These mistakes show up constantly when investors bring their books in for cleanup. Most of them are fixable but create expensive problems when they go uncorrected for years.

Commingling personal and rental funds tops the list. Investors buy groceries on the same debit card they use to pay a plumber at their rental. By year end, it’s impossible to separate what’s deductible from what isn’t. Every property, or at minimum every entity, needs its own bank account and credit card. Running everything through one mixed account also weakens the liability protection an LLC is supposed to provide if commingling comes up in a lawsuit.

Not tracking expenses by property is the second big one. All rental expenses get dumped into a single category and the investor has no idea which property is actually profitable. You might have three properties where two subsidize the third and never know. Each property needs its own class, location, or sub-account in QuickBooks. Property-level profit and loss is the whole point of keeping books for rentals.

Missing depreciation costs more than any other error. Investors claim mortgage interest, property taxes, and repairs but forget the biggest deduction of all. Residential rentals depreciate over 27.5 years and commercial over 39, based on the building value excluding land. Capital improvements like roofs, HVAC systems, and major renovations depreciate separately from the building. On larger properties, a cost segregation study can accelerate depreciation and generate significant current-year deductions. Leaving depreciation off returns means overpaying taxes every year the property is held.

Not keeping receipts creates audit risk that’s entirely avoidable. The IRS requires documentation for deductions and without receipts those deductions get disallowed. Paper receipts fade and get lost. Use an app that captures receipts with your phone and stores them digitally tied to the expense in your accounting software. It takes a few minutes per week and saves deductions that would otherwise disappear.

Confusing repairs with improvements causes problems in both directions. Repairs get expensed in the current year. Improvements get capitalized and depreciated. Replacing a broken faucet is a repair. Remodeling the kitchen is an improvement. Classifying improvements as repairs triggers IRS scrutiny and potential penalties. Classifying repairs as improvements means spreading a deduction over decades that should have been taken immediately. The distinction matters and most investors get it wrong without help.

Skipping 1099 filings is common and creates unnecessary exposure. Any non-corporate contractor or service provider paid $600 or more during the year needs a 1099-NEC. That includes handymen, landscapers, painters, property managers, and other unincorporated vendors. Penalties run $290 per form or higher plus interest. Collecting W-9s from every vendor before you pay them makes January filing straightforward. Waiting until year end to chase down tax IDs turns into a scramble and some vendors won’t respond, leaving you stuck.

A few other problems show up regularly. Not separating land from building on the books means depreciation gets calculated wrong from the start. Using cash basis accounting when security deposits pile up distorts the picture. Entering mortgage payments as a single expense instead of splitting principal and interest overstates deductions and hides equity buildup.

Most of these mistakes are easy to avoid with the right setup from day one. Cleaning them up later is more expensive than doing it right from the start. If you own two or three properties and find yourself guessing at what went where, working with a bookkeeper familiar with real estate investor accounting pays for itself quickly in caught deductions and cleaner tax returns. The Pasadena bookkeepers at A Squared bring real estate accounting experience from the institutional side to investors running smaller portfolios.