"House Hacking" Calculator: How a Multi-Unit Property Changes Your Mortgage Math in 2026

House hacking is simple:

You live in one unit and rent the others.

But the math behind it is often misunderstood.

A normal mortgage calculator assumes you pay everything alone.
A house hacking calculator factors in rental income, which can change affordability fast.

This guide explains how house hacking works in 2026 and how a calculator built for it shows what you can really afford.

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What Is House Hacking? (Quick Explanation)

House hacking means buying:

• A duplex, triplex, or fourplex
• A home with an ADU (accessory dwelling unit)

You:

• Live in one unit
• Rent out the others
• Use rent to cover part of the mortgage

The goal is not profit at first.
The goal is lower monthly cost.

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Why Normal Mortgage Calculators Get This Wrong

Standard calculators:

• Ignore rental income
• Assume one household pays the full mortgage
• Show higher monthly stress

For multi-unit buyers, this makes good deals look impossible.

That’s where a house hacking mortgage calculator matters.

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How Lenders View Rental Income (2026 Rules)

Most lenders:

• Count 50%–75% of expected rent
• Require a lease or market rent estimate
• Still check your personal income

They don’t count 100% of rent, but they do count enough to help.

A good calculator must reflect this.

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Example: Single-Family vs Duplex

Single-family home

• Price: $400,000
• Mortgage payment: $2,400
• You pay: $2,400

Duplex (house hack)

• Same price: $400,000
• Your unit: live in
• Other unit rent: $1,400

Lender counts 75% of rent:

$1,050

Effective monthly cost:

$2,400 − $1,050 = $1,350

That’s the power of house hacking.

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How a House Hacking Calculator Should Work

A proper calculator should let users:

• Enter number of units
• Add expected rent per unit
• Apply lender rent rules (50–75%)
• Show real monthly out-of-pocket cost

Without this, the numbers lie.

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Example: “Afford More” Without Risk

Without house hacking

• Safe housing budget: $1,800
• Max home price: ~$300,000

With house hacking

• Rental offset: $1,000
• Same budget
• New buying power: $420,000+

Same income.
Same budget.
Different structure.

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ADUs: The Quiet House Hack

ADUs are popular in 2026 because:

• They add rental income
• They increase property value
• They often need less space

A house hacking calculator should include:

• Main unit mortgage
• ADU rent
• Shared expenses
• Net monthly cost

Ignoring ADUs misses a major trend.

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Costs House Hackers Must Not Ignore

Rental income helps — but costs still exist.

Always include:

• Maintenance
• Vacancies
• Repairs
• Higher insurance
• Property taxes on full value

A good calculator shows net benefit, not just rent.

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When House Hacking Makes Sense

House hacking works best if:

• You plan to stay a few years
• You’re comfortable sharing property
• Local rent supports the numbers
• Zoning allows it

It’s not passive income.
It’s active ownership.

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Common House Hacking Mistakes

• Counting 100% of rent
• Ignoring vacancy risk
• Forgetting maintenance
• Buying too much property

A calculator helps avoid these early.

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What Makes a Great House Hacking Calculator

The best tools in 2026:

• Separate personal payment vs net cost
• Use conservative rent rules
• Show lender view and real-life view
• Allow scenario testing

This builds confidence before you buy.

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Final Thoughts

House hacking doesn’t mean “free housing.”

It means:

• Shared costs
• Smarter structure
• Lower personal burden

A house hacking calculator shows how rental income changes your mortgage math — and why multi-unit homes can open doors that single-family homes can’t.

Used correctly, it’s not a shortcut.
It’s a strategy.

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Disclaimer: Rental income rules vary by lender and location. This guide is for education only. Always confirm qualification rules with a licensed lender before buying.