The client
A first-time buyer purchasing in Lethbridge planned the classic house-hack: live upstairs, rent the legal basement suite, and let the tenant carry a meaningful share of the mortgage. On the buyer's income alone, the purchase did not qualify. With the suite's lease counted, it did — with roughly two-tenths of a point to spare on GDS.
Borrower
Single, salaried, first-time buyer
$78,000/year
Purchase
$375,000, legal basement suite, Lethbridge
10% down ($37,500)
Suite lease
$1,300/month, signed
50% add-back is this lender's published treatment (illustrative)
Other debt
$180/month
Student loan
First lender tried
Would not count any suite income
Treatment varies by lender on owner-occupied secondary-suite purchases
The problem
On the buyer's income alone, both ratios miss: GDS runs to 42.6% against the 39% insured-mortgage cap, and TDS to 45.4% against the 44% cap — even with a signed suite lease already in hand, because the first lender simply does not credit secondary-suite income on this loan program. That is a lender-by-lender choice, not a rule.
The math without the suite
- ▸Housing costs: P&I at the qualifying rate ($2,371) + property tax ($250) + heat ($150) = $2,771/month
- ▸GDS: $2,771 ÷ the buyer’s $6,500 income = 42.6% — over the 39% ceiling
- ▸TDS adds the $180 student loan: $2,951 ÷ $6,500 = 45.4% — over the 44% ceiling
Nothing about the borrower's own file was in question — same income, same clean credit, same legal suite with a signed lease. What changed between lenders was entirely on the underwriting side: one policy is built to read a legal secondary-suite lease as qualifying income, and one is not, however well the lease is documented.
The numbers
At 10% down this is an insured purchase, so insured-mortgage rules govern the whole structure, not just the ratio caps.
| Structuring the insured purchase | Amount |
|---|---|
| Purchase price | $375,000 |
| Down payment (10%) | −$37,500 |
| Base mortgage (90% LTV) | $337,500 |
| CMHC premium — 3.10% in the 85.01–90% LTV band, capitalized | +$10,462 |
| Total insured mortgage | $347,962 |
The minimum down payment on a $375,000 purchase is $18,750 (5% of the first $500,000); this file put down more than double that minimum to land in the cheaper 85.01-90% premium band instead of the top band.
| Rate & payments | Figure |
|---|---|
| Contract rate (illustrative, not a quote) | 4.69% |
| Minimum qualifying rate — greater of contract + 2% and 5.25% | 6.69% |
| Monthly P&I at the qualifying rate — the ratios run on this | $2,371 |
| Monthly P&I at the contract rate — what would actually be paid | $1,963 |
The 50% add-back, before and after
The 50% suite-income add-back treatment is illustrative of one common secondary-suite policy; each lender publishes its own.
| Ratio | Without the suite | With the 50% add-back |
|---|---|---|
| Income used | $6,500 | $6,500 + $650 = $7,150 |
| GDS ($2,771 housing costs) | 42.6% ✗ | 38.8% ✓ |
| TDS ($2,771 + $180 student loan) | 45.4% ✗ | 41.3% ✓ |
The margin on GDS — 0.2 points under the 39% cap — is thin. A pass is a pass, but there was no room in this file for a surprise at underwriting.
The solution
An RECA-licensed Alberta mortgage associate diagnosed the decline as a policy mismatch rather than a borrower problem, then matched the file to a lender whose insured secondary-suite guidelines publish a 50% add-back for a legal, permitted basement suite.
With GDS clearing by only 0.2 points, the submission went in complete on the first pass, leaving nothing for underwriting to raise as a follow-up condition:
The municipal permit did as much work as the lease itself. An unpermitted or non-conforming suite is a materially different risk to most insured lenders, whatever the rent roll says — confirming legal status before submission avoided a condition that could easily have arrived mid-file instead.
Alberta has no verified transfer-tax fact to cite, so the closing-cost conversation with the client stayed qualitative — land titles registration fees and legal costs were flagged as a range to confirm with the lawyer, not quoted as a specific dollar figure.
The outcome
Approved and funded: insured at 90% LTV, with the legal suite's lease counted at the lender's published 50% add-back. The buyer moved in upstairs with the tenant already in place downstairs, and the file cleared with just enough room on GDS to survive the underwriter's second look.
A 0.2-point margin on a 39% cap is not a number to build a repeatable process around. The lesson this file leaves for the next one is procedural: confirm the add-back percentage and the legal-suite documentation requirement with the specific lender before the client is told the deal works, not after.
What to take from this file
- 01A legal secondary suite's income is a lender-policy question before it is a borrower-qualification question. Confirm which lenders on your shelf will add it back — and by how much — before you pick where the file goes.
- 02The qualifying rate, not the contract rate, decides both ratios. This file qualifies at 6.69% and would actually pay at 4.69%.
- 03A thin margin on one ratio means the whole package has to be clean. With GDS clearing by 0.2 points, every document went in on the first submission, not as a follow-up condition.
- 04Alberta closing costs stay qualitative. With no verified provincial transfer-tax figure, land titles registration fees are a range to confirm with the lawyer, not a number to promise the client.
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.
- ▸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:
- ▸4.69% contract rate — rates move daily; not a quote.
- ▸50% suite-income add-back — each lender publishes its own secondary-suite treatment.
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.