How to Underwrite a Rental Property: Residential (1-4 Units) vs Commercial (5+ Units)

The difference between a good real estate deal and a money pit usually comes down to one thing: underwriting. It's not glamorous, it's not exciting, but it's the single most important skill you'll develop as a rental property investor.

Underwriting is simply the process of analyzing a property's financials to determine whether it's a good investment at the asking price. But the way you underwrite a duplex looks very different from how you underwrite a 20-unit apartment building. The numbers are different, the lenders are different, and the sources you use to verify assumptions are different.

This guide breaks down exactly how to underwrite both — so whether you're looking at your first duplex or scaling into multifamily, you'll know what to look for and where to find the data.

Why Underwriting Matters

Every property listing looks good in the seller's marketing materials. But sellers are motivated to present the best possible picture — they'll use "pro forma" rents (what rents could be, not what they are), understate expenses, and gloss over deferred maintenance.

Your job as a buyer is to build your own financial model based on verifiable data. If the numbers still work after your conservative analysis, you've found a deal. If they don't — you walk away and save yourself years of headaches.

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Part 1: Underwriting Residential Properties (1-4 Units)

Residential properties — single-family rentals, duplexes, triplexes, and fourplexes — are where most Canadian investors start. The underwriting process is straightforward, but you need to be disciplined about verifying every assumption.

Step 1: Determine Market Rent

This is your most important input. If you get rents wrong, nothing else matters.

Where to check current market rents:

• Facebook Marketplace — Search "apartments for rent" or "rooms for rent" in your target area. Filter by number of bedrooms and price range. This is the most up-to-date source for what's actually being listed right now.
• Kijiji — Canada's largest classifieds site. Search under Real Estate → For Rent. Filter by city, bedrooms, and price. Look at listings posted in the last 7-14 days for the most current data.
• Rentals.ca — Aggregates listings and publishes monthly rent reports by city and unit type. Good for trend data.
• CMHC Rental Market Report — Published semi-annually (April and October). Gives average rents by bedroom count and neighbourhood. The data lags 3-6 months, but it's the most authoritative source.
• Zumper / PadMapper — Additional listing sites to cross-reference.

How to use these sources:

1. Search for comparable units: same neighbourhood, same bedroom count, similar condition.
2. Collect 5-10 comparable listings.
3. Discard the highest and lowest outliers.
4. Use the median of the remaining listings as your estimated market rent.
5. If the property needs renovation, estimate rent at the current condition — not what it could be after improvements (unless you're budgeting for those improvements separately).

Pro tip: Take screenshots of your comps with dates. You'll need them when talking to lenders and partners.

Step 2: Estimate All Expenses

This is where most new investors make mistakes — they underestimate expenses. Here's the full list for a residential rental:

The biggest mistake: Not including CapEx reserves. A furnace costs $5,000-$8,000 to replace. A roof costs $10,000-$20,000. If you're not setting aside money monthly for these inevitabilities, you'll be blindsided.

Step 3: Calculate Cash Flow

Once you have rent and expenses, the math is simple:

Gross Rent (monthly)
- Vacancy Allowance
= Effective Gross Income (EGI)
- Total Operating Expenses
= Net Operating Income (NOI)
- Mortgage Payment (P+I)
= Cash Flow

What's a good cash flow? Most investors target $200-$400/month per unit after all expenses and mortgage. If you're below $100/unit, the margins are too thin — one unexpected repair wipes out your annual return.

Step 4: Run the Key Metrics

• Cash-on-Cash Return = Annual Cash Flow / Total Cash Invested. Target: 8-12%+
• Cap Rate = NOI / Purchase Price. Gives you an unlevered return and comparison point between properties.
• Gross Rent Multiplier (GRM) = Purchase Price / Annual Gross Rent. Quick screening tool — lower is better.
• The 1% Rule (screening only): Monthly rent should be ~1% of purchase price. In expensive Canadian markets (Toronto, Vancouver), this is nearly impossible — use it as a rough filter, not a hard rule.

Step 5: Stress Test

Before committing, ask:

• What if rates rise 2% at renewal? (Canadian mortgages are 5-year terms — you will face this)
• What if vacancy doubles?
• What if a major repair ($10K+) hits in year one?

If the deal survives these scenarios with positive or break-even cash flow, it's resilient. If it goes deeply negative, the margins are too thin.

Use our free residential calculator tools to model these scenarios quickly.

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Part 2: Underwriting Commercial Properties (5+ Units)

Once you cross the 5-unit threshold, you're in commercial real estate territory. The rules change significantly:

• Lenders value the property based on its income, not comparable sales
• DSCR (Debt Service Coverage Ratio) becomes the primary qualification metric
• Down payments are typically 20-25% (sometimes higher)
• CMHC offers MLI Select insurance for qualifying multifamily buildings (can reduce down payment to 5-15%)
• The seller provides a full financial package: rent roll, trailing financials, utility statements

The Commercial Underwriting Framework

Commercial underwriting is more structured than residential. Here's the framework:

1. Revenue Analysis

Start with the current rent roll — a line-by-line list of every unit, its current rent, and lease status.

Then determine:

• Market rent per unit — Is the building above or below market? Use the same sources (Kijiji, Facebook Marketplace, CMHC data), but also reference CMHC's Housing Market Information Portal for vacancy rates at the neighbourhood/zone level.
• Other income — Parking, laundry, storage, pet fees. These add up on larger buildings.
• Vacancy rate — Use CMHC's semi-annual survey data for your specific CMA (Census Metropolitan Area). Don't just guess 5% — verify with actual data.

Gross Potential Rent (all units at market rent)
+ Other Income
- Vacancy & Bad Debt Allowance
= Effective Gross Income (EGI)

2. Expense Analysis

Commercial properties have a more detailed expense structure:

Key principle: Use trailing 12-month actuals as your baseline, then adjust upward for items you believe are understated. Never use the seller's pro forma without verification.

3. Net Operating Income (NOI)

EGI - Total Operating Expenses = NOI

NOI is the single most important number in commercial real estate. It determines:

• The property's value (NOI / Cap Rate = Value)
• Whether you qualify for financing (NOI / Debt Service = DSCR)
• Your return as an investor

4. Valuation

Commercial properties are valued by their income, using cap rates:

Property Value = NOI / Market Cap Rate

Find market cap rates through:

• Recent multifamily sales in the area (brokers can provide)
• CBRE, Colliers, or JLL market reports
• Municipal assessment data (though this often lags)

If the asking price implies a cap rate well below market, the property may be overpriced. If it implies a cap rate above market, investigate why — there may be hidden problems.

5. Debt Analysis (DSCR)

Lenders for commercial properties care primarily about DSCR:

DSCR = NOI / Annual Debt Service

• Most conventional lenders require DSCR of 1.20-1.30 (meaning NOI is 20-30% above the mortgage payment)
• CMHC MLI Select requires minimum 1.10 DSCR but offers better rates and higher LTV for qualifying buildings (energy efficient, accessible, or affordable)
• Typical terms: 25-30 year amortization, 5-year term, 75-80% LTV

6. Cash-on-Cash and Equity Build

After the debt analysis, calculate:

• Cash-on-Cash Return — Same as residential, but target 8-15% for the additional risk and complexity
• Equity build — How much principal are you paying down annually through the mortgage?
• Total return — Cash flow + equity build + any value increase from rent growth

Using the Multifamily Underwriter Tool

For commercial deals, we built the Multifamily Underwriter — a purpose-built tool that handles the full commercial analysis:

• Buy & Hold Underwriter — Input your rent roll, expenses, financing terms, and market cap rate. The tool calculates NOI, property value, DSCR, cash-on-cash, and tells you whether the deal works at the asking price.
• Value Add Underwriter — Model renovation scenarios: what happens to your returns if you invest $X in improvements and raise rents by $Y? See how value-add plays out over your hold period.

The tool pulls together all the calculations above into a single dashboard — no spreadsheet required. You input the data from your due diligence (rent roll, expenses from trailing financials, lender terms) and the model runs the full analysis.

Key inputs the tool needs:

• Purchase price and financing terms (rate, amortization, LTV)
• Current rent roll (rent per unit)
• Vacancy rate (source from CMHC Housing Market Information Portal)
• Operating expenses broken down by category
• Market cap rate (from recent sales comps)

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Residential vs Commercial: Key Differences at a Glance

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Common Underwriting Mistakes

Whether residential or commercial, these mistakes sink deals:

1. Using asking rents instead of market rents. Always verify independently — sellers inflate rent assumptions.
2. Ignoring CapEx reserves. A building with no reserve fund is a ticking time bomb.
3. Trusting the seller's expense numbers. Request actual utility bills, tax assessments, and maintenance invoices. If the seller won't provide them, walk away.
4. Not accounting for the mortgage stress test. Canadian lenders qualify you at the higher of your contract rate + 2% or the qualifying rate. Budget accordingly.
5. Falling in love with the property. Underwriting is emotional protection. If the numbers don't work, no amount of "potential" changes that. Move on.

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Start Underwriting Today

Good underwriting is the foundation of successful real estate investing. Whether you're analyzing a duplex on Kijiji or a 30-unit apartment building, the principles are the same: verify revenue, account for all expenses, stress-test your assumptions, and only proceed when the numbers work under conservative conditions.

Ready to run the numbers?

• Use our residential calculator tools for 1-4 unit properties — mortgage payments, cash flow, affordability, and more.
• Use the Multifamily Underwriter for commercial deals (5+ units) — full rent roll analysis, DSCR, cap rate valuation, and value-add modelling.
• Take our Real Estate Investing Course to learn the full framework, from finding deals to building a portfolio.

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Not financial advice. For educational purposes only. Consult a licensed financial advisor and real estate professional before making investment decisions.