The client
Buyers in London put $108,000 (15%) down on a $720,000 duplex with a basement suite already renting for $1,650/month, against $11,500/month of their own primary income. The suite has existed since long before the city's current zoning bylaw was adopted.
Purchase price
$720,000
London duplex
Down payment
$108,000 (15%)
Insured file
Basement suite rent
$1,650/month
Legal non-conforming, pre-dates current zoning
Primary income
$11,500/month
Before any add-back
Other debt
$300/mo car loan
Unchanged throughout
The problem
The lender's standard secondary-suite checklist asks for a building permit proving the suite was legally registered as a new unit. This suite doesn't have one, and never needed one: it was built years before the city's current zoning bylaw took effect, which makes it legal non-conforming — grandfathered and lawful to keep renting exactly as it is, but never subject to a permit process for a use that already existed when the rule changed.
Legal non-conforming vs. a newly permitted suite
- ▸A newly registered secondary suite needs a building permit issued under the current bylaw
- ▸A legal non-conforming suite predates the bylaw entirely -- it was never built 'under' a permit process that didn't yet apply to it
- ▸The document that actually proves grandfathered status is a municipal compliance letter, not a permit
A reviewer working from a checklist built for the first case can easily read the second as a red flag, when the underlying rental income treatment the lender actually applies doesn't change at all once the suite's status is confirmed the right way.
The numbers
Once the suite's legal status was properly documented, the file's math was the same as any owner-occupied purchase with a secondary suite.
| The insured purchase with add-back income | Amount |
|---|---|
| Purchase price | $720,000 |
| Down payment (15%) | $108,000 |
| Base mortgage | $612,000 |
| CMHC premium — 2.80% at 80.01-85% LTV | +$17,136 |
| Total insured mortgage | $629,136 |
| Qualifying income with the suite's add-back | Figure |
|---|---|
| Minimum qualifying rate on a 4.60% contract rate | 6.60% |
| Payment at the qualifying rate, 25 years | $4,252/mo |
| Add-back — 50% of $1,650 suite rent | +$825/mo |
| Qualifying income ($11,500 + $825) | $12,325/mo |
| GDS (payment + $340 tax + $150 heat) ÷ qualifying income | 38.5% |
| TDS (GDS numerator + $300 car loan) ÷ qualifying income | 40.9% |
Both ratios sit inside CMHC's 39% GDS and 44% TDS maximums — and they would have looked identical whether the suite were newly permitted or, as here, legal non-conforming. The legal-status question never touched the math; it only touched which document proved the suite could be counted at all.
The solution
A mortgage agent went to the City of London's planning department rather than treating the missing permit as a dead end.
First, confirmed the suite predated the current zoning bylaw. Property records and the sellers' own disclosure showed the suite had existed, continuously, for well over a decade before the bylaw's adoption date.
Second, requested a legal non-conforming use compliance letter from the municipality. This is the specific document London's planning department issues to confirm a use predates a bylaw change and remains lawfully grandfathered — distinct from, and not a substitute for seeking, a building permit for a use that would be new today.
Third, submitted the letter in place of the building permit the lender's checklist expected. Paired with the existing lease and 12 months of rent deposits, it gave the lender everything its policy actually required, just not in the document type it had defaulted to asking for.
The outcome
The lender accepted the compliance letter in place of a building permit and added back 50% of the $1,650 rent in full. GDS settled at 38.5% and TDS at 40.9%, both inside CMHC's maximums, and Ontario's land transfer tax on the purchase came to $10,875.
The 50% add-back percentage is this lender's own policy; other lenders offset rental income against carrying costs instead, which can produce a different qualifying number for the same suite.
What to take from this file
- 01Legal non-conforming is not the same as illegal or unpermitted. A use that predates a zoning bylaw is grandfathered by definition, and doesn't need a permit it was never subject to in the first place.
- 02The right document is a compliance letter, not a building permit. Asking the municipality for the wrong document type wastes weeks a purchase file often doesn't have.
- 03A suite's legal status and its income treatment are two separate questions. Once status is confirmed, the add-back or offset math is identical to any other secondary suite.
- 04Zoning bylaws change; existing uses often don't have to. Check the suite's history against the bylaw's adoption date before assuming a missing permit means a missing legality.
- 05Confirm which convention a given lender uses -- add-back or offset -- before qualifying a client on either number. The two can produce materially different qualifying income for the same rent.
Sources
Every regulatory figure in this file traces to one of these primary sources. Client details and anything that varies by lender are illustrative, as flagged below.
- ▸CMHC — Purchase (Mortgage Loan Insurance) — default-insurance premium schedule by LTV band (25-year amortization).
- ▸CMHC — CMHC Home Start — minimum down payment tiers (5%/10%) and the $1.5M insured price cap.
- ▸OSFI — Minimum qualifying rate for uninsured mortgages — the minimum qualifying rate — greater of contract rate + 2% or 5.25%.
- ▸CMHC — CMHC Reviews Underwriting Criteria — GDS 39% / TDS 44% maximums and the 600 credit-score floor for insured files.
- ▸Ontario.ca — Calculating Land Transfer Tax / Land Transfer Tax Refunds for First-Time Homebuyers — Ontario's marginal land transfer tax brackets and first-time-buyer refund.
- ▸Provincial/territorial mortgage-broker legislation fetched directly (bclaws.gov.bc.ca, legisquebec.gouv.qc.ca, fcaa.gov.sk.ca, web2.gov.mb.ca, nslegislature.ca, assembly.nl.ca) plus FCNB's own site for NB and CanLII's index for PE — see notes for per-province method — provincial mortgage regulators and licence titles.
Illustrative in this file — lender-specific, not rules:
- ▸4.60% contract rate — rates move daily; not a quote.
- ▸the 50% add-back percentage — each lender sets its own rental-income treatment; some add back a percentage of gross rent, others offset it against costs instead.
Authority & provenance
How this case file was built
We publish the origin, the verification method and the reviewer for every case file, so you can judge how far to trust it before you rely on it with a client.
Where it comes from
Derived from files handled by Treadstone’s fulfillment desk and from scenarios contributed by partner brokerages. Names, employers, exact amounts and dates are changed so no client or file is identifiable.
Provenance: Composite — a pattern seen repeatedly on fulfilled files, not a single transaction.
What is verified
Every regulatory figure traces to a primary source listed above and was checked against it on the date shown. The arithmetic is recomputed by machine on every rebuild.
Anything that varies by lender is labelled illustrative rather than stated as a rule.
Who reviewed it
Reviewed for Canadian regulatory accuracy before publication, and re-checked whenever a cited rule changes.
Reviewed by: Nicholas Parson, Treadstone Associates — reviews every case file before publication.
This case file is professional reference material for licensed mortgage professionals. It is not advice to a borrower, and it is not a lender commitment. Insurer rules, qualifying rates and provincial taxes change — confirm the current position with the insurer, regulator or lender before you rely on any figure here in a live file.