simply staying put.