Using Home Equity to Build an ADU: How Much Is Actually Usable

Before a homeowner should be comparing a HELOC to a refinance to a construction mortgage, there is a more basic question sitting underneath all three: how much equity is actually there to work with? Most people answer this with a rough guess, usually some version of "the house is probably worth $150,000 more than I owe, so I've got $150,000 to play with." That guess is almost always wrong, in both directions, and it is the reason so many ADU conversations stall out after a homeowner gets a quote and finds the financing doesn't reach. Equity is not what you think you have. It is a specific, calculable number, and lenders will only let you touch a portion of it. Understanding that number properly, before picking a product, is what actually determines whether a build is affordable. For the full menu of financing programs once the equity math is clear, see the complete ADU financing guide.

What "equity" actually means

Equity is current appraised value minus what is still owed on the property. It is not purchase price minus what is owed, which is the mental math most homeowners actually run. If a home was bought for $500,000 five years ago and the mortgage balance has been paid down to $420,000, the instinct is to think there's $80,000 of equity. But if that home now appraises at $750,000, the real equity position is $330,000. Appreciation, not paydown, is usually doing most of the work in Ontario markets that have seen meaningful price growth over the last several years, and a lot of homeowners are sitting on far more usable equity than they assume simply because they're anchoring to the wrong starting number.

The reverse also happens. A homeowner who bought recently at a peak price, or in a market that has softened, can have less equity than the purchase-price math suggests, or in a thin-equity scenario, less than they need to fund a build at all. Either way, the only number that matters to a lender is today's appraised value against today's balance, confirmed by an actual appraisal, not a homeowner's estimate or an automated online valuation. That appraisal is the first real step in any ADU financing conversation, because everything downstream, the HELOC room, the refinance proceeds, the insured program eligibility, is sized against that one number.

Why you can't access all of it: the 80% ceiling

Even once the real equity number is known, a lender will not advance against all of it. The standard ceiling in Canada for a conventional, uninsured mortgage or HELOC is 80% combined loan-to-value, meaning the first mortgage and any HELOC secured against the same property together cannot exceed 80% of the appraised value. This is not an arbitrary lender preference. It is a regulatory guardrail, and it exists for two reasons. First, it leaves a buffer: if property values soften, the lender's security position stays intact even after a price decline, which protects both the lender and, indirectly, the borrower from being underwater. Second, mortgage default insurance from CMHC and the other insurers, the mechanism that allows lending above 80% LTV in the first place, generally isn't available on a refinance under normal rules. Insurance exists mainly for purchases with less than 20% down. Take equity out through a refinance or a HELOC on a property you already own, and you are typically capped at 80%, full stop, because there is no insurer standing behind the portion above that line. That is exactly why the 2025 insured refinance program, covered further down, is such a notable exception. It is a purpose-built carve-out to a rule that otherwise holds firm.

A worked example: usable equity at 80% LTV

Take a home appraised at $750,000 with a mortgage balance of $380,000. The 80% combined ceiling on that appraised value is $600,000. Subtract the existing $380,000 owed, and the usable equity available through a HELOC and the first mortgage together is $220,000. That is the number that actually matters for sizing an ADU build, not the raw equity position of $370,000 (the $750,000 value minus the $380,000 owed), and not a flat "20% of home value" guess. The gap between what a homeowner assumes is available and what the 80% ceiling actually permits is usually the first surprise in this conversation.

It also matters where that $220,000 comes from. A HELOC on its own, without being paired with an amortizing first mortgage, is typically capped at 65% of appraised value, not 80%. On this same $750,000 home, 65% is $487,500. If the existing $380,000 mortgage already sits on title, a revolving HELOC limited to that 65% ceiling only has room for about $107,500 above the existing balance. It is only when the HELOC is structured alongside the first mortgage, with the two balances combined and measured against the 80% ceiling instead of the HELOC's own 65% limit, that the full $220,000 becomes available. That difference, roughly $107,500 versus $220,000 on the exact same property, is the reason how the HELOC is set up matters as much as whether one is set up at all. The mechanics of drawing against that combined structure versus a straight refinance or a construction draw facility are covered in the HELOC-versus-refinance-versus-construction-mortgage comparison, once the usable equity number itself is settled.

The 2025 insured refinance program changes what "usable" means

Everything above assumes the equity being measured already exists today. The insured refinance program introduced federally in January 2025 breaks that assumption in a specific and useful way. It allows an owner-occupied refinance for the purpose of building a secondary suite to go to 90% loan-to-value instead of the standard 80%, and, more importantly, it bases that 90% against the as-complete appraised value, the property's projected value once the ADU is finished, rather than what the home is worth today.

Run the same $750,000 home with the same $380,000 mortgage through this program. Suppose an appraiser projects that adding a secondary suite brings the completed property to $920,000. Ninety percent of $920,000 is $828,000. Subtract the $380,000 owed, and the available proceeds are $448,000, more than double the $220,000 available under the standard 80%-of-today's-value calculation. The extra $228,000 in this example isn't equity the homeowner has built up through paydown or past appreciation. It is equity the build itself is projected to create, unlocked in advance because the insurer is willing to lend against the future value rather than requiring it to exist first. CMHC premiums apply on the insured amount and add to the carrying cost, and the projected as-complete value has to come from an appraiser, not the borrower's own estimate of what the finished unit will be worth, but for a build that doesn't pencil against today's equity, this is frequently the program that makes it possible at all.

Why running the real numbers beats guessing at 20%

The reason to work through this before falling in love with a floor plan is that the difference between "20% of home value" and the actual usable-equity figure, calculated against a real appraisal and the correct ceiling for the correct product, routinely swings by six figures on an average Ontario property. A homeowner who assumes $150,000 is available might actually have access to $220,000 through a properly structured combined HELOC, or over $400,000 through the insured refinance program if the build itself justifies the as-complete value. Alternatively, a homeowner assuming $150,000 based on a rough purchase-price calculation might find the real number, after an actual appraisal and the 80% ceiling, is closer to $60,000. Both directions matter, and both are common. The only way to know which one applies to a specific property is to run the numbers against the actual appraised value, the actual mortgage balance, and the specific program being considered, rather than estimating. The ADU Financing calculator is built for exactly this step, to run your own equity numbers across every program before deciding whether, or how, a build makes sense.