The heart of any real estate deal is the mathematical formula that determines whether a property is a "money pit" or a "wealth builder." For a house hacker, this calculation is divided into two parts: what you owe (the expenses) and what you earn (the income). To truly understand if a deal works, you must look past the sticker price and dive into the monthly cash flow.

Breaking Down PITI: The Cost of Ownership

PITI is the acronym for the four components of a monthly mortgage payment. Understanding each is vital because they represent the "floor" of your expenses.

Principal: The portion of the payment that goes toward paying down the actual balance of the loan.
Interest: The cost of borrowing the money from the lender.
Taxes: Property taxes levied by the local government. These can vary significantly by neighborhood and are often adjusted after a sale .
Insurance: Homeowners insurance, which protects the structure. For a multi-family home, this may be slightly higher than a single-family home because it covers multiple dwellings.

When you use an FHA loan, you will also have an additional cost: Mortgage Insurance Premium (MIP). Because you are putting down less than 20%, the FHA requires this insurance to protect the lender in case you default . This must be added to your PITI calculation to get an accurate "Total Monthly Payment."

Gross Rental Income vs. Net Income

Gross Rental Income is the total amount of rent you collect from your tenants before any expenses are taken out. For a house hacker living in a triplex, this would be the rent from Unit 2 plus the rent from Unit 3.

However, you cannot rely on the Gross Rental Income to pay your bills. You must calculate your Net Operating Income (NOI). This is the income left after you subtract operating expenses but before you pay the mortgage.

The "Hidden" Operating Expenses

Many beginners make the mistake of thinking: Rent - Mortgage = Profit. This is a dangerous assumption. You must account for the following:

Maintenance (5-10% of Gross Rent): These are small, recurring fixes like leaky faucets, broken light fixtures, or painting a room between tenants .
Capital Expenditures (CapEx): These are big-ticket items that don't happen every year but are inevitable. Think of a new roof, a new furnace, or replacing a water heater. You should set aside a percentage of rent every month into a "reserve fund" so you aren't hit with a $10,000 bill you can't afford.
Vacancy Rate (5-10% of Gross Rent): No property stays 100% occupied forever. You must assume that, on average, your units will be empty for about one month every two years while you clean, advertise, and find a new tenant .
Utilities: In some multi-family setups, there is only one water or gas meter. If you cannot "sub-meter" the units, you (the landlord) may be responsible for paying these bills .

Case Study: The Duplex Deal

Let’s look at a practical example of a duplex purchase using an FHA loan.

Purchase Price: $400,000
Down Payment (3.5%): $14,000
Loan Amount: $386,000
Interest Rate: 6.5%
Estimated PITI (including MIP): $3,100/month

The Income Side:

Unit 1: You live here (Rent = $0)
Unit 2: Rented for $1,800/month
Gross Rental Income: $1,800

The Expense Side (Non-Mortgage):

Vacancy (5%): $90
Maintenance/CapEx (10%): $180
Total Operating Expenses: $270

The Final Calculation:

Total Monthly Cost: $3,100 (Mortgage) + $270 (Reserves) = $3,370
Rental Income: $1,800
Your Net Housing Cost: $1,570

In this scenario, you are paying $1,570 out of pocket to live in a $400,000 property. If a comparable single-family home in that neighborhood would cost you $2,800 a month to own, you are "saving" $1,230 every month. This is the essence of a successful house hack.

The Importance of Cash Reserves

Lenders, especially for multi-family properties, often require you to have "cash reserves." This is a set amount of money sitting in your bank account after the down payment and closing costs are paid. Most lenders want to see 4 to 8 months of mortgage payments, taxes, and insurance in reserve . This ensures that if a tenant moves out or a major repair is needed, you won't immediately default on the loan.

Tax Implications and Schedule E

Becoming a landlord changes your relationship with the IRS. You are required to report all rental income, but you also get to deduct expenses. According to IRS Publication 527, you can deduct:

Mortgage interest
Property taxes
Insurance premiums
Repairs and maintenance
Depreciation (a non-cash expense that accounts for the "wear and tear" on the building)

Because you live in one of the units, you must "pro-rate" these expenses. If you live in 50% of the house, you can only deduct 50% of the shared expenses (like a roof repair) on your Schedule E. The other 50% is considered a personal expense .

FAQ: Math and Money

1. What is a "Net Tangible Benefit"?
This is a term used during refinancing. For an FHA streamline refinance, the new loan must provide a clear benefit, such as lowering your monthly principal and interest payment or shortening the loan term .

2. Can I use my IRA to buy the property?
You can use a self-directed IRA to buy real estate, but the rules are extremely strict. You cannot live in the property, and your family members cannot live in it either . For house hacking, an IRA is generally not the right tool because of the owner-occupancy requirement.

3. What happens if the tenant doesn't pay?
You are still responsible for the full mortgage payment. This is why banks look at your Debt-to-Income (DTI) ratio so closely; they want to know you can handle the payment even if the rental income stops .

4. Is the interest on a HELOC tax-deductible?
Only if the funds are used to "buy, build, or substantially improve" the home that secures the loan .

5. How does depreciation work?
Depreciation allows you to deduct the cost of the building (not the land) over 27.5 years. It is a powerful way to reduce your taxable income even if the property is actually increasing in value .