2 rules to consider when deciding how much mortgage you can afford, according to a financial planner

Whether you're a first-time homebuyer or a seasoned real estate investor, buying a home involves a lot of paperwork and patience. And if you want to ensure you're making the right financial decision, it's also important to do the math before your heart is set on a particular house.

While your lender will tell you the maximum loan amount you qualify for, you should be taking a really close look at your budget to understand how much you can comfortably afford. Financial advisors have a few rules to follow, but it's also up to you to understand your comfort level when taking on debt.

CNBC Select spoke with Mark Reyes, CFP and Albert financial advice expert, about the two rules you should follow when taking out a mortgage — and when it might be OK to break them.

When you get pre-approved for a mortgage, the lender will tell you how much loan you can qualify for based on your entire financial picture. Typically, this is a great starting point, especially since some lenders offer a quick pre-approval process. For instance, Ally Bank and Better.com Mortgage can process your pre-approval online in a matter of minutes.

That said, the pre-approval letter shouldn't be your only source of guidance. There are other essential factors you should consider when calculating how much house you could afford, such as your salary. And according to Reyes, the ideal mortgage size should be no more than three times your annual salary.

That's not to say you should always opt for the most expensive mortgage you can qualify for. If you settle on something below your max, you'll have more wiggle room to put money into a high-yield savings account or pay for other costs like home renovations.

If you want to focus your search even more, take the time to think about your monthly spending. While the Consumer Financial Protection Bureau (CFPB) reports that banks will qualify mortgage amounts that are up to 43% of a borrower's monthly income, you might not want to take on that much debt.

"You want to make sure that your monthly mortgage is no more than 28% of your gross monthly income," says Reyes.

So if you bring home $5,000 per month (before taxes), your monthly mortgage payment should be no more than $1,400.

"With a general budget, you want to have 50% of your income going toward utilities, mortgage and other essentials," says Reyes. Keeping your mortgage payment under 30% of your income ensures you have plenty of room for the rest of your needs.

The "three times your salary" rule and the "less than 30% of your monthly income" rule are both helpful guidelines. But the amount you feel comfortable spending on your mortgage payments could differ depending on where you live and your other financial goals.

You should also consider what the market is like where you live, says Reyes. The "three times your salary" rule might not be realistic for people who live in areas with a high cost of living.

If it seems like you might need to take out a bigger mortgage to afford to buy a home, Reyes recommends that you make sure you're in good financial standing in other areas of your life. It's important to have significant emergency savings set aside to make up for the fact that your budget will be stretched a little thin. You should also have ample retirement savings and a separate stash of cash to cover your move-in and closing costs.

But bigger mortgages are not always desirable, explains Reyes. If your mortgage represents too big of a chunk of your income, a lender might charge higher interest rates and other fees to compensate for the higher risk you could default.

Don't spend all your time daydreaming about listings you find online. Do research to learn what kinds of mortgage loans are out there, including FHA, conventional, VA and USDA loan programs. (Here are four tips to help you qualify for a mortgage.) Get pre-approved by a lender before you start shopping, so you know your price range, and you'll be ready to make an offer on the spot if need be.

It's also important to know your credit score. Having a FICO score of 760 or higher will qualify you for the best mortgage rates, so take a few months and build your credit if you can. And then do everything you can to keep it in good standing.

If you're not sure where your credit score currently stands, sign up for a free or paid credit monitoring service to check your score.

CreditWise® from Capital One is a free credit monitoring service that anyone — regardless of whether they are a Capital One cardholder — can use. Receive an updated VantageScore credit score from TransUnion every week and credit report updates from TransUnion and Experian in real-time. Use the credit score simulator to check the potential effect that certain actions, such as paying off debt or closing a credit card, may have on your credit score. In the months leading up to applying for your mortgage, you'll want to be extra careful about closing accounts and racking up debt, as it can decrease your score and make your mortgage more expensive.

PrivacyGuard™, one of CNBC Select's top choices for best credit monitoring services, offers well-rounded coverage, including alerts from all three credit bureaus whenever there's new information as well as monthly credit score and report updates, depending on the plan you choose.

Besides using a pre-approval letter from your lender, you can follow the annual salary and monthly income to determine how much mortgage you can afford. However, remember that every situation is unique. Factor in where you live and any additional financial obligations that you have to figure out how much you're comfortable spending on a mortgage.

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