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@linduca1216 wrote:

 For total area, should the remaining part of the home square footage be divided by 50%

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Assuming the taxpayer uses the entire home, you would use the square footage of the entire home.

As you mentioned, you enter the expenses that were actually paid by the taxpayer.  You mentioned they paid 50% of the utilities, but that would apply to whatever percentage of the mortgage interest, real estate taxes, etc. that they pay for.  And you likely would only enter 50% of the Basis for depreciation.