When only part of a property earns rent, the expense file needs more detail than a pile of whole-house bills. A reasonable allocation connects each claimed amount to the rental activity.
Start with the rooms and periods involved. The same percentage will not necessarily suit every bill.
Separate three kinds of cost
A repair entirely inside the rented unit may be directly attributable to that unit. A repair in the owner's private bedroom is personal. A shared heating bill needs an allocation supported by the circumstances.
Keep a simple floor plan showing the relevant areas and explain how shared rooms are treated. Record when the space became available for rent and any periods of personal use. Seasonal use may require both an area and a time adjustment.
If a supported rental allocation is 25% for a full year, a $2,000 shared expense produces a $500 rental portion before other deductibility restrictions. A different use pattern requires a different calculation.
Record the reason for the percentage
Save the measurements, dates and calculation with the bill. Do not choose a percentage merely because it produces an attractive deduction. Revisit it when the tenant's space or your own use changes.
A below-market family cost-sharing arrangement requires separate analysis. Receiving money from a relative does not automatically establish a commercial rental activity or a deductible rental loss.
Check rules beyond the allocation
A reasonable percentage cannot turn a capital improvement into a current expense. Nor can it override the expense restrictions for non-compliant short-term rentals.
Renting a home or part of one can also raise change-of-use questions. Review those before claiming capital cost allowance or assuming the eventual sale remains fully exempt.
The aim is a file that another person can reproduce: which cost, which space, which period and why that share belongs to the rental.
