Updated October 18, 2026. Quick answer: Line 8 starts Part II with your Schedule C income, which caps the deduction. Lines 9 to 11 list the home expenses that would have been personal deductions anyway: casualty losses, mortgage interest and real estate taxes. Lines 12 to 15 total them, apply the business percentage, and subtract from line 8.
Line 8: the income starting point
The form’s caption reads “Enter the amount from Schedule C, line 29, plus any gain derived from the business use of your home, minus any loss from the trade or business not derived from the business use of your home.” The instructions say “If all the gross income from your trade or business is from the business use of your home, enter on line 8 the amount from Schedule(s) C, line 29, plus any gain derived from the business use of your home”. They continue with the loss that is subtracted. If you file more than one Form 8829, the instructions say “include only the income earned and the deductions attributable to that income during the period you owned the home for which Part I was completed.”
If only some of the income comes from your home, the instructions say “If some of the income is from a place of business other than your home, you must first determine the part of your gross income” and add “In making this determination, consider the amount of time you spend at each location as well as other facts.” The Schedule C line 29 figure itself is covered on the Schedule C lines 27a to 29 page.
Columns (a) and (b): direct and indirect expenses
The form says “See instructions for columns (a) and (b) before completing lines 9” and the following lines. The instructions define the two columns. “Direct expenses benefit only the business part of your home.” “Enter 100% of your direct expenses on the appropriate line in column (a).” “Indirect expenses are for keeping up and running your entire home.” “Generally, enter 100% of your indirect expenses on the appropriate line in column (b).”
There is an exception: “If the business percentage of an indirect expense is different from the percentage on line 7, enter only the business part of the expense on the appropriate line in column (a), and leave that line in column (b) blank.” Also, “Other expenses not allocable to the business use of your home, such as salaries, supplies, and advertising, are deductible elsewhere on Schedule C and should not be entered on Form 8829.”
Lines 9 to 11: expenses that would be personal anyway
The instructions say “Use lines 9, 10, and 11 for business use of the home expenses that would have been deductible as a personal expense if you had not used your home for business.” The captions are “Casualty losses (see instructions)”, “Deductible mortgage interest (see instructions)” and “Real estate taxes (see instructions)”.
On the standard deduction the instructions say “If you claim the standard deduction, you will not include any mortgage interest or real estate taxes on lines 10 and 11; instead, you will claim the entire business use of the home portion of those expenses using lines 16 and 17.” The same paragraph says “You may prefer to itemize your deductions on Schedule A to claim amounts on lines 9, 10, and 11”. The standard deduction and itemizing choice is explained at Form 1040 line 12.
Line 9: casualty losses
For itemizers the instructions begin “Complete a worksheet version of Section A of Form 4684 treating all your casualty losses (and gains) as personal expenses.” They add “Do not file this worksheet version of Form 4684; instead, keep it for your records.” The Form 4684 rules are covered on the Form 4684 page and in casualty losses after the 2017 tax law. The instructions send casualty losses beyond this line to the line 29 instructions, covered on the lines 28 to 36 page.
Line 10: mortgage interest
The instructions say “If you are claiming the standard deduction, do not report an amount on line 10.” If you itemize, the first step is “Treat all the mortgage interest you paid as a personal expense and figure the amount that would be deductible as an itemized deduction on Schedule A.” The second step is “Include in column (b) of line 10 the amount of deductible mortgage interest figured in Step 1 that is attributable to the home in which you conducted the business.”
Direct entries are ruled out: “do not claim mortgage interest in column (a) as a direct expense, even if you use a separate structure in your home in connection with your trade or business.”
Line 11: real estate taxes
The instructions say “If you are claiming the standard deduction, do not report an amount on line 11.” If you itemize, they say “If the total of your state and local income (or if elected on your Schedule A, general sales) taxes, real estate taxes, and personal property taxes is not more than $10,000 ($5,000 if married filing separately), enter all the real estate taxes attributable to the home in which you conducted business in column (b) of line 11.”
When the total is higher the instructions say “If you do not meet the condition of Step 1, use the following worksheet to figure the amount to include in column (a) of line 11.” That is the Line 11 Worksheet in the instructions. It is a worksheet with several yes and no steps, so use the worksheet in the instructions for the figures rather than relying on this page.
Lines 12 to 15: totals and the limit
The captions in order are “Add lines 9, 10, and 11” for line 12, “Multiply line 12, column (b), by line 7” for line 13, “Add line 12, column (a), and line 13” for line 14, and “Subtract line 14 from line 8” for line 15, with a floor of zero printed on the form.
Where the number comes from
Line 8 comes from your Schedule C and any related gains or losses. Lines 9 to 11 come from your own records of casualty losses, mortgage interest and real estate taxes, and the business percentage on line 7 comes from lines 1 to 7. The business income itself is reported on Schedule C.
Where it goes next
Line 14 is added again on line 34 (caption “Add lines 14, 27, and 33”), and line 15 is used on line 27 (caption “Allowable operating expenses. Enter the smaller of line 15 or line 26”) and line 28 (caption “Limit on excess casualty losses and depreciation. Subtract line 27 from line 15”). Those lines are explained on the lines 16 to 27 and lines 28 to 36 pages.
Edition and line numbers
This page reads the 2025 Form 8829, Expenses for Business Use of Your Home and its 2025 instructions, the current files irs.gov served on 2026-10-18. They are the files for the tax year 2025 return, filed in 2026. The form’s own header says “File only with Schedule C (Form 1040).” Check the tax year printed on the form you are filling in and use the line numbers on that year’s form, because a different year’s form can number or word its lines differently.
Common mistakes
- Entering mortgage interest or real estate taxes on lines 10 and 11 while taking the standard deduction. The instructions say “If you claim the standard deduction, you will not include any mortgage interest or real estate taxes on lines 10 and 11”
- Putting mortgage interest in the direct column. The instructions say “do not claim mortgage interest in column (a) as a direct expense”
- Entering ordinary business costs on Form 8829. The instructions say such expenses “should not be entered on Form 8829.”
Related: Form 8829, Schedule C lines 27a to 29, Form 4684, Form 1040 line 12, and Form 8829 lines 1 to 7.
Sources
- Instructions for Form 8829, Expenses for Business Use of Your Home (2025): https://www.irs.gov/pub/irs-pdf/i8829.pdf
- Form 8829, Expenses for Business Use of Your Home (2025): https://www.irs.gov/pub/irs-pdf/f8829.pdf
Read from the IRS documents linked above on October 18, 2026. General information, not tax advice. Your own facts decide the outcome, and a preparer, the account provider or the IRS is the right place to confirm anything consequential.