Updated October 18, 2026. Quick answer: Lines 28 to 33 handle excess casualty losses and depreciation, which are allowed only up to what is left after operating expenses. Line 34 adds the three allowed amounts, line 35 pulls out any casualty loss portion for Form 4684, and line 36 is the allowable expense that goes to Schedule C, line 30.
Form lines read one part at a time: Form 4684 Section B Part I and Form 4684 Section B Part II.
Line 28: the limit
The caption reads “Limit on excess casualty losses and depreciation. Subtract line 27 from line 15”. Line 15 is explained on the lines 8 to 15 page and line 27 on the lines 16 to 27 page.
Lines 29 to 31: the three amounts
Line 29 is captioned “Excess casualty losses (see instructions)”. The instructions say “Multiply the casualty losses attributable to the home in which you conducted business that are in excess of the amount reported on line 9 (if any) by the business percentage of those losses and enter the result.”
Line 30 is captioned “Depreciation of your home from line 42 below”, so it comes from Part III, covered on the lines 37 to 44 page.
Line 31 is captioned “Carryover of prior year excess casualty losses and depreciation (see instructions)”. The instructions say “Enter any amount from your 2024 Form 8829, line 44.” and “If you did not file a 2024 Form 8829, then your carryover of prior year excess casualty losses and depreciation is the amount of excess casualty losses and depreciation shown in Part IV of the last Form 8829, if any, that you filed to claim a deduction for business use of the home.”
Lines 32 and 33: total and allowable
Line 32 is captioned “Add lines 29 through 31” and line 33 is captioned “Allowable excess casualty losses and depreciation. Enter the smaller of line 28 or line 32”. Line 44 later says “Excess casualty losses and depreciation. Subtract line 33 from line 32”.
Line 34: add the allowed amounts
Line 34 is captioned “Add lines 14, 27, and 33”. Those are the three allowed pieces: the casualty, mortgage and tax amount from line 14, the operating expenses from line 27, and the excess casualty and depreciation from line 33.
Line 35: the casualty loss portion
Line 35 is captioned “Casualty loss portion, if any, from lines 14 and 33. Carry amount to Form 4684. See instructions”. The instructions say “Enter this amount on Form 4684, line 27” and write “See Form 8829” above line 27. The Form 4684 rules are on the Form 4684 page, and the effect of the 2017 law on personal casualty losses is at casualty losses after the 2017 tax law.
Line 36: allowable expenses for business use of your home
The caption is “Allowable expenses for business use of your home. Subtract line 35 from line 34. Enter here and on Schedule C, line 30.” The form adds “If your home was used for more than one business, see instructions”. The instructions say “If your home was used in more than one business, allocate the amount shown on line 36 to each business using any method that is reasonable under the circumstances.” They finish “For each business, enter on Schedule C, line 30, only the amount allocated to that business.”
Where the number comes from
Every line here is calculated from other lines of the form, except line 29, which comes from your own casualty loss records, and line 31, which comes from last year’s Form 8829. Depreciation arrives from Part III.
Where it goes next
Line 36 goes to Schedule C, line 30, covered on the Schedule C line 30 page. The instructions note, in the iteration steps, “Then use the results from the Form 8829 to determine the deduction for expenses for business use of your home on Schedule C, line 30.” Any amounts above the limits go to Part IV, explained on the lines 37 to 44 page.
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 a casualty loss on line 29 that is already on line 9. The instructions say to multiply the losses “that are in excess of the amount reported on line 9 (if any)”
- Putting line 36 on Schedule C for several businesses without splitting it. The instructions say “allocate the amount shown on line 36 to each business using any method that is reasonable under the circumstances.”
- Forgetting the Form 4684 step on line 35. The instructions say “Enter this amount on Form 4684, line 27”
Related: Form 8829, Form 4684, casualty losses after the 2017 tax law, Schedule C line 30, and Form 8829 lines 37 to 44.
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.