Why It Matters
Here's what the number tells you: add up your full PITI payment (principal, interest, taxes, insurance) plus a maintenance reserve, then subtract every dollar of rent you collect from tenants in the other units. The result is your effective housing cost — what the property actually costs you to occupy each month.
On a duplex where PITI is $2,600 and the other unit rents for $1,400, your effective housing cost is $1,200. If you were renting the same neighborhood apartment, you might pay $1,800. The $600 gap is money that compounds — saved every month, invested or applied to principal, building wealth while you sleep.
This number is the core metric of every house-hack-duplex, house-hack-triplex, and house-hack-fourplex strategy. Run it before you make an offer. Run it conservatively — use a realistic vacancy factor, not best-case rent.
At a Glance
- What it is: Monthly shelter cost after rental income offsets mortgage, taxes, insurance, and maintenance
- Formula: Effective Housing Cost = PITI + Maintenance − Rental Income from Other Units
- Why it matters: Converts a house hack from a lifestyle choice into a quantified investment decision
- Best-case outcome: Zero or negative effective cost — tenants pay your full housing expense
- Common mistake: Using gross rent without vacancy reserve, overstating the offset
- Applies to: Duplexes, triplexes, fourplexes, and single-family rentals with ADUs
Effective Housing Cost = PITI + Maintenance − Rental Income from Other Units
How It Works
Start with full PITI. Principal and interest are fixed from your loan terms. Property taxes and insurance vary by market and owner-occupied-insurance product. Use the actual PITI from your lender's loan estimate — not a rounded estimate. On a $400,000 owner-occupied loan at 7%, P&I alone runs about $2,661/month before taxes and insurance stack on.
Add a maintenance reserve. Many first-time house hackers skip this and then absorb repair costs as surprise expenses. A standard reserve is 5-10% of gross rental income — on $1,500/month in rent, set aside $75–$150 monthly. Some investors use 1% of the property's annual value split into monthly reserves. Either method works; the goal is capturing real cost.
Subtract rental income from the other units. This is where a house-hack-duplex earns its math. If the other unit rents for $1,500 and you're holding 5% for vacancy ($75), your effective offset is $1,425. Subtract that from your total PITI-plus-reserve and you have your effective housing cost.
Apply a vacancy factor. Rental income is not guaranteed every month. A 5% vacancy factor on an annual basis equals roughly 18 days of vacancy per year. For a monthly calculation, reduce gross rent by 5% before applying it to your formula. Ignoring vacancy makes your effective housing cost look better than it performs.
A house-hack-sfr works the same way. Single-family rentals with an accessory dwelling unit, a basement apartment, or rooms rented individually all reduce your effective housing cost by the amount tenants pay. The formula is identical — what changes is the unit count and the rental income figure.
Real-World Example
Kwame is under contract on a duplex in Columbus, Ohio — purchase price $347,000, 5% down via FHA owner-occupant financing. Here's the math before closing:
Monthly PITI (estimated):
- Principal + Interest (30yr, 6.75%): $2,149
- Property taxes (monthly): $393
- Insurance (owner-occupied-insurance policy): $127
- PITI total: $2,669
Maintenance reserve: 8% of gross rent = $136/month (based on $1,700 target rent for the other unit)
Total all-in cost: $2,805/month
Rental offset:
- Market rent for other unit: $1,700
- Less 5% vacancy factor: −$85
- Net rental offset: $1,615
Effective housing cost: $2,805 − $1,615 = $1,190/month
Kwame was paying $1,475 for a one-bedroom apartment before. His effective housing cost drops $285/month — $3,420 saved per year — while he builds equity in a $347,000 asset and depreciates the rental portion on his taxes.
Six months in, he finds a tenant who signs a two-year lease at $1,750. His effective housing cost drops to under $1,050. The same duplex, the same mortgage — but better numbers because he locked in stronger rent.
Pros & Cons
- Converts house hacking from a gut-feel lifestyle choice into a concrete, comparable monthly number
- Creates a direct apples-to-apples comparison against what you'd pay to rent in the same area
- Works at any property size — duplex, triplex, fourplex, or SFR with ADU — with the same formula
- Reveals instantly whether a deal underwrites at market rents or only works with optimistic assumptions
- Depends entirely on rent accuracy — plug in aspirational rent and you get a misleading number
- Ignores capital expenditure reserves (roof, HVAC, appliance replacement) unless you build them in deliberately
- Doesn't account for owner time — managing tenants, coordinating repairs, and handling vacancies has a real cost in hours
- A single unit vacancy on a duplex doubles your effective housing cost that month
Watch Out
Vacancy is not optional. The single most common effective-housing-cost error is using 100% occupancy in the denominator. Every unit sits empty between tenants. Use 5% minimum — more in soft rental markets. If your deal only works with full occupancy at top rent, the deal doesn't work.
Maintenance reserves are real money. A $1,500/month rental unit will need a new water heater, a fresh coat of paint, and worn flooring addressed within a few years. Skipping the maintenance reserve makes your effective housing cost look lower than it really is — until the bill arrives.
FHA multi-unit loans require occupancy. If you finance a duplex, triplex, or fourplex using FHA owner-occupant rates, you are required to live in one unit. Moving out within the first year violates your loan agreement. Factor this into your exit and lifestyle plan before closing.
Rate assumptions drift. If you're analyzing at today's rate but closing six months from now, rate movement affects PITI — which shifts your effective housing cost in either direction. Lock early or model a range.
Ask an Investor
The Takeaway
Effective housing cost is the number that turns a house hack from an abstract idea into an investment with a monthly price tag you can compare to anything. Run PITI plus maintenance, subtract rent with a vacancy haircut, and you know exactly what shelter costs you after tenants contribute. A well-underwritten duplex or fourplex can bring that number to zero — or below it, where your tenants are funding not just your housing but your savings. That's the house hacker's core equation, and it starts with running this number right.