Multi-unit conversions: the basement secondary suite math

Whether a legal basement suite makes sense — with the actual numbers

The strongest leverage available to small-scale real estate investors in Ottawa right now is the legal basement secondary suite. Bill 23 and Ottawa's municipal implementation have made these much easier to permit than five years ago, and the rental math can transform the cap rate on a single-family home.

Let me walk through the rules, the typical costs, and a worked example.

The rules in 2026

Under Bill 23 (More Homes Built Faster Act) and amendments to the Planning Act, most residential lots in Ontario are now permitted up to three residential units "as of right" — without a zoning amendment. This means:

• The primary dwelling
• A secondary suite (within the primary dwelling, typically a basement apartment)
• A third unit (garden suite or laneway in some municipalities)

The City of Ottawa permits secondary suites as of right in most residential zones. "As of right" only removes the requirement for a zoning amendment. You still need:

• Building permit
• Compliance with the Ontario Building Code
• Fire separation between units (typically 45-minute rated assemblies)
• Egress windows in bedrooms
• Separate entrance
• Smoke and carbon monoxide alarms interconnected
• Compliance with Ontario Fire Code (updated January 1, 2026)
• Adequate ceiling height (typically 6'5" minimum in most areas)
• Off-street parking per zoning by-law

Typical cost to add a legal basement secondary suite in Ottawa

For a home that already has an unfinished or partially finished basement:

• Design and permit: $3,000–$5,000
• Walk-out or window-well egress: $4,000–$15,000 depending on whether wall cutting is required
• Rough plumbing and electrical: $12,000–$20,000
• Drywall, flooring, paint: $15,000–$25,000
• Kitchen (basic): $8,000–$15,000
• Bathroom: $9,000–$14,000
• Fire separation and code upgrades: $4,000–$10,000
• HVAC adjustments (separate metering or zoned heat): $3,000–$8,000

Total range: $60,000 to $120,000 for a finished, legal, code-compliant basement secondary suite.

If your basement is already finished but not to code, plan on $20K–$50K to bring it into compliance.

Rental rates for Ottawa basement suites

• One-bedroom basement suite, suburban Ottawa: $1,400–$1,700/month
• Two-bedroom basement suite, suburban Ottawa: $1,650–$2,000/month
• Basement suite in central neighbourhood (Hintonburg, Westboro): $1,750–$2,200/month
• Larger above-grade walk-out suite: $1,900–$2,500/month

Lower end if utilities are included; higher end if tenant pays.

Worked example: Barrhaven detached with legal basement suite

You buy a Barrhaven detached for $780,000. Comparable rents for the entire home as single-family: $3,400/month.

Cost to add a legal two-bedroom basement suite: $85,000.

After conversion:

• Main floor rent (3-bed up): $2,800/month
• Basement rent: $1,800/month
• Gross monthly: $4,600 vs $3,400 as single-family
• Annual gross: $55,200 vs $40,800

That's $14,400/year additional gross revenue on an $85,000 investment. Simple payback: 5.9 years. Effective cap rate on the basement-suite addition: roughly 17% before tax.

The total property economics:

• Combined NOI on $865K all-in basis (purchase + reno): roughly $35,000
• Effective cap rate: 4.05%

That's substantially better than the 2.8–3.5% cap rate range on equivalent single-family rentals in 2026 Ottawa.

Risks and considerations

1. Tenant conflict. Two tenancies in one building means twice the tenant management. Pick tenants carefully, lay out shared expectations in writing.

2. Mortgage and insurance. Some lenders will treat the property as multi-family and underwrite differently. Notify your insurer; rates will adjust.

3. Resale. A property with a legal income suite typically sells at a premium of 5–10% over equivalent single-family, but the buyer pool is smaller. You're selling to investors or to multi-generational households.

4. Capital cost recovery. CRA permits you to claim CCA on the rental portion of the property. Talk to your accountant before claiming — there are recapture implications on sale.

5. HST. If you build a brand-new garden suite or substantially renovate, HST may apply. Consult a tax professional.

Where this works best in Ottawa

Three patterns that produce the strongest math:

1. Older Orleans, Barrhaven, or Kanata detached with high basements and existing rough-in for plumbing. Cheaper conversion, suburban rents that still cover the math.

2. Hintonburg or Old Ottawa South older singles where central rents are higher and the property has resale character even as a multi-unit.

3. Glebe and Westboro semis where converting to a duplex (main floor unit + basement unit) is sometimes possible. Higher rents make even expensive conversions math out.

Where this doesn't work

• New construction with finished basements designed as primary living space. The conversion cost is high and the resale market for "converted into a duplex" new builds is weak.
• Condos. Most condo declarations prohibit this entirely.
• Properties with limited basement ceiling height or no walkout potential.

My advice

If you're sitting on a single-family detached in an Ottawa neighbourhood where you'll be staying 5+ years, adding a legal basement suite is one of the best risk-adjusted improvements you can make. The math works, the resale value is preserved, and it provides ongoing income.

If you're considering buying a property specifically for conversion, run the numbers carefully. Walk through the property with a contractor who has done legal Ottawa basement suite builds before you firm up. I'm happy to refer you to two or three contractors I trust.

To talk through your specific property or potential purchase: 613-262-6545 or fil@613realtor.ca.